NIST Special Publication 800-175B

# Guideline for Using Cryptographic Standards in the Federal Government:
*Cryptographic Mechanisms*

Elaine Barker

COMPUTER SECURITY

NIST
National Institute of Standards and Technology
U.S. Department of Commerce

# NIST Special Publication 800-175B

# Guideline for Using Cryptographic Standards in the Federal Government:
*Cryptographic Mechanisms*

Elaine Barker
Computer Security Division
Information Technology Laboratory

March 2016

U.S. Department of Commerce
Penny Pritzker, Secretary

National Institute of Standards and Technology
Willie May, Under Secretary of Commerce for Standards and Technology and Director

NIST SP 800-175B (DRAFT) Guideline for Using Crypto Standards: Cryptographic Mechanisms

Authority

This publication has been developed by NIST in accordance with its statutory responsibilities under the Federal Information Security Modernization Act (FISMA) of 2014, 44 U.S.C. § 3541 et seq., Public Law (P.L.) 113-283. NIST is responsible for developing information security standards and guidelines, including minimum requirements for federal information systems, but such standards and guidelines shall not apply to national security systems without the express approval of appropriate federal officials exercising policy authority over such systems. This guideline is consistent with the requirements of the Office of Management and Budget (OMB) Circular A-130.

Nothing in this publication should be taken to contradict the standards and guidelines made mandatory and binding on federal agencies by the Secretary of Commerce under statutory authority. Nor should these guidelines be interpreted as altering or superseding the existing authorities of the Secretary of Commerce, Director of the OMB, or any other federal official. This publication may be used by nongovernmental organizations on a voluntary basis and is not subject to copyright in the United States. Attribution would, however, be appreciated by NIST.

National Institute of Standards and Technology Special Publication 800-175B Natl. Inst. Stand. Technol. Spec. Publ. 800-175B, 77 pages (March 2016) CODEN: NSPUE2

Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by NIST, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

There may be references in this publication to other publications currently under development by NIST in accordance with its assigned statutory responsibilities. The information in this publication, including concepts and methodologies, may be used by federal agencies even before the completion of such companion publications. Thus, until each publication is completed, current requirements, guidelines, and procedures, where they exist, remain operative. For planning and transition purposes, federal agencies may wish to closely follow the development of these new publications by NIST.

Organizations are encouraged to review all draft publications during public comment periods and provide feedback to NIST. Many NIST cybersecurity publications, other than the ones noted above, are available at http://csrc.nist.gov/publications.

Public comment period: March 11, 2016 through April 29, 2016 National Institute of Standards and Technology Attn: Computer Security Division, Information Technology Laboratory 100 Bureau Drive (Mail Stop 8930) Gaithersburg, MD 20899-8930 Email: SP800-175@nist.gov

NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

Reports on Computer Systems Technology

The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology (NIST) promotes the U.S. economy and public welfare by providing technical leadership for the Nation’s measurement and standards infrastructure. ITL develops tests, test methods, reference data, proof of concept implementations, and technical analyses to advance the development and productive use of information technology. ITL’s responsibilities include the development of management, administrative, technical, and physical standards and guidelines for the cost-effective security and privacy of other than national security-related information in federal information systems. The Special Publication 800-series reports on ITL’s research, guidelines, and outreach efforts in information system security, and its collaborative activities with industry, government, and academic organizations.

Abstract

This document is intended to provide guidance to the Federal Government for using cryptography and NIST’s cryptographic standards to protect sensitive, but unclassified digitized information during transmission and while in storage. The cryptographic methods and services to be used are discussed.

Keywords

Asymmetric-key algorithm, authentication, confidentiality, cryptography, digital signatures, encryption, integrity, key agreement, key derivation, key management, key transport, key wrapping, message authentication codes, non-repudiation, Public Key Infrastructure, random bit generation, symmetric-key algorithm.

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NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

Acknowledgments

The author wishes to thank the authors of SP 800-21 from which this document was derived, Annabelle Lee and William C. Barker, along with those colleagues that reviewed drafts of this document and contributed to its development. The author also gratefully acknowledges and appreciates the many comments from the public and private sectors whose thoughtful and constructive comments improved the quality and usefulness of this publication.

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NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

Table of Contents

SECTION 1: INTRODUCTION ............................................................................................ 1 1.1 Background and Purpose ......................................................................................... 1 1.2 Audience ................................................................................................................... 2 1.3 Scope......................................................................................................................... 2 1.4 Background............................................................................................................... 2 1.5 Terms and Definitions............................................................................................... 3 1.6 Acronyms.................................................................................................................. 9 1.7 Content................................................................................................................... 10 SECTION 2: STANDARDS AND GUIDELINES ................................................................ 12 2.1 Benefits of Standards .............................................................................................. 12 2.2 Federal Information Processing Standards and Special Publications .................... 13 2.2.1 The Use of FIPS and SPs .................................................................................... 13 2.2.2 FIPS Waivers........................................................................................................ 14 2.3 Other Standards Organizations ............................................................................... 14 2.3.1 American National Standards Institute (ANSI).................................................... 14 2.3.2 Institute of Electrical and Electronics Engineers (IEEE) Standards Association 15 2.3.3 Internet Engineering Task Force (IETF).............................................................. 16 2.3.4 International Organization for Standardization (ISO).......................................... 17 2.3.5 Trusted Computing Group (TCG)......................................................................... 18 SECTION 3: CRYPTOGRAPHIC ALGORITHMS ................................................................ 19 3.1 Cryptographic Hash Functions ................................................................................. 19 3.2 Symmetric-Key Algorithms........................................................................................ 20 3.2.1 Block Cipher Algorithms ...................................................................................... 21 3.2.1.1 Data Encryption Standard (DES)..........................................................................22 3.2.1.2 Triple Data Encryption Algorithm (TDEA)...........................................................22 3.2.1.3 SKIPJACK.............................................................................................................22 3.2.1.4 Advanced Encryption Standard (AES).................................................................23 3.2.1.5 Modes of Operation.............................................................................................23 3.2.2 Hash-based Symmetric-key Algorithms ............................................................... 23 3.3 Asymmetric-Key Algorithms...................................................................................... 23 3.3.1 DSA...................................................................................................................... 25 3.3.2 ECDSA................................................................................................................. 25 3.3.3 RSA ..................................................................................................................... 26 3.3.4 Diffie-Hellman and MQV ..................................................................................... 26 3.4 Algorithm Security Strength ..................................................................................... 26 3.5 Algorithm Lifetime ................................................................................................... 27 SECTION 4: CRYPTOGRAPHIC SERVICES .................................................................... 28 4.1 Data Confidentiality.................................................................................................. 28 4.2 Data Integrity and Source Authentication............................................................... 29

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4.2.1 Hash Functions........................................................................................................... 29 4.2.2 Message Authentication Code Algorithms............................................................... 30 4.2.2.1 MACs Based on Block Cipher Algorithms...................................................31 4.2.2.2 MACs Based on Hash Functions..................................................................31 4.2.3 Digital Signature Algorithms ................................................................................... 32 4.3 Combining Confidentiality and Authentication in a Block-Cipher Mode of Operation......................................................................................................................... 34 4.4 Random Bit Generation ................................................................................................. 34 4.5 Symmetric vs. Asymmetric Cryptography .................................................................... 36 SECTION 5: KEY MANAGEMENT ..................................................................................... 37 5.1 General Key Management Guidance ............................................................................ 37 5.1.1 Recommendation for Key Management........................................................... 37 5.1.2 Security Requirements for Cryptographic Modules......................................... 39 5.1.3 Transitions to New Cryptographic Algorithms and Key Lengths.................... 39 5.2 Cryptographic Key Management Systems.................................................................... 40 5.2.1 Key Management Framework ......................................................................... 40 5.2.2 Key Management System Profile.................................................................... 41 5.2.3 Public Key Infrastructure ................................................................................. 41 5.2.3.1 PKI Components, Relying Parties and Their Responsibilities................42 5.2.3.2 Basic Certificate Verification Process ....................................................44 5.2.3.3 CA Certificate Policies and Certificate Practice Statements ..................44 5.2.3.4 Federal Public Key Infrastructure ..........................................................45 5.3 Key Establishment ......................................................................................................... 45 5.3.1 Key Generation ................................................................................................ 45 5.3.2 Key Derivation ................................................................................................. 46 5.3.3 Key Agreement ................................................................................................. 47 5.3.4 Key Transport................................................................................................... 48 5.3.4.1 SP 800-56A Key Transport....................................................................48 5.3.4.2 SP 800-56B Key Transport ....................................................................49 5.3.5 Key Wrapping .................................................................................................. 50 5.3.6 Derivation of a Key from a Password .............................................................. 51 5.4 Key Management Issues ................................................................................................ 51 5.4.1 Manual vs. Automated Key Establishment ...................................................... 51 5.4.2 Selecting and Operating a CKMS .................................................................... 51 5.4.3 Storing and Protecting Keys............................................................................ 51 5.4.4 Cryptoperiods .................................................................................................. 52 5.4.5 Use Validated Algorithms and Cryptographic Modules .................................. 52 5.4.6 Control of Keying Material ............................................................................. 53 5.4.7 Compromises.................................................................................................... 53

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5.4.8 Accountability and Auditing .................................................................................... 54 SECTION 6: OTHER ISSUES ................................................................................................. 55 6.1 Required Security Strength .................................................................................... 55 6.2 Interoperability ....................................................................................................... 55 6.3 When Algorithms are no Longer Approved ............................................................ 56 6.4 Registration Authorities (RAs) .............................................................................. 56 6.5 Cross Certification .................................................................................................. 56 Appendix A: References ................................................................................................. 57

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SECTION 1: INTRODUCTION

1.1 Background and Purpose

In today's environment of increasingly open and interconnected systems and networks and the use of mobile devices, network and data security are essential for the optimum safe use of this information technology. Cryptographic techniques should be considered for the protection of data that is sensitive, has a high value, or is vulnerable to unauthorized disclosure or undetected modification during transmission or while in storage.

Cryptography is a branch of mathematics that is based on the transformation of data and can be used to provide several security services: confidentiality, data integrity authentication, and source authentication, and also to support non-repudiation.

• Confidentiality is the property whereby sensitive information is not disclosed to unauthorized entities. Confidentiality can be provided by a cryptographic process called encryption.
• Data integrity is a property whereby data has not been altered in an unauthorized manner since it was created, transmitted or stored. The process of determining the integrity of the data is called data integrity authentication.
• Source authentication is a process that provides assurance of the source of information to a receiving entity; source authentication can also be considered as identity authentication (i.e., providing assurance of an entity's identity). A special case of source authentication is called non-repudiation, whereby support for assurance of the source of the information is provided to a third party.

This document is one part in a series of documents intended to provide guidance to the Federal Government for using cryptography to protect its sensitive, but unclassified digitized information during transmission and while in storage; hereafter, the shortened term "sensitive" will be used to refer to this class of information. Other sectors are invited to use this guidance on a voluntary basis. The following are the initial publications to be included in the SP 800-175 series. Additional documents may be provided in the future.

• SP 800-175A will provide guidance on the determination of requirements for using cryptography. It will include the laws and regulations for the protection of the Federal Government's sensitive information, guidance for the conduct of risk assessments to determine what needs to be protected and how best to protect that information, and a discussion of the required security-related documents (e.g., various policy and practice documents). DOCUMENT UNDER DEVELOPMENT.
• SP 800-175B (this document) discusses the cryptographic methods and services available for the protection of the Federal Government's sensitive information and provides an overview of NIST's cryptographic standards.

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NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

1.2 Audience

This document is intended for federal employees and others who are responsible for providing and using cryptographic services to meet identified security requirements. This document might be used by:

• Program managers responsible for selecting and integrating cryptographic mechanisms into a system,
• A technical specialist requested to select one or more cryptographic methods/techniques to meet a specified requirement,
• A procurement specialist developing a solicitation for a system, network or service that will require cryptographic methods to perform security functionality, and
• Users of cryptographic services.

The goal is to provide these individuals with sufficient information to allow them to make informed decisions about the cryptographic methods that will meet their specific needs to protect the confidentiality and integrity of data that is transmitted and/or stored in a system or network, as well as to obtain assurance of its authenticity.

This document is not intended to provide information on the federal procurement process or to provide a technical discussion on the mathematics of cryptography and cryptographic algorithms.

1.3 Scope

This document limits its discussion of cryptographic methods to those that conform to Federal Information Processing Standards (FIPS) and NIST Special Publications (SPs), which are collectively discussed as NIST "standards" in this document. While the Federal Government is required to use these standards, when applicable, industry and national and international standards bodies have also adopted these cryptographic methods.

This document provides information on selecting and using cryptography in new or existing systems.

1.4 Background

The use of cryptography relies upon two basic components: an algorithm (or cryptographic methodology) and a key. The algorithm is a mathematical function, and the key is a parameter used during the cryptographic process. The algorithm and key are used together to apply cryptographic protection to data (e.g., to encrypt the data or to generate a digital signature) and to remove or check the protection (e.g., to decrypt the encrypted data or to verify the digital signature). The security of the cryptographic protection relies on the secrecy of the key, while the algorithm specification is publicly available.

In order to use a cryptographic algorithm, cryptographic keys must be “in place”, i.e., keys must be established for and/or between parties that intend to use cryptography. Keys may be established either manually (e.g., via a trusted courier) or using an automated method. However, when an automated method is used, authentication is required for the

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76 participating entities that relies on an established trust infrastructure, such as a Public Key 77 Infrastructure (PKI) or on a manually distributed authentication key. 78 In general, keys used for one purpose (e.g., the generation of digital signatures) must not 79 be used for another purpose (e.g., for key establishment) because the use of the same key 80 for two different cryptographic processes may weaken the security provided by one or both 81 of the processes. See Section 5.2 in SP 800-57, Part 1^1 for further information.

82 1.5 Terms and Definitions 83 The following terms and definitions are used in this document. In general, the definitions 84 are drawn from FIPS and NIST Special Publications.

^1 SP 800-57, Part 1: Recommendation for Key Management: General Guideline.

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NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

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NIST SP 800-175B (DRAFT) GUIDELINE FOR USING CRYPTO STANDARDS: CRYPTOGRAPHIC MECHANISMS

| Signature generation | The use of a digital signature algorithm and a private key to generate a digital signature on data. | | Signature verification | The use of a digital signature and a public key to verify a digital signature. | | Source authentication | A process that provides assurance of the source of information. | | Static key pair | A long-term key pair for which the public key is often provided in a public-key certificate. | | Symmetric key | A single cryptographic key that is used with a symmetric (secret key) algorithm. | | Symmetric (secret key) algorithm | A cryptographic algorithm that uses the same secret key for an operation and its complement (e.g., encryption and decryption). |

85 1.6 Acronyms 86 AES Advanced Encryption Standard; specified in FIPS 197. 87 ANS American National Standard. 88 ANSI American National Standard Institute. 89 ASC Accredited Standards Committee. 90 CA Certification Authority. 91 CBC Cipher Block Chaining mode; specified in SP 800-38A. 92 CFB Cipher Feedback mode; specified in SP 800-38A. 93 CKMS Cryptographic Key Management System. 94 CP Certificate Policy. 95 CPS Certification Practice Statement. 96 CRL Certificate Revocation List. 97 CTR Counter mode; specified in SP 800-38A. 98 DES Data Encryption Standard; originally specified in FIPS 46; now provided in 99 SP 800-67. 100 DH Diffie-Hellman algorithm. 101 DNSSEC Domain Name System Security Extensions. 102 DRBG Deterministic Random Bit Generator; specified in SP 800-90A. 103 DSA Digital Signature Algorithm; specified in FIPS 186. 104 ECB Electronic Codebook mode; specified in SP 800-38A. 105 ECDSA Elliptic Curve Digital Signature Algorithm. 106 EMC Electromagnetic Compatibility. 107 FCKMS Federal Cryptographic Key Management System. 108 FIPS Federal Information Processing Standard. 109 FISMA Federal Information Security Management Act. 110 GCM Galois Counter Mode; specified in SP 800-38D. 111 HMAC Keyed-Hash Message Authentication Code; specified in FIPS 198. 112 IEC International Electrotechnical Commission.

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113 IEEE Institute of Electrical and Electronics Engineers. 114 IETF Internet Engineering Task Force. 115 EMI Electromagnetic Interference. 116 INCITS International Committee for Information Technology Standards. 117 IPSEC Internet Protocol Security. 118 ISO International Standards Organization. 119 IT Information Technology. 120 MAC Message Authentication Code. 121 MQV Menezes-Qu-Vanstone algorithm; specified in SP 800-56A. 122 NRBG Non-deterministic Random Bit Generator. 123 NIST National Institute of Standards and Technology. 124 OFB Output Feedback mode; specified in SP 800-38A. 125 OTAR Over-the-Air-Rekeying. 126 PKI Public Key Infrastructure. 127 RA Registration Authority. 128 RBG Random Bit Generator. 129 RFC Request for Comment. 130 RSA Rivest, Shamir, Adleman. 131 SHA Secure Hash Algorithm. 132 SP Special Publication. 133 SSH Secure Shell protocol. 134 TCG Trusted Computing Group. 135 TDEA Triple Data Encryption Algorithm; specified in SP 800-67. 136 TLS Transport Layer Security.

137 1.7 Content 138 This document is organized into the following sections: 139 * Section 1 provides an introduction to the SP 800-175 series of publications and to 140 this document in particular, and provides a glossary of terms and a list of acronyms. 141 * Section 2 discusses the importance of standards, as well as the national and 142 international standards bodies concerned with cryptography. 143 * Section 3 introduces the approved algorithms used for encryption, digital signature 144 and key-establishment, and provides discussions on security strengths and 145 algorithm lifetime. 146 * Section 4 discusses the services that cryptography can provide: data confidentiality, 147 data integrity authentication, source authentication and support for non-repudiation. 148 * Section 5 discusses the key management required for the use of cryptography, 149 providing general guidance and discussions on key-management systems, key- 150 establishment mechanisms and random bit generation. 151 * Section 6 discusses additional issues associated with the use of cryptography. 152 There is one appendix in this document:

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153

• Appendix A lists applicable Federal Information Processing Standards, 154 recommendations, and guidelines. 155

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SECTION 2: STANDARDS AND GUIDELINES

2.1 Benefits of Standards

Standards define common practices, methods, and measures/metrics. Standards provide solutions that have been evaluated by experts in relevant areas, reviewed by the public and subsequently accepted by a wide community of users. By using standards, organizations can reduce costs and protect their investments in technology.

Standards provide the following benefits:

• Interoperability. Products developed to a specific standard may be used to provide interoperability with other products that conform to the same standard. For example, by using the same cryptographic encryption algorithm, data that was encrypted using vendor A’s product may be decrypted using vendor B’s product. The use of a common standards-based cryptographic algorithm is necessary, but may not be sufficient to ensure product interoperability. Other common standards, such as communications protocol standards, may also be necessary.

  By ensuring interoperability among the products of different vendors, standards permit an organization to select from various available products to find the most cost-effective solution.

• Security. Standards may be used to establish a common approved level of security. For example, most agency managers are not cryptographic security experts, and, by using an approved cryptographic algorithm and key length, a manager knows that the algorithm has been found to be adequate for the protection of sensitive government data and has been subjected to a significant period of public analysis and comment.

• Quality. Standards may be used to assure the quality of a product. Standards may:

  • Specify how a feature is to be implemented,
  • Require self-tests to ensure that the product is still functioning correctly, and
  • Require specific documentation to assure proper implementation and product-change management.

  Many NIST standards have associated conformance tests and specify the conformance requirements. The conformance tests may be administered by NIST-accredited laboratories and provide validation that the NIST standard was correctly implemented.

• Common Form of Reference. A NIST standard may become a common form of reference to be used in testing/evaluating a vendor’s product. For example, FIPS 140² contains security and integrity requirements for any cryptographic module implementing cryptographic operations.

² FIPS 140, Security Requirements for Cryptographic Modules.

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193 • Cost Savings. A standard can save money by providing a single commonly 194 accepted specification. Without standards, users may be required to become 195 experts in every information technology (IT) product that is being considered for 196 procurement. Also, without standards, products may not interoperate with different 197 products purchased by other users. This will result in a significant waste of money 198 or in the delay of implementing IT.

199 2.2 Federal Information Processing Standards and Special Publications

200 2.2.1 The Use of FIPS and SPs

201 A Federal Information Processing Standard (FIPS) is mandatory for the Federal 202 Government whenever the type of service provided by that standard is required by a federal 203 agency for the protection of sensitive information. For example, FIPS 197³ contains a 204 specific set of technical security requirements for the AES algorithm. Whenever AES is 205 used by an agency, the implementation and use must conform to FIPS 197. A FIPS is 206 approved via a signature by the Secretary of Commerce.

207 A NIST Special Publication (SP) is similar to a FIPS, but is not mandatory unless a 208 particular government agency (e.g., OMB) makes it so. An SP does not need the approval 209 of the Secretary of Commerce.

210 Although the requirements for the use of a FIPS and an SP are different, both types of 211 publications have been subjected to the same review process by the federal agencies and 212 the public. The approval process for a FIPS is more formal than that of an SP, and 213 subsequently takes longer for the initial approval and the approval of any subsequent 214 revisions.

215 When a federal agency requires the use of cryptography (e.g., for encryption), an approved 216 algorithm must be used; approval is indicated by inclusion in a FIPS or SP. For example, 217 two approved algorithms for encryption are AES (as specified in FIPS 197) and TDEA 218 (as specified in SP 800-67⁴). Whenever encryption is used by a federal agency for the 219 protection of sensitive information, either AES or TDEA must be used. Whenever AES is 220 to be used, it must be implemented as specified in FIPS 197; whenever TDEA is to be used, 221 it must be implemented as specified in SP 800-67. In addition to using approved 222 algorithms, federal agencies are required to use only implementations of these algorithms 223 that have been validated and are included in validated cryptographic modules (see Section 224 5.4.5 for further discussion).

225 When developing a specification or the criteria for the selection of a cryptographic 226 mechanism or service, cryptographic algorithms specified in FIPSs and SPs must be used, 227 when available. Some guidelines may be used to specify the functions that the algorithm

³ FIPS 197, the Advanced Encryption Standard. ⁴ SP 800-67, Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher.

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228 will perform (e.g., FIPS 199^5 or SP 800-53^6). Other NIST standards specify the operation 229 and use of specific types of algorithms (e.g., AES, DSA) and the level of independent 230 testing required for classes of security environments (e.g., FIPS 140). 231 Appendix A contains a list of FIPS and SPs that apply to the implementation of 232 cryptography in the Federal Government. Note that when a FIPS is revised, its number is 233 commonly followed by a revision number that indicates the number of times that it has 234 been revised (e.g., "FIPS 186-4" is used to indicate the fourth revision of FIPS 186); this 235 practice is not used in the main body of this document; the reader must refer to the latest 236 version of the FIPS or SP that has been officially approved (see 237 http://csrc.nist.gov/publications/index.html; note that this site may also contain clearly 238 marked draft publications). 239 2.2.2 FIPS Waivers 240 In the past, a waiver was sometimes issued by an agency to indicate that the use of a FIPS 241 was not required by that agency. However, the Federal Information Security Management 242 Act (FISMA) of 2002 (P.L. 107-347) eliminated previously authorized provisions for 243 waivers from FIPS (see SP 800-175A for a discussion). 244 2.3 Other Standards Organizations 245 NIST develops standards, recommendations, and guidelines that are used by vendors who 246 are developing security products, components, and modules. These products may be 247 acquired and used by federal government agencies. In addition, there are other groups that 248 develop and promulgate standards. These organizations are briefly described below. 249 2.3.1 American National Standards Institute (ANSI)^7 250 The American National Standards Institute (ANSI) is the administrator and coordinator of 251 the United States (U.S.) private-sector voluntary standardization system. ANSI does not 252 develop American National Standards itself; rather, it facilitates the development of 253 standards by establishing consensus among qualified groups. 254 Several ANSI committees have developed standards that use cryptography, but the primary 255 committee that has developed standards for the cryptographic algorithms themselves is 256 Accredited Standards Committee (ASC) X9, which is a financial-industry committee^8. 257 Many of the standards developed within ASC X9 have been adopted within NIST standards 258 (e.g., the Elliptic Curve Digital Signature Algorithm specified in American National 259 Standard X9.62^9 has been adopted in FIPS 186); likewise, ASC X9 has approved the use

^5 FIPS 199, Standards for Security Categorization of Federal Information and Information Systems. ^6 SP 800-53, Recommended Security Controls for Federal Information Systems. ^7 Further information is available at the ANSI web site: www.ansi.org. ^8 Further information is available at the ANSI X9 web site: x9.org. ^9 ANS X9.62, Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA).

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260 of NIST standards via a registry of approved standards from non-ASC X9 sources (e.g., 261 AES, as specified in FIPS 197). 262 A number of ASC X9 standards have also been incorporated into the standards of other 263 standards bodies, such as the International Standards Organization (ISO) (see Section 264 2.3.4) via a Technical Advisory Group (TAG) called the International Committee on 265 Information Technology Standards (INCITS). INCITS has been responsible for assuring 266 that U.S. standards (e.g., both those developed by NIST and those developed within ASC 267 X9) are incorporated within ISO standards. 268 2.3.2 Institute of Electrical and Electronics Engineers (IEEE) Standards 269 Association^10 270 IEEE is an international, professional association that is dedicated to advancing 271 technological innovation and excellence. The technical objectives of the IEEE focus on 272 advancing the theory and practice of electrical, electronics and computer engineering, and 273 computer science. IEEE develops and disseminates voluntary, consensus-based industry 274 standards involving leading-edge electro-technology. IEEE supports international 275 standardization and encourages the development of globally acceptable standards. 276 The Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) is 277 an organization within IEEE that develops global standards. It has more than one thousand 278 active standards, some of which are related to cryptography. 279 IEEE P1363^11 is the only IEEE standard that focus on cryptography. It includes a series of 280 standards on public-key cryptography. IEEE P1363 was developed at the same time as the 281 ANSI public-key cryptographic standards, such as ANS X9.31^12, X9.42^13, X9.44^14, 282 X9.62^15, and X9.63^16, which were developed in ASC X9 (see Section 2.4.1). 283 * The first part of the IEEE P1363 standard was published in 2000 and revised in 284 2004 as IEEE P1363a^17. It includes the basic public-key cryptography schemes, 285 such as RSA encryption, signatures, the Digital Signature Algorithm (DSA), and 286 key establishment using Diffie-Hellman (DH) and Menezes-Qu-Vanstone (MQV) 287 over finite fields and elliptic curves.

^10 Further information is available at the IEEE-SA web site: standards.ieee.org. ^11 IEEE P1363: Standard Specifications for Public-Key Cryptography. ^12 ANS X9.31, Digital Signatures Using Reversible Public Key Cryptography for the Financial Services Industry (rDSA), which has now been withdrawn. ^13 ANS X9.42, Agreement of Symmetric Keys Using Discrete Logarithm Cryptography, which has now been withdrawn. ^14 ANS X9.44, Key Establishment Using Integer Factorization Cryptography. ^15 ANS X9.62, The Elliptic Curve Digital Signature Algorithm (ECDSA). ^16 ANS X9.63, Key Agreement and Key Transport Using Elliptic Curve Cryptography. ^17 IEEE P1363a, Standard Specifications for Public Key Cryptography - Amendment 1: Additional Techniques.

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• IEEE P1363.1^18^, which was published in 2008, specifies NTRU encryption and signature schemes.
• IEEE P1363.2^19^ was also published in 2008. It specifies password-authenticated key agreement and password-authenticated key retrieval schemes.

The schemes specified in IEEE P1363.1 and P1363.2 are not included in the NIST standards.

Cryptographic schemes are used in IEEE standards for different applications. One of the more notable is the IEEE 802 LAN/MAN group of standards, which are widely used computer networking standards for both wired (Ethernet) and wireless (IEEE 802.11^20^) networks. Cryptographic algorithms are used to protect wireless communications. The CCM mode for authentication and confidentiality specified in SP 800-38C was adopted from IEEE 802.11. Other AES modes of operations (e.g., GCM, which is specified in SP 800-38D) are also used in IEEE 802 standards. IEEE 802 standards also use the SHA-1 and SHA-2 family of hash functions specified in FIPS 180 and used in HMAC, as specified in FIPS 198.

XTS, a block cipher mode of operation specified in SP 800-38E, was adopted from IEEE P1619^21^.

2.3.3 Internet Engineering Task Force (IETF)

The Internet Engineering Task Force (IETF) is an international community of network designers, operators, vendors, researchers, and technologists that work on the Internet architecture, and its techniques and protocols. The official technical specifications and recommendations of the IETF are called Request for Comments (RFCs).

The technical work of the IETF is done in its working groups, which are organized by topic into several areas, such as routing, transport and security. In the security area, different working groups work on security mechanisms for different protocols or applications. For example,

1. The PKIX (Public-Key Infrastructure X.509) Working Group (PKIX-WG) developed technical specifications and recommendations to support a Public Key Infrastructure, based on the X.509 protocol, which is used to build a trust and authentication services infrastructure,
2. The IPSEC (Internet Protocol Security) working group developed a protocol and other technical recommendations for secure routing between network devices, and
3. The TLS (Transport Layer Security) working group has been specifying a communication protocol and technical recommendations to provide security services for communication between a server and a client, etc.

^18^ IEEE P1363.1, Public-Key Cryptographic Techniques Based on Hard Problems over Lattices. ^19^ IEEE P1363.2, Password-Based Public-Key Cryptography. ^20^ IEEE 802.11, Wireless Local Area Networks. ^21^ IEEE P1619, Standard for Cryptographic Protection of Data on Block-Oriented Storage Devices.

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323 NIST-approved cryptographic algorithms, such as block cipher modes of operation, hash 324 functions, key establishment schemes, and digital signatures are used in various IETF 325 protocols. For example, RFC 5288 specifies the AES Galois Counter Mode (GCM) Cipher 326 Suites for TLS, based on SP 800-38D. 327 Further information is available at the IETF web site: http://ietf.org.

328 2.3.4 International Organization for Standardization (ISO)22 329 ISO is a non-governmental, worldwide federation of national standards bodies. Its mission 330 is to develop international standards that help to make industry more efficient and effective. 331 ISO standards cover almost all aspects of technology and business, from food safety to 332 computers, and from agriculture to healthcare. Experts from all over the world develop the 333 standards that are required by their sector, using a consensus process. 334 ISO/IEC JTC 1 is a joint technical committee of the International Organization for 335 Standardization (ISO) and the International Electrotechnical Commission (IEC). ISO/IEC 336 JTC 1 SC 27 is the subcommittee for IT security. Working group 2 (WG2) is the group 337 developing standards for cryptography and security mechanisms. It usually has more than 338 twenty active projects to develop either a revision of an existing standard or a new standard. 339 Each standard consists of multiple parts, and each part includes multiple algorithms and/or 340 mechanisms. 341 The cryptographic algorithms and schemes in FIPS and SPs are usually included in 342 ISO/IEC JTC 1 standards, along with many other algorithms submitted by other countries. 343 The following is a list of ISO/IEC standards that include cryptographic algorithms and 344 schemes specified in NIST standards.

345 1. ISO/IEC 9797-1:2011, Information technology -- Security techniques -- Message 346 Authentication Codes (MACs) -- Part 1: Mechanisms using a block cipher. 347 2. ISO/IEC 9797-2:2011, Information technology -- Security techniques -- Message 348 Authentication Codes (MACs) -- Part 2: Mechanisms using a dedicated hash- 349 function. 350 3. ISO/IEC 10116:2006, Information technology -- Security techniques -- Modes of 351 operation for an n-bit block cipher. 352 4. ISO/IEC 10118-3:2004, Information technology -- Security techniques -- Hash- 353 functions -- Part 3: Dedicated hash-functions. 354 5. ISO/IEC 11770-3:2008, Information technology -- Security techniques -- Key 355 management -- Part 3: Mechanisms using asymmetric techniques. 356 6. ISO/IEC CD 11770-6 "Information technology -- Security techniques -- Key 357 management -- Part 6: Key derivation. 358 7. ISO/IEC 14888-2: 2008, Information technology -- Security techniques -- Digital 359 signatures with appendix -- Part 2: Integer factorization based mechanisms.

22 Further information is available at the ISO web site: www.iso.org.

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360 8. ISO/IEC CD 14888-3, Information technology -- Security techniques -- Digital 361 signatures with appendix -- Part 3: Discrete logarithm based mechanisms. 362 9. ISO/IEC 18033-3:2010, Information technology -- Security techniques -- 363 Encryption algorithms -- Part 3: Block ciphers. 364 10. ISO/IEC 19772:2009, Information technology -- Security techniques -- 365 Authenticated encryption.

366 2.3.5 Trusted Computing Group (TCG) 367 The Trusted Computing Group (TCG) develops and promotes a set of industry standards 368 that build upon roots of trust. Roots of Trust (RoTs) are hardware, firmware, and 369 software components that are inherently trusted to perform specific, and vital, security 370 functions. Because misbehavior by an RoT cannot be detected, they must be secure by 371 design. To ensure that they are reliable and resistant to tampering, RoTs are often 372 implemented in, or protected by, hardware. 373 Industry standards developed by the TCG define the capabilities of a set of fundamental 374 roots of trust, and describe how to use those roots of trust in a variety of architectures and 375 use cases. Many of the use cases supported by TCG technologies and specifications 376 focus on one or more of the following areas: 1) device identity, 2) cryptographic key or 377 credential storage, and 3) attestation of the system state. 378 Technologies supporting TCG-developed standards are deployed enterprise-class clients 379 and servers, storage devices, embedded systems, and virtualized devices. Families of 380 relevant TCG standards and specifications include: 381 • Trusted Platform Module (TPM): A TPM is a cryptographic module that can, 382 among other features, establish device identity in a platform, provide secure 383 storage for keys and credentials, and support the measurement and reporting of 384 the system state. The TPM 2.0 Library Specification provides the general 385 architecture and command set for TPMs, with platform-specific specifications 386 detailing how a TPM can be implemented in a particular classes of systems. 387 ISO/IEC JTC 1 has approved the TPM Library Specification as ISO/IEC 388 11889:2015 Parts 1-4. 389 • Trusted Network Connect (TNC): The TCG's TNC Work Group defines 390 specifications that allow network administrators to enforce policies regarding 391 endpoint integrity on devices connected to a network. These specifications were 392 the basis for much of the work in the IETF's Network Endpoint Assessment 393 (NEA) working group, and are highly complimentary to the on-going work in the 394 IETF Security Automation and Continuous Monitoring (SACM) working group. 395 • Storage: The TCG's Storage Work Group defines specifications that enable 396 standards-based mechanisms to protect data on storage devices, and manage these 397 devices and capabilities. The TCG's storage specifications break out from a 398 common core specification into two Security Subsystem Classes (SSCs): the Opal 399 SSC, intended for client devices (e.g., tablets, notebooks, desktops), and the 400 Enterprise SSC, intended for high-performance storage systems (e.g., servers).

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SECTION 3: CRYPTOGRAPHIC ALGORITHMS

This document describes three types of cryptographic algorithms: cryptographic hash functions, symmetric-key algorithms and asymmetric-key algorithms, discussed in Sections 3.1, 3.2 and 3.3, respectively. Other topics to be introduced in this section include the concept of algorithm security strength and algorithm lifetime (see Sections 3.4 and 3.5, respectively).

3.1 Cryptographic Hash Functions

A hash function (also called a hash algorithm) is a cryptographic primitive algorithm that produces a condensed representation of its input (e.g., a message). A hash function takes an input of arbitrary length and outputs a value with a predetermined length. Common names for the output of a hash function include hash value and message digest.

A cryptographic hash function is a one-way function that is extremely difficult to invert. That is, it is not practical to reverse the process from the hash value back to the input.

Figure 1 depicts the process of generating and verifying a hash value.

Generation Verification M1 Hash Function H1 M2 Hash Function H2 Compare H1

Figure 1: Hash Function Generation and Verification

A hash function is used as follows:

• Hash Generation:
  1. Hash value ($H_1$) is generated on data ($M_1$) using the hash function.
  2. $M_1$ and $H_1$ are then saved or transmitted.
• Hash Verification:
  1. Hash value ($H_2$) is generated on the received or retrieved data ($M_2$) using the same hash function that generated $H_1$.
  2. $H_1$ and $H_2$ are compared. If $H_1 = H_2$, then it can be assumed that $M_1$ has not changed during storage or transmission.

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426 The above description is for the simplest use of a hash function. Hash functions are usually 427 used in higher-level algorithms, including: 428 • Keyed-hash message authentication code algorithms (Sections 3.2.2 and 4.2.2.2), 429 • Digital signature algorithms (Section 4.2.3), 430 • Key derivation functions (e.g., for key establishment) (Section 5.3.2), and 431 • Random bit generators (Section 4.4). 432 When these higher-level algorithms are used with a key, they could be considered as 433 symmetric-key algorithms (see Section 3.2 for further discussion). 434 Approved hash functions for Federal Government use are specified in FIPS 180^23 and 435 FIPS 202^24. 436 • FIPS 180 specifies the SHA-1, SHA-224, SHA-256, SHA-384, SHA-512, SHA- 437 512/224 and SHA-512/256 hash functions. Additional guidance for the use of these 438 hash functions is provided in SP 800-106^25 and SP 800-107^26. 439 Note that attacks on SHA-1 have indicated that SHA-1 provides less security than 440 originally thought when generating digital signatures (see Section 4.2.3) and is now 441 disallowed for that purpose. However, SHA-1 may continue to be used for most 442 other hash-function applications (see SP 800-131A^27). 443 • FIPS 202 specifies SHA3-224, SHA3-256, SHA3-384 and SHA3-512. This FIPS 444 also specifies two extendable-output functions (SHAKE128 and SHAKE256), 445 which are not, in themselves, considered to be hash functions; guidance on their 446 use will be provided in the future.

447 3.2 Symmetric-Key Algorithms 448 Symmetric-key algorithms (often called secret-key algorithms) use a single key to both 449 apply cryptographic protection and to remove or check the protection. For example, the 450 key used to encrypt data (i.e., apply protection) is also used to decrypt the encrypted data 451 (i.e., remove the protection); in the case of encryption, the original data is called the 452 plaintext, while the encrypted form of the data is called the ciphertext. The key must be 453 kept secret if the data is to remain protected. 454 Several classes of symmetric-key algorithms have been approved: those based on block 455 cipher algorithms (e.g., AES) and those based on the use of hash functions (e.g., a keyed- 456 hash message authentication code based on SHA-1).

23 FIPS 180: Secure Hash Standard. 24 FIPS 202: SHA-3 Standard: Permutation-Based Hash and Extendable Output Functions. 25 SP 800-106: Randomized Hashing for Digital Signatures. 26 SP 800-107: Recommendations for Applications Using Approved Hash Algorithms. 27 SP 800-131A: Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths.

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457 Symmetric-key algorithms are used for: 458 • Encryption to provide data confidentiality (see Section 4.1), 459 • Authentication to provide assurance of data integrity and the source of the data 460 (see Section 4.2), 461 • Key derivation (see Section 5.3.2), 462 • Key wrapping (see Section 5.3.5), and 463 • Random bit generation (see Section 4.4). 464 When using a symmetric-key algorithm, a unique key needs to be generated for each 465 cryptographic relationship^28 and used for each purpose (e.g., encryption, data integrity 466 authentication and key wrapping). Technically, the same key can be used for multiple 467 purposes when the same algorithm is used, but this is usually ill-advised, as the use of the 468 same key for two different cryptographic processes (e.g., HMAC and key derivation using 469 the same hash function) may weaken the security provided by one or both of the processes. 470 However, exceptions to this rule have been approved (see Section 4.3). 471 As an example of the number of keys required for the use of symmetric-key algorithms, 472 suppose that there are four entities (A, B, C, and D) that need to communicate using 473 encryption, with each pair of entities using a different encryption key. There are six 474 possible pair-wise relationships (A-B, A-C, A-D, B-C, B-D, and C-D), so, at least six keys 475 are required^29. If, instead, there are 1000 entities that wish to communicate with each other, 476 there are 499,500 possible pair-wise relationships, and at least one unique key would be 477 required for each relationship. If more than one algorithm, key length or purpose is to be 478 supported (e.g., both encryption and key wrapping), then additional keys will be needed. 479 Each entity must keep all its symmetric keys secret and protect their integrity. The 480 requirement for a large number of keying relationships is a significant problem; methods 481 for mitigating this problem are discussed in Section 5. 482 Several symmetric-key algorithms have been approved by NIST for the protection of 483 sensitive data. However, some of these algorithms are no longer approved for applying 484 cryptographic protection (e.g., encryption), but may continue to be used for processing 485 already-protected information (e.g., decryption), providing that the risk of doing so is 486 acceptable (e.g., there is reason to believe that a key was not compromised). See SP 800- 487 57, Part 1 and SP 800-131A for more information about the acceptability of using different 488 cryptographic algorithms. 489 3.2.1 Block Cipher Algorithms 490 A block cipher algorithm is used with a single key in an approved mode of operation to both apply 491 cryptographic protection (e.g., encrypt) and to subsequently process the protected information (e.g.,

28 A cryptographic relationship exists when two or more parties can communicate using the same key and algorithm. A relationship may be one-to-one or one-to-many (e.g., broadcast). 29 Although only six cryptographic relationships are used in the example, different keys may be required by some protocols for each communication direction, i.e., a different key may be required for communications sent from A to B than is used for communications sent from B to A.

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492 decrypt). Several block cipher algorithms have been approved by NIST as cryptographic primitives, 493 some of which may no longer be approved for applying cryptographic protection. However, they 494 may still be needed for processing information that was previously protected (e.g., they may be 495 needed for decrypting previously encrypted information). 496 The block cipher algorithms are discussed in Sections 3.2.1.1 through 3.2.1.4. The 497 approved modes of operation are discussed in Section 3.2.1.5. 498 3.2.1.1 Data Encryption Standard (DES) 499 The Data Encryption Standard (DES) became effective in July 1977, and was the first 500 NIST-approved cryptographic algorithm. It was reaffirmed several times, but due to 501 advances in computer power and speeds, the strength of the DES algorithm is no longer 502 sufficient to adequately protect Federal Government information. Therefore, DES was 503 withdrawn as an approved algorithm in 2005 (i.e., the use of DES is no longer approved 504 for encryption or otherwise applying cryptographic protection). However, the DES 505 “cryptographic engine” continues to be used as a component function of TDEA (see the 506 next section). 507 3.2.1.2 Triple Data Encryption Algorithm (TDEA) 508 The Triple Data Encryption Algorithm (TDEA), also known as Triple DES, uses the DES 509 cryptographic engine to transform data in three operations. TDEA is specified in SP 800- 510 67^30. 511 TDEA encrypts data in blocks of 64 bits, using three keys that define a key bundle. The 512 use of TDEA using three distinctly different (i.e., mathematically independent) keys is 513 approved and is commonly known as three-key TDEA (also referred to as 3TDEA or 514 3TDES). 515 Other variations of TDEA, where two or three of the keys are identical, are no longer 516 approved for applying cryptographic protection because of increased computing power or 517 weaknesses in the algorithm. 518 3.2.1.3 SKIPJACK 519 SKIPJACK is referenced in FIPS 185^31 and specified in a classified document. SKIPJACK 520 is no longer considered adequate for the protection of federal information and has been 521 withdrawn as a FIPS. The use of SKIPJACK for applying cryptographic protection (e.g., 522 encryption) is not approved, although it is permissible to use the algorithm for decrypting 523 information.

30 SP 800-67: Recommendation for the Triple Data Encryption Algorithm (TDEA) Block Cipher. 31 FIPS 185: Escrowed Encryption Standard.

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3.2.1.4 Advanced Encryption Standard (AES) The Advanced Encryption Standard (AES) was developed as a replacement for DES and is the preferred block cipher algorithm for new products. AES is specified in FIPS 197^32. AES operates on 128-bit blocks of data, using 128, 192 or 256-bit keys. Note that the use of the longer key lengths affects algorithm performance (e.g., the speed), though not by very much. Also, note that the performance of AES is significantly better than that of TDEA.

3.2.1.5 Modes of Operation With a symmetric-key block cipher algorithm, the same input block will always produce the same output block when the same key is used. For example, if the multiple blocks in a typical message are encrypted without using a mode designed for the purpose, an adversary could easily substitute individual blocks, possibly without detection. Furthermore, certain kinds of data patterns in the plaintext, such as repeated blocks, would be apparent in the ciphertext. Therefore, block cipher modes-of-operation have been specified to address this problem by combining the cryptographic primitive algorithm with variable starting values (commonly known as initialization vectors) and rules that successively use the block cipher algorithm to perform a cryptographic service (e.g., the encryption of a message). Approved modes for block cipher algorithms have been specified in the SP 800-38 series of publications and include modes for:

• Encryption, as specified in SP 800-38A, SP 800-38E and SP 800-38G (see Section 4.1),
• Authentication, as specified in SP 800-38B (see Section 4.2.2.1),
• Authenticated encryption, as specified in SP 800-38C and SP 800-38D (see Section 4.3), and
• Key wrapping, as specified in SP 800-38F (see Section 5.3.5).

3.2.2 Hash-based Symmetric-key Algorithms A symmetric-key algorithm based on the use of a hash function has been specified in FIPS 198^33. This algorithm, known as HMAC, has been approved for use with any approved hash function specified in FIPS 180 or FIPS 202. Guidance on the use of the hash functions specified in FIPS 180 for HMAC is provided in SP 800-107.

3.3 Asymmetric-Key Algorithms Asymmetric-key algorithms (often called public-key algorithms) use a pair of keys (i.e., a key pair): a public key and a private key that are mathematically related to each other. The public key may be made public without reducing the security of the process, but the private

^32 FIPS 197: Advanced Encryption Standard. ^33 FIPS 198: Keyed Hash Message Authentication Code (HMAC).

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key must remain secret if the data is to retain its cryptographic protection. Even though there is a relationship between the two keys, the private key cannot easily be determined based on knowledge of the public key.

One of the keys of the key pair is used to apply cryptographic protection, and the other key is used to remove or verify that protection. The key to use depends on the algorithm used and the service to be provided. For example, a digital signature is computed using a private key, and the signature is verified using the public key (i.e., the protection is applied using the private key and verified using the corresponding public key). For those asymmetric algorithms also capable of encryption^34, the encryption is performed using the public key, and the decryption is performed using the private key (i.e., the protection is applied using the public key and removed using the private key).

Asymmetric-key algorithms are used primarily for data integrity authentication and source authentication (see Section 4.2), and for key establishment (see Section 5.3). These algorithms tend to be much slower than symmetric-key algorithms, so are not used to process large amounts of data. However, when used for key establishment (see Section 5), there are methods that combine the use of symmetric and asymmetric algorithms to reduce the number of keys required for establishing cryptographic relationships.

Like symmetric-key algorithms, the key pair for an asymmetric-key should be generated for each purpose (e.g., one key pair for generating and verifying digital signatures, and a different key pair for key establishment). Technically, it is sometimes possible to use the same key pair for more than one purpose, but this is ill-advised, as the use of the same key pair for two different cryptographic purposes (e.g., digital signatures and key establishment) may weaken the security provided by one or both of the processes.

The use of asymmetric-key algorithms requires the establishment of fewer initial keys than the use of symmetric-key algorithms. As an example, suppose that an entity wants to generate digital signatures and participate in a key-establishment process using its own key pair^35; a key pair needs to be generated for each purpose. If there are six entities that intend to both generate digital signatures and participate in the key-establishment process, then six key pairs are needed for digital signature generation, and another six key pairs are needed for key establishment, for a total of twelve key pairs. For 1000 entities, 1000 key pairs of each would be needed for each purpose, for a total of 2000 key pairs. A unique key pair does not need to be generated for each relationship; recall that for symmetric-key algorithms, a unique key needs to be generated for each relationship (see Section 3.2). If multiple public-key algorithms or key lengths are to be used for either process, then additional key pairs will be required.

The private key is retained by the entity who “owns” the key pair; it must be kept secret and its integrity protected. The public key is usually distributed to other entities and requires integrity protection; this is often accomplished by using a public-key certificate,

^34 Not all public-key algorithms are capable of multiple functions, e.g., both encryption and decryption, and the generation and verification of digital signatures. ^35 Note that some key-establishment schemes do not require that all parties have key pairs, so some parties will not need a key pair for key establishment.

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597 as discussed in Section 5.2.3. When a public-key certificate is used, the certificate provides 598 the integrity protection for the public key, so the burden of key protection by each entity is 599 limited to only those private keys owned by the entity. 600 Some asymmetric-key algorithms use domain parameters, which are additional values 601 necessary for the use of the cryptographic algorithm. These values are mathematically 602 related to each other and to the keys with which they will be used. Domain parameters are 603 usually public and are used by a community of users for a substantial period of time. These 604 domain parameters are either contained within or referenced by a certificate containing a 605 public key. 606 The secure use of asymmetric-key algorithms is dependent on users obtaining certain 607 assurances: 608 • Assurance of domain-parameter validity (for those algorithms requiring domain 609 parameters) provides confidence that the domain parameters are mathematically 610 correct, 611 • Assurance of public-key validity provides confidence that the public key appears 612 to be a suitable key, and 613 • Assurance of private-key possession provides confidence that the party that is 614 supposedly the owner of the private key really has the key. 615 3.3.1 DSA 616 The Digital Signature Algorithm (DSA) is approved and specified in FIPS 186. This 617 algorithm is used to generate and verify digital signatures using finite-field mathematics 618 (i.e., the mathematics that most of us are familiar with). FIPS 186 defines methods for 619 generating DSA domain parameters and key pairs, and specifies the key lengths to be used 620 for secure interoperability and the algorithms to be used for digital-signature generation 621 and verification. 622 3.3.2 ECDSA 623 The Elliptic Curve Digital Signature Algorithm (ECDSA) is approved within FIPS 186, 624 but actually specified within American National Standard (ANS) X9.62^36. The basic 625 signature and verification algorithms are the same as those used for DSA, except that the 626 mathematics is based on the use of elliptic curves, rather than finite fields (i.e., the rules 627 for combining numbers is different than commonly used). FIPS 186 provides guidance for 628 the use of ECDSA within the Federal Government, as well as providing recommended 629 elliptic curves to facilitate interoperability and security. An advantage of using ECDSA is 630 that the key lengths are considerably shorter than those used for DSA and RSA, requiring 631 less storage space and transmission bandwidth, and the execution of the algorithm is 632 generally faster than DSA and RSA 633 ANS X9.62 includes specifications for the generation of the ECDSA domain parameters 634 and key pairs, as well as the algorithms for digital signature generation and verification.

36 ANS X9.62: Public Key Cryptography for the Financial Services Industry, The Elliptic Curve Digital Signature Algorithm (ECDSA).

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635 FIPS 186 defines the key lengths to be used for secure interoperability, provides additional 636 guidance on the use of random bit generators to generate the key pairs, and recommends 637 elliptic curves for use by the Federal Government. Note that the same elliptic curves are 638 also included in ANS X9.62.

639 3.3.3 RSA 640 The RSA algorithm is approved for the generation and verification of digital signatures in 641 FIPS 186] and specified in PKCS 1^37 and ANS X9.31^38. FIPS 186 includes restrictions on 642 the use of RSA to generate digital signatures, methods to generate RSA key pairs, and 643 defines the key lengths to be used for secure interoperability.

644 The RSA primitive can be used for key establishment, as well as for the generation and 645 verification of digital signatures. It's use for key establishment is specified in SP 800- 646 56B^39; that publication specifies approved methods for both key agreement and key 647 transport (see Section 5.3 for further information on key establishment, key agreement and 648 key transport).

649 The key pairs used for RSA digital-signature generation and verification, and for RSA key 650 establishment are generated in the same way, but need to be different for each purpose.

651 3.3.4 Diffie-Hellman and MQV 652 Diffie-Hellman (DH) and MQV^40 are two classes of key-establishment algorithms used for 653 key agreement (see Section 5.3.3). The use of these algorithms for key agreement is 654 specified in SP 800-56A^41 using both finite-field and elliptic-curve mathematics for each. 655 For elliptic-curve key pairs and domain parameters, the methods for generating those key 656 pairs and domain parameters are specified in ANS X9.62 using the same methods used to 657 generate ECDSA key pairs and domain parameters.

658 659 The recommended elliptic curves for elliptic-curve DH and MQV are the same as those 660 provided in FIPS 186 for ECDSA.

661 3.4 Algorithm Security Strength 662 The security strength of a cryptographic algorithm is measured by an attacker's difficulty 663 in breaking the algorithm. Breaking a cryptographic algorithm can be defined as defeating 664 some aspect of the protection that the algorithm is intended to provide. For example, a

37 Public Key Cryptography Standard #1.

38 ANS X9.31, Digital Signatures Using Reversible Public Key Cryptography For The Financial Services Industry (RDSA). This standard has been withdrawn as an ANSI standard.

39 SP 800-56B: Recommendation for Pair-wise Key Establishment Schemes Using Integer Factorization Cryptography.

40 Menezes–Qu–Vanstone

41 SP 800-56A: Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography.

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665 block cipher encryption algorithm that is used to protect the confidentiality of data is 666 broken if, with an acceptable amount of work, it is possible to determine the value of its 667 key or to recover the plaintext from the ciphertext without knowledge of the key. 668 SP 800-57, Part 1 provides the current estimates for the security strengths that can be 669 provided by the approved cryptographic algorithms; these strengths have been determined 670 with respect to specific key lengths. 671 The approved security strengths for federal applications are 112, 128, 192 and 256 bits. 672 Appropriate algorithms, key lengths, and key generation and handling methods need to be 673 used to actually support those security strengths, and is further discussed in Section 5.1.4. 674 3.5 Algorithm Lifetime 675 Over time, algorithms may be successfully attacked so that the algorithm no longer 676 provides the desired protection. The attack could be on the algorithm itself, or could be on 677 the algorithm with a specific key length. In the latter case, the use of a longer key may 678 prevent a successful attack, or at least delay it for a period of time. 679 When selecting the algorithms and key lengths to be used for an application, the length of 680 time for which the data needs to be protected should be taken into account so that a suitable 681 algorithm and key length is used. SP 800-57, Part 1 provides a current estimate of the time 682 frames during which the approved algorithms and key lengths are considered to be secure. 683 The algorithms and key lengths used for cryptographic protection need to fall within the 684 estimated time frame. However, these estimates are just that – estimates. It is possible that 685 an advance in technology or cryptanalysis could occur prior to the end date of that time 686 frame (e.g., the use of quantum computers and algorithms). It is often the case that these 687 advances are initially impractical or limited in their threat. It is recommended that an 688 organization have a transition strategy for addressing this problem if it occurs, including 689 assessing the risk for the compromise of the organization's data, and transitioning to a new 690 algorithm or key length, as appropriate. 691

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692 693 SECTION 4: CRYPTOGRAPHIC SERVICES

694 All sensitive information requires integrity protection, and confidentiality protection may 695 be required as well. This section discusses the cryptographic services that can be provided 696 for the protection of sensitive data other than keys. These services include data 697 confidentiality, data integrity authentication and source authentication, including non- 698 repudiation. The protection and management of the keys used while providing these 699 cryptographic services are discussed in Section 5.

700 Ideally, cryptographic services would be provided using as few algorithms as possible. For 701 example, AES could be used to provide confidentiality (Section 4.1), data integrity 702 authentication (Section 4.2), key wrapping (Section 5.3.5) and as the basis for a random bit 703 generator (see Section 4.4). However, this may not be as practical as it first appears, as 704 other algorithms may also be available that are needed for different applications and that 705 provide other security properties.

706 4.1 Data Confidentiality

707 Encryption is used to provide confidentiality for data. The unprotected form of the data is 708 called plaintext. Encryption transforms the data into ciphertext, and ciphertext can be 709 transformed back into plaintext using decryption. Data encryption and decryption are 710 provided using symmetric-key block cipher algorithms. The approved symmetric-key 711 algorithms for data encryption are: AES and TDEA (see Section 3.2.1.4 and Section 712 3.2.1.2, respectively). Decryption of the ciphertext is performed using the algorithm and 713 key that were used to encrypt the plaintext. Unauthorized recipients of the ciphertext who 714 know the cryptographic algorithm but do not have the correct key should not be able to 715 decrypt the ciphertext. However, anyone who has the key and the cryptographic algorithm 716 can easily decrypt the ciphertext and obtain the original plaintext.

Key Plaintext Encryption Ciphertext Key Ciphertext Decryption Plaintext

717 718 Figure 2: Encryption and Decryption

719 Figure 2 depicts the encryption and decryption processes. The plaintext and a key are used 720 by the encryption process to produce the ciphertext. To decrypt, the ciphertext and the same 721 key are used by the decryption process to recover the plaintext data.

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722 Note that asymmetric-key algorithms could also be used to encrypt and decrypt data, but 723 because these algorithms are slow in comparison to block cipher algorithms, they are not 724 normally used to encrypt and decrypt general data; they can, however, be used to protect 725 keys, as discussed in Section 5. 726 As discussed in Section 3.2.1.5, encryption is performed using a block cipher algorithm 727 and a mode of operation. The approved modes of operation for encryption are specified 728 in:

• SP 800-38A for AES and TDEA: the Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Counter (CTR), and Output Feedback (OFB) modes,
• SP 800-38E for AES: the XTS-AES mode (for protecting the confidentiality of data on storage devices only), and
• SP 800-38G for AES: the FF1 and FF3 modes for Format Preserving Encryption.

735 Additional modes that provide both confidentiality and authentication (as discussed in 736 Section 4.2) are discussed in Section 4.3.

4.2 Data Integrity and Source Authentication

738 Data integrity (often referred to as simply integrity) is concerned with whether or not 739 something (e.g., some data) has changed between two specified times (e.g., between the 740 time when the data was created, stored and/or transmitted, and the time when it was 741 retrieved and/or received). The absolute integrity of the data cannot be guaranteed, but the 742 computation of a data integrity code on the data when it is created, before storage or before 743 transmission will allow the detection of any changes with a high probability when that code 744 is later verified, thus providing a measure of assurance of data integrity. In cryptographic 745 literature, this process is called message (or data) authentication. 746 Source authentication is a process used to provide assurance of the source of information. 747 Source authentication includes identity authentication, which provides assurance to one of 748 the parties in a communication (say, Bob) that he is receiving data from or providing data 749 to another specific party (say, Alice). Depending on the method used, source authentication 750 could also support non-repudiation, whereby both Bob and some third party (say, Carl) 751 have some assurance that the data came from Alice. 752 Cryptography can be used to provide these services, but the same algorithm may not 753 provide all of them. Hash functions, as discussed in Section 4.2.1, can be used to provide 754 some assurance of data integrity. Message Authentication Code (MAC) algorithms, as 755 discussed in Section 4.2.2, can provide both data integrity and source authentication 756 services. Digital signature algorithms can be used to provide data integrity and source 757 authentication services, as well as supporting non-repudiation, but at a higher performance 758 cost (see Section 4.2.3).

4.2.1 Hash Functions

760 A hash function is used to generate a hash value that can provide some assurance of the 761 integrity of the data over which the hash value is generated. However, since no

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cryptographic key is used, there is no assurance that the data has not been altered by an adversary and a new hash value computed. This method for providing integrity protection is not recommended unless there is a very low risk of this scenario (e.g., when data is provided by a trusted source, and the hash value is used to determine changes that may occur because of a degraded transmission medium).

4.2.2 Message Authentication Code Algorithms A Message Authentication Code algorithm and a cryptographic key are used to generate a message authentication code (MAC) that can be used to provide assurance of data integrity and source authentication. A MAC is a cryptographic checksum on the data that can provide assurance that the data has not changed or been altered since some point in time, and that the MAC was computed by the party or parties sharing the key. Typically, MACs are used between two or more parties that share the same secret key to authenticate information exchanged between those parties; the use of MACs to provide data integrity and source authentication depends on limiting knowledge of the secret key to only those parties. Since a MAC key is shared among a community of users (e.g., two or more parties), only those parties sharing the key can compute a correct MAC on given data.

MAC Generation MAC Verification K K M1 MAC Algorithm MAC1 M2 MAC Algorithm MAC2 Compare MAC1

Figure 3: Message Authentication and Verification Figure 3 depicts the use of MACs:

• A MAC (MAC1) is computed on data (M1) using a key (K). M1 and MAC1 are then saved or transmitted.
• At a later time, the integrity of the retrieved or received data is checked by labeling the retrieved or received data as M2 and computing a MAC (MAC2) on M2 using the same key (K).
• If MAC1 is the same as MAC2, then it can be assumed that M2 is the same as the original data (M1) (i.e., M1 = M2). The verifying party also knows that only a party that shares the key could have correctly generated the MAC.

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789 For example, if two parties (e.g., parties A and B) share a key, party A generates the MAC 790 and sends it to party B, and party B successfully verifies the received MAC, then party B 791 knows that party A generated the original MAC, and source authentication has been 792 accomplished. However, if three parties share the key (e.g., A, B and C), party A generates 793 the MAC to be sent to party B, and party B successfully verifies the received MAC; party 794 B knows that either party A or party C generated the original MAC, but has no proof of 795 which one. Note that this may be acceptable for some applications. 796 MACs are used to detect data modifications that occur between the initial generation of the 797 MAC and the verification of the received MAC. They do not detect errors that occur before 798 the MAC is originally generated. 799 Assurance of data integrity is frequently provided using non-cryptographic techniques 800 known as error detection codes. However, these codes can be altered by an adversary to 801 the adversary’s benefit. The use of an approved cryptographic mechanism, such as a 802 MAC, addresses this problem. That is, the assurance of integrity provided by a MAC is 803 based on the assumption that it is not likely that anyone could correctly generate a MAC 804 without knowing the cryptographic key. An adversary without knowledge of the key will 805 be unable to modify data and then generate a verifiable MAC on the modified data. It is 806 therefore crucial that MAC keys be kept secret. 807 Two types of algorithms for computing a MAC have been approved for Federal 808 Government use: MAC algorithms that are based on symmetric-key block cipher 809 algorithms, and MAC algorithms that are based on hash functions. 810 4.2.2.1 MACs Based on Block Cipher Algorithms 811 The SP 800-38 series of publications includes modes for the generation of MACs: 812 • SP 800-38B^42 defines the CMAC mode for computing a MAC using the NIST- 813 approved block-cipher algorithms: AES and TDEA. 814 • SP 800-38D^43 defines the GMAC mode for the computation of a MAC using AES. 815 • Modes providing both confidentiality (i.e., encryption) and authentication (i.e., 816 computing a MAC) in a single operation are also defined (see Section 4.3). 817 4.2.2.2 MACs Based on Hash Functions 818 FIPS 198^44 defines a MAC (HMAC) that uses a cryptographic hash function in 819 combination with a secret key. HMAC must be used with an approved cryptographic hash

42 SP 800-38B: Recommendation for Block Cipher Modes of Operation: The CMAC Mode for Authentication. 43 SP 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC. 44 FIPS 198-1: The Keyed-Hash Message Authentication Code (HMAC).

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function (see Section 4.2.1). The security associated with the use of HMAC is discussed in SP 800-107^45.

4.2.3 Digital Signature Algorithms

A digital signature algorithm is used with a pair of keys – a private key and a public key – to generate and verify digital signatures. The private key is used to generate signatures and must be known only by the signer (the key-pair owner); the public key is used to verify the signatures. Because of the design of the algorithm, and the methods for generating key pairs, the public key cannot easily be used to determine the private key. Because two keys are required for the generation and verification process, digital signature algorithms are classified as asymmetric-key algorithms.

A digital signature is represented in a computer as a string of bits and is an electronic analogue of a hand-written signature that can be verified by anyone with access to the public key. The signature can be used to provide assurance of data integrity and source authentication, and to support non-repudiation.

Each signer possesses a private and public key pair. Signature generation (with a verifiable digital signature) can be performed only by the party that has access to the private key. Anyone that knows the public key can verify the signature by employing the associated public key. The security of a digital-signature system is dependent on maintaining the secrecy of the signer’s private key. Therefore, signers must guard against the unauthorized acquisition of their private keys.

Digital signatures offer protection that is not available by using alternative signature techniques. One such alternative is a digitized signature. A digitized signature is generated by converting a visual form of a handwritten signature to an electronic image (e.g., by scanning it into a computer). Although a digitized signature resembles its handwritten counterpart when printed, it does not provide the same protection as a digital signature. Digitized signatures can be forged and can be duplicated and appended to other electronic data; digitized signatures cannot be used to determine if information has been altered after it is signed. Digital signatures, however, are computed on each message using a private key known only by the signer. Each different message signed by the signer will have a different digital signature. Even small changes to the message will result in a different signature. If an adversary does not know the private key, the adversary cannot generate a valid signature (i.e., a signature that can be verified using the public key that corresponds to the private key used to generate the signature).

Figure 4 depicts the generation and verification of digital signatures. A digital signature algorithm includes a signature generation process and a signature verification process:

• Signature generation:
  • A hash function (see Section 3.1) is used in the signature generation process to obtain a hash value, which is a condensed version of the data to be signed (i.e., shown as M1 for signature generation in Figure 4).

45 SP 800-107: Recommendation for Applications Using Approved Hash Algorithms.

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859 o The hash value is then input to the signature generation process, along with a 860 private key, to generate the digital signature (shown as $DS_1$ in Figure 4). 861 o The digital signature ($DS_1$) is provided to the verifier, along with the signed data 862 ($M_1$). 863 Signature verification: The receiver of the data and signature verifies the signature 864 as follows using the signatory's public key to process the received signature: 865 o The received data ($M_2$) is hashed using the same hash function to produce 866 another hash value. 867 o The newly computed hash value and the received signature ($DS_2$) are input to 868 the signature verification process, along with the the signer's public key. The 869 output of this process is an indication of whether or not the signature is valid or 870 invalid for the received message ($M_2$). 871 Note that the details of the signature generation and verification processes are different for 872 each approved algorithm. Also, note that $M_2$ is used in the verification process rather than 873 $M_1$, and $D_2$ is used rather than $D_1$ because of the possibility that $M_1$ and $D_1$ could have been 874 deliberately or accidentally modified before the verification process performed by the 875 receiver.

Signature Generation Signature Verification $M_1$ $M_2$ Hash Function Hash Function Hash Value Hash Value Private Key Signature Generation Public Key Signature Verification $DS_2$ Digital Signature ($DS_1$) Valid/Invalid

876 877 Figure 4: Digital Signature Generation and Verification 878 FIPS 186 specifies methods for generating and verifying digital signatures using 879 asymmetric (public-key) cryptography. The FIPS includes three digital signature 880 algorithms: 881 The Digital Signature Algorithm (DSA) (see Section 3.3.1),

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• The Elliptic Curve Digital Signature Algorithm (ECDSA) (see Section 3.3.2), and
• RSA (see Section 3.3.3).

The digital signature algorithms are used in conjunction with the hash functions specified in FIPS 180^46 and FIPS 202. Each of these algorithms requires obtaining assurances about the domain parameters and/or keys used, as discussed in Section 3.3; SP 800-89^47 provides methods for obtaining these required assurances when using digital signatures. In many cases, determining when a digital signature was generated is important. For example, it may be important to determine whether a document was signed before a certain date, e.g., which of two wills was signed closest to and prior to the date that a person died. SP 800-102^48 provides guidance on establishing when a digital signature was generated.

4.3 Combining Confidentiality and Authentication in a Block-Cipher Mode of Operation

Confidentiality and authentication can be provided using either two separate block-cipher algorithms (e.g., AES in the CBC mode for encryption and HMAC for authentication) or in a single block-cipher mode of operation. Note that in this discussion, authentication is used to obtain both an assurance of data integrity and of the source of the data that has been cryptographically protected.

If encryption and authentication are performed as two separate operations (see Sections 4.1 and 4.2, respectively), two distinct keys are required. If care is not taken in performing these operations (e.g., performing the operations in the right order), vulnerabilities can be introduced that may allow attacks.

An alternative is to use modes that both encrypt and authenticate in a single operation using a single key; such a mode is called an “authenticated-encryption” mode. Using such modes requires fewer keys and is generally faster than using two separate operations. Two authenticated-encryption modes have been defined for AES (no such mode has been defined for TDEA):

• SP 800-38C^49 specifies the CCM mode, and
• SP 800-38D^50 defines the Galois/Counter mode (GCM).

4.4 Random Bit Generation

Cryptography and security applications make extensive use of random numbers and random bits. For cryptography, random values are needed to generate cryptographic keys.

^46 FIPS 180: Secure Hash Standard. ^47 SP 800-89: Recommendation for Obtaining Assurances for Digital Signature Applications. ^48 SP 800-102: Recommendation for Digital Signature Timeliness. ^49 SP 800-38C: Recommendation for Block Cipher Modes of Operation: the CCM Mode for Authentication and Confidentiality. ^50 SP 800-38D: Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC.

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913 The term “entropy” is used to describe the amount of randomness in a value, and the 914 amount of entropy determines how hard it is to guess that value. 915 There are two classes of random bit generators (RBGs): Non-Deterministic Random Bit 916 Generators (NRBGs), sometimes called true random number (or bit) generators, and 917 Deterministic Random Bit Generators (DRBGs), sometimes called pseudorandom bit (or 918 number) generators. Each RBG is dependent on the use of an entropy source to provide 919 unpredictable bits that are outside of human control; these bits are acquired from some 920 physical source, such as thermal noise, ring oscillators or hard-drive seek times. An NRBG 921 is dependent on the availability of new, unused entropy bits produced by the entropy source 922 for every NRBG output. A DRBG is initially “seeded” with entropy produced by an 923 entropy source or using an approved method that depends on an entropy source (e.g., an 924 NRBG); depending on the application, the DRBG may or may not receive additional 925 entropy (e.g., by being reseeded). 926 Several publications have been developed or are currently under development for random- 927 bit generation: 928 • SP 800-90A^51 specifies approved DRBG algorithms, based on the use of hash 929 functions and block-cipher algorithms; DRBGs must be initialized from a 930 randomness source that provides sufficient entropy for the security strength(s) to 931 be supported by the DRBG. 932 • SP 800-90B^52, which is currently under development, discusses entropy sources, 933 including health tests to determine that the entropy source has not failed and tests 934 to estimate how much entropy that the entropy source can provide reliably. 935 • SP 800-90C^53 provides constructions for the design and implementation of NRBGs 936 and DRBGs from the algorithms in SP 800-90A and the entropy sources designed 937 in accordance with SP 800-90B. Note that the NRBGs are constructed to include a 938 DRBG algorithm from SP 800-90A to provide a fallback capability if an entropy 939 source failure is not immediately detected. 940 • SP 800-22^54 discusses some aspects of selecting and testing random and 941 pseudorandom number generators. This document includes some criteria for 942 characterizing and selecting appropriate generators, discusses statistical testing and 943 its relation to cryptanalysis and provides some recommended statistical tests. These 944 tests may be useful as a first step in determining whether or not a generator is 945 suitable for a particular cryptographic application. However, for federal 946 applications, the RBGs must be validated for compliance to FIPS 140 and the 947 appropriate parts of SP 800-90.

51 SP 800-90A: Random Number Generation Using Deterministic Random Bit Generator Mechanisms. 52 SP 800-90B: Entropy Sources. 53 SP 800-90C: Random Bit Generator (RBG) Constructions. 54 SP 800-22: A Statistical Test Suite for Random and Pseudorandom Number Generators for Cryptographic Applications.

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4.5 Symmetric vs. Asymmetric Cryptography

As discussed in Sections 3.2 and 3.3, when large numbers of cryptographic relationships are required, the number of initial symmetric keys that will be required may be significantly larger than the number of public/private key pairs required.

However, the primary advantage of symmetric-key cryptography is speed. Symmetric-key algorithms are generally significantly faster than asymmetric-key algorithms, and the keys are shorter in length for the same security strength; the key length may be an important consideration if memory for storing the keys, or the bandwidth for transporting the keys is limited. In addition, advances in cryptanalysis and computational efficiency have tended to reduce the level of protection provided by public-key cryptography more rapidly than that provided by symmetric-key cryptography. Also, in a potential post-quantum “world”, the currently approved asymmetric-key algorithms will not provide adequate protection.

Since asymmetric-key (i.e., public-key) cryptography requires fewer keys overall, and symmetric-key cryptography is significantly faster, a hybrid approach is often used, whereby asymmetric-key algorithms are used for the generation and verification of digital signatures and for key establishment, while symmetric-key algorithms are used for all other purposes (e.g., encryption), especially those involving the protection of large amounts of data. For example, an asymmetric-key system can be used to establish a symmetric key via a key-agreement or key-transport process (see Sections 5.3.3 and 5.3.4, respectively), after which the symmetric key is used to encrypt files or messages.

In some situations, asymmetric-key cryptography is not necessary, and symmetric-key cryptography alone is sufficient. This includes environments where secure symmetric-key establishment can take place using symmetric keys already shared between entities, environments where a single authority knows and manages all the keys, and in single-user environments.

In general, asymmetric cryptography is best suited for an open, multi-user environment.

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SECTION 5: KEY MANAGEMENT

The proper management of cryptographic keys is essential to the effective use of cryptography for security. Keys are analogous to the combination of a safe. If a safe combination becomes known by an adversary, that safe provides no security against penetration by that adversary. Similarly, poor key management may easily compromise strong algorithms. Ultimately, the security of information protected by cryptography directly depends on the strength of the keys, the effectiveness of mechanisms and protocols associated with keys, and the protection afforded to the keys themselves. All keys need to be protected against modification (i.e., their integrity needs to be preserved), and secret and private keys (i.e., keys used by symmetric and asymmetric algorithms, respectively) need to be protected against unauthorized disclosure (i.e., their confidentiality needs to be maintained).

Key management provides the foundation for the secure generation, storage, distribution/establishment, use and destruction of keys, and is essential at all phases of a key’s life. Cryptography can be used to protect large amounts of data. If a strong algorithm is used to encrypt the data using keys that are properly generated, then the protection of that data can subsequently be reduced to just protecting the keys, i.e. the security of information protected by cryptography directly depends on the protection afforded the keys. Therefore, a Cryptographic Key Management System (CKMS) is required for managing the keys.

5.1 General Key Management Guidance

Several publications have been developed to provide general key-management guidance: SP 800-57 (see Section 5.1.1), FIPS 140 (see Section 51.2), and SP 800-131A (see Section 5.1.3).

5.1.1 Recommendation for Key Management

SP 800-57^55 provides general guidance on the management of cryptographic keys: their generation, use, and eventual destruction. Related topics, such as algorithm selection and appropriate key size, and cryptographic policy are also included in SP 800-57, which consists of three parts:

• SP 800-57, Part 1, General Guidance, contains basic key-management guidance, including:
  • The protection required for keying material;
  • Key life-cycle responsibilities;
  • Key backup, archiving and recovery;
  • Changing keys;
  • Cryptoperiods (i.e., the appropriate lengths of time that keys are to be used);

55 SP 800-57: Recommendation for Key Management.

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o Accountability and auditing; o Contingency planning; and o Key compromise recovery (e.g., by generating new keys).

Federal agencies have a variety of information that they have determined to require cryptographic protection; the sensitivity of the information and the periods of time that the protection is required also vary. To this end, NIST has established four^56 security strengths for the protection of information: 112, 128, 192 and 256 bits^57. These security strengths have been assigned to the approved cryptographic algorithms and key sizes, and dates have been projected during which the use of these algorithms and key sizes is anticipated to be secure. For further information, see [SP 800-131A](SP 800-131A).

Agencies need to determine the length of time that cryptographic protection is required before selecting an algorithm and key size with the appropriate security strength.

• [SP 800-57, Part 2](SP 800-57, Part 2), Best Practices for Key Management Organizations, contains: o A generic key-management infrastructure, o Guidance for the development of organizational key-management policy statements and key-management practices statements, o An identification of key-management information that needs to be incorporated into security plans for general support systems and major applications that employ cryptography, and o An identification of key-management information that needs to be documented for all federal applications of cryptography.
• [SP 800-57, Part 3](SP 800-57, Part 3), Application-Specific Key Management Guidance, addresses the key management issues associated with currently available cryptographic mechanisms, such as the Public Key infrastructure (PKI), Internet Protocol Security (IPsec), the Transport Layer Security protocol (TLS), Secure/Multipart Internet Mail Extensions (S/MIME), Kerberos, Over-the-Air Rekeying (OTAR), Domain Name System Security Extensions (DNSSEC), Encrypted File Systems and the Secure Shell (SSH) protocol.

Specific guidance is provided regarding: o The recommended and/or allowable algorithm suites and key sizes, o Recommendations for the use of the mechanism in its current form for the protections of federal government information, and

56 A fifth security strength was originally defined to provide 80 bits of security strength, but this strength is no longer adequate for the protection of Federal information. 57 A fifth security strength (i.e., 80 bits of security) was acceptable for applying cryptographic protection (e.g., encryption) prior to 2014. However, this strength is no adequate.

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o Security considerations that may affect the effectiveness of key-management processes and the cryptographic mechanisms using keys that are generated and managed by those key-management processes. Note that in the case of TLS, a reference is provided to a separate publication – SP 800-52^58 – that provides extensive details for using TLS. New key-management techniques and mechanisms are constantly being developed, and existing key-management mechanisms and techniques are constantly being refined. While the security-guidance information contained in Part 3 will be updated as mechanisms and techniques evolve, new products and technical specifications can always be expected that are not reflected in the current version of the document. Therefore, the context provided may include status information, such as version numbers or implementation status at the time that the document was published.

5.1.2 Security Requirements for Cryptographic Modules

FIPS 140^59 provides minimum security requirements for cryptographic modules that embody or support cryptography in federal information systems. A cryptographic module performs the actual cryptographic computations for a security system protecting sensitive information. The security requirements cover areas related to the secure design and implementation of a cryptographic module, including the module specification; cryptographic module ports and interfaces; roles, services and authentication; finite-state models; physical security; the operational environment; cryptographic key management; electromagnetic interference/electromagnetic compatibility (EMI/EMC); self-tests; design assurance; and the mitigation of attacks. FIPS 140 is applicable to all federal agencies that use cryptography to protect sensitive information in computer and telecommunications systems. Further information about FIPS 40 and the validation of cryptographic modules is available at http://csrc.nist.gov/groups/STM/cmvp/index.html.

5.1.3 Transitions to New Cryptographic Algorithms and Key Lengths

With the development and publication of SP 800-57, Part 1, NIST provided recommendations for transitioning to new cryptographic algorithms and key lengths because of algorithm breaks or the availability of more powerful computers that could be used to efficiently search for cryptographic keys. SP 800-131A was developed to provide more specific guidance for such transitions. Each algorithm and service is addressed in SP

58 SP 800-52: Guidelines for the Selection, Configuration, and Use of Transport Layer Security (TLS) Implementations. 59 FIPS 140: Security Requirements for Cryptographic Modules.

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1077 800-131A, indicating whether its use is acceptable⁰, deprecated⁶¹, restricted⁶², allowed 1078 only for legacy applications⁶³, or disallowed. 1079 5.2 Cryptographic Key Management Systems 1080 Several publications have been developed for the development of key-management 1081 systems: SP 800-130⁶⁴ (see Section 5.2.1), SP 800-152⁵ (see Section 5.2.2) and 1082 documents relating to the Public Key Infrastructure used for asymmetric-key cryptography 1083 (see Section 5.2.3). 1084 A CKMS includes policies, procedures, components and devices that are used to protect, 1085 manage and distribute cryptographic keys and associated information (called metadata). A 1086 CKMS includes all devices or subsystems that can access a key or its metadata. The 1087 devices could be computers, cell phones, tablets, or other smart devices, such as cars, alarm 1088 systems, or refrigerators. 1089 5.2.1 Key Management Framework 1090 SP 800-130 contains topics that should be considered by a CKMS designer when 1091 developing a CKMS design specification. Topics include security policies, cryptographic 1092 keys and metadata, interoperability and transitioning, security controls, testing and system 1093 assurances, disaster recovery, and security assessments. 1094 For each topic, SP 800-130 specifies one or more documentation requirements that need to 1095 be addressed by the designer. SP 800-130 is intended to assist in: 1096 • The definition of the CKMS design by requiring the specification of significant 1097 CKMS capabilities, 1098 • Encouraging CKMS designers to consider the factors needed in a comprehensive 1099 CKMS, 1100 • Logically comparing different CKMSs and their capabilities, 1101 • Performing security assessments by requiring the specification of implemented and 1102 supported CKMS capabilities, and 1103 • Forming the basis for the development of Profiles that specify the specific 1104 requirements for the CKMS to be used by an organization.

⁶⁰ No security risk is known at present. ⁶¹ The use of the algorithm and key length is allowed, but the user must accept some risk. ⁶² The use of the algorithm is discouraged, and there are additional restrictions required for use. ⁶³ The algorithm and key length may be used to process already-protected information, but there may be a risk in doing so. ⁴ SP 800-130: A Framework for Designing Cryptographic Key Management Systems. ⁶⁵ SP 800-152: A Profile for U. S. Federal Cryptographic Key Management Systems (CKMS).

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1105 5.2.2 Key Management System Profile 1106 SP 800-152 contains requirements for the design, implementation, procurement, 1107 installation, configuration, management, operation and use of a CKMS by and for U.S. 1108 federal organizations and their contractors. The Profile is based on SP 800-130 (see Section 1109 5.2.1). SP 800-152 specifies requirements, makes recommendations for federal 1110 organizations having special security needs and desiring to augment the base security and 1111 key-management services, and suggests additional features that may be desirable to 1112 implement and use. 1113 In addition to providing design requirements to be incorporated into a CKMS design, SP 1114 800-152 provides requirements for a Federal CKMS (FCKMS) to be operated by a service 1115 provider that may be a federal agency or a third party operating an FCKMS under contract 1116 for one or more federal agencies and their contractors. 1117 This Profile is intended to: 1118 * Assist CKMS designers and implementers in supporting appropriate cryptographic 1119 algorithms and keys, selecting the metadata associated with the keys, and selecting 1120 protocols for protecting sensitive U.S. federal computing applications and data; 1121 * Establish requirements for testing, procurement, installation, configuration, 1122 administration, operation, maintenance and usage of the FCKMS; 1123 * Facilitate an easy comparison of one CKMS with another by analyzing their designs 1124 and implementations in order to understand how each meets the Framework and 1125 Profile requirements; and 1126 * Assist in understanding what is needed to evaluate, procure, install, configure, 1127 administer, operate, and use an FCKMS that manages the cryptographic keys that 1128 protect sensitive and valuable data obtained, processed, stored, and used by U.S. 1129 federal organizations and their contractors.

1130 5.2.3 Public Key Infrastructure 1131 A PKI is a security infrastructure that creates and manages public-key certificates to 1132 facilitate the use of public-key (i.e., asymmetric-key) cryptography. To achieve this goal, 1133 a PKI needs to perform two basic tasks: 1134 1. Generate and distribute public key certificates that bind public keys to the identifier 1135 associated with the owner of the corresponding private key^66 and to other required 1136 information after validating the accuracy of the information to be bound, and 1137 2. Maintain and distribute certificate-status information for unexpired and revoked 1138 certificates. 1139 Two types of certificates are commonly used: certificates used to distribute the public keys 1140 that are used to verify digital signatures, and certificates used to distribute public keys used 1141 for key management (i.e., key establishment). Each certificate associated with digital

^66 The identifier could be the true identity of the owner, or could be an alias or a pseudonym used to represent the owner.

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1142 signatures provides the public keys of one of the three digital-signature algorithms 1143 approved in FIPS 186: DSA, ECDSA or RSA (see Section 3.3). Certificates that convey 1144 the public keys to be used for key establishment may be of two types: those that provide a 1145 key-agreement public key (see Section 5.3.3), and those that provide a key-transport public 1146 key (see Section 5.3.4). Key-usage bits in a certificate indicate the purpose for which the 1147 public key is intended to be used. 1148 As discussed in Section 3.3, public keys can be made available to anyone. However, a 1149 private key must be maintained under the exclusive control of the owner of that private 1150 key^67 (i.e., the user that is authorized to use the private key). 1151 * If a private key that is used to generate digital signatures is lost, the owner can no 1152 longer generate digital signatures; some policies may permit users to maintain 1153 backup copies of the private key for continuity of operations, but this is not 1154 encouraged, so an alternative is to simply generate new key pairs and certificates. 1155 * If the private key used to generate digital signatures is compromised, relying parties 1156 can no longer trust the digital signatures generated using that private key (e.g., 1157 someone may be using the signature to provide false information). 1158 * If a private key used for key establishment is lost (e.g., a key used for key transport 1159 or key agreement), then further key establishment processes cannot be 1160 accomplished until the key is recovered or replaced; if the key is needed to recover 1161 data protected by the key, then that data is lost unless the key can be recovered. For 1162 example, if the key is used to transport a decryption key for encrypted data, and the 1163 key is lost, then the encrypted data cannot be decrypted. To ensure that access to 1164 critical data is not lost, PKIs often backup the private key-establishment key for 1165 possible recovery. 1166 * If a private key used for key establishment is compromised, then any transactions 1167 involving that key cannot be trusted (e.g., someone other than the true owner of the 1168 private key may be attempting to enter into a supposedly "secure" transaction for 1169 some illicit purpose).

1170 5.2.3.1 PKI Components, Relying Parties and Their Responsibilities 1171 For scalability, PKIs are usually implemented with a set of complementary components, 1172 each focused on specific aspects of the PKI process. The main PKI tasks are assigned to 1173 the following logical components; other components are also used to support the PKI, but 1174 are not discussed here (see SP 800-32^68 for further discussion): 1175 * Certification authorities (CAs) generate certificates and certificate-status 1176 information, and

^67 An exception could be some other trusted entity, such as the owner's organization. In these cases, the organization could be considered to be the real owner of the key. ^68 SP 800-32: Introduction to Public Key Technology and the Federal PKI Infrastructure.

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1177 • Registration authorities (RAs) verify the identity of users applying for a 1178 certificate^69 and authenticate other information to be included in the certificate. 1179 In general, a PKI operates as follows: 1180 1. An entity applies to an RA to request a certificate. 1181 2. The RA verifies the identity of the applicant, and 2) verifies the information to be 1182 inserted in the certificate. 1183 3. If the checks made by the RA in step 2 indicate that the information to be inserted 1184 in the certificate is valid, then the RA sends the public key and other relevant 1185 information to the CA to request that a certificate be generated. 1186 4. Upon receiving the certificate request from the RA, the CA creates a digital 1187 certificate, returns the certificate to the RA and deposits the certificate in a 1188 repository. 1189 5. When a relying party interacts with another entity that has a public-key certificate, 1190 the relying party needs to obtain the other entity’s certificate, either directly or from 1191 the CA’s repository. After acquiring the certificate, the relying entity verifies the 1192 signature on the certificate. Assuming that the certificate is “good,” then the relying 1193 party can proceed safely with its interaction with the certificate’s owner. 1194 Most of the interaction involved with using a certificate is transparent to the user. However, 1195 a user or a system administrator may be responsible for obtaining and installing a 1196 certificate. Thereafter, an application (e.g., a browser) uses the certificate to interact with 1197 other entities, and the user may not be aware of these actions. An exception might be when 1198 a certificate has expired or been revoked, in which case a message may be displayed to 1199 indicate this status. 1200 Certificates expire at a predetermined time unless revoked prior to the expiration date. 1201 Certificates can be revoked for a variety of reasons, including the compromise of the 1202 private key corresponding to the public key in the certificate, and the owner of the 1203 certificate leaving the organization. When a certificate has been revoked, a system will 1204 quite often display the certificate-revocation message and perhaps include the reason for 1205 the revocation. Depending on the application implementation and the revocation reason, 1206 the application could disallow further actions, or could allow the user to indicate whether 1207 to ignore the warning and continue operations, or to simply discontinue operations. This 1208 warning must not be taken lightly. Ignoring the warning means that the user is accepting 1209 the risks associated with doing so. For example, if a warning indicates a compromised 1210 digital signature certificate, there is a possibility that someone other than the claimed owner 1211 of the certificate actually used the private key corresponding to the public key to sign data. 1212 Depending on the data, it may not be prudent to ignore the warning. A user should consult 1213 with his organization to determine how to respond to this warning.

^69 The certificate could be for the user or for a device for which the user is authorized to obtain a certificate.

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1214 5.2.3.2 Basic Certificate Verification Process 1215 A PKI consists of at least one CA with its subscribers, as shown in Figure 5. Each of the 1216 subscribers (e.g., User 1, User 2 and User 3) obtains a certificate containing their public 1217 key and other information, which is signed by their CA. All CA subscribers are provided 1218 with the public key of the CA. 1219 As a basic example of how this works, suppose that User 3 signs a document and sends it 1220 to User 1, who needs to verify the contents and source of the signed document. This is 1221 accomplished as follows: 1222 1. User 1 obtains the certificate containing the 1223 public key that corresponds to the private key 1224 used to sign the document, i.e., User 1 obtains 1225 User 3’s certificate. Either User 3 supplies 1226 that certificate, or the certificate is obtained 1227 from some other source, e.g., the CA. 1228 2. User 1 verifies User 3’s certificate using the 1229 CA’s public key. 1230 3. User 1 then employs the public key in User 1231 3’s certificate to verify the signature on the 1232 document received from User 3. If the 1233 signature is successfully verified, then User 1 1234 knows that User 3 generated the signature, 1235 and no unauthorized modifications were 1236 made to the document after the signature was 1237 generated.

CA User 1 User 2 User 3 Figure 5: Basic Certificate Verification Example

1238 Note that other more-complicated scenarios exist when users subscribing to different CAs 1239 need to interact using CAs that have cross certified by signing a certificate for each other.

1240 5.2.3.3 CA Certificate Policies and Certificate Practice Statements 1241 Each CA has a Certificate Policy and a Certificate Practices Statement. As defined by 1242 ITU^70 Recommendation X.509, a Certificate Policy (CP) is “a named set of rules that 1243 indicates the applicability of a certificate to a particular community and/or class of 1244 application with common security requirements.” The CP defines the expectations and 1245 requirements of the relying party community that will trust the certificates issued by the 1246 CAs using that policy. A CP addresses such issues as key generation and storage; certificate 1247 generation; key escrow^71 and recovery; certificate status services, including Certificate 1248 Revocation List (CRL) generation and distribution; and system management functions, 1249 such as security audits, configuration management, and archiving.

^70 ITU is the abbreviation of the International Telecommunication Union. ^71 Saving a key or information that allows the key to be reconstructed so that the key can be recovered if ever needed (e.g., by being lost or corrupted).

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1250 A Certification Practice Statement (CPS) describes how a specific CA issues and manages 1251 public-key certificates. The CPS is derived from the applicable CP for the community or 1252 application in which the CA participates. 1253 A Federal Public Key Infrastructure (FPKI) has been established for use by the Federal 1254 Government (see Section 5.2.3.4 for further information). 1255 DRAFT NISTIR 7924^72 identifies a baseline set of security controls and practices to 1256 support the secure issuance of certificates. NISTIR 7924 is designed to be used as a 1257 template and guide for writing a CP for a specific community, or a CPS for a specific CA. 1258 5.2.3.4 Federal Public Key Infrastructure 1259 A Federal Public Key Infrastructure (FPKI) provides the Federal Government with a 1260 common infrastructure to administer digital certificates and public-private key pairs. The 1261 network portion of the FPKI (commonly referred to as the “Bridge”) consists of “Principal 1262 CAs” designated by various agencies. Each CA within the bridge is cross-certified with 1263 every other CA within the bridge, thus establishing a conduit for trust relationships among 1264 all CAs within the FPKI. Each Principal CA may also be associated with other CAs that 1265 are not part of the bridge. For more information about the FPKI, including its certificate 1266 policy and certificate practices statement, see http://www.idmanagement.gov/federal- 1267 public-key-infrastructure. 1268 5.3 Key Establishment 1269 Key establishment is the means by which keys are generated and provided to the entities 1270 that are authorized to use them. An entity may be a person, organization, device or process. 1271 Scenarios for which key establishment could be performed include the following: 1272 * A single entity could generate a key (see Section 5.3.1) and use it without providing 1273 it to other entities (e.g., for protecting locally stored data), 1274 * A key could be derived from a key that is already shared between two or more 1275 entities (see Section 5.3.2), 1276 * Two entities could generate a key using contributions (i.e., data) from each entity 1277 using an automated protocol that incorporates a key-agreement scheme (see Section 1278 5.3.3), or 1279 * A single entity could generate a key and provide it to one or more other entities, 1280 either by a manual means (e.g., a courier or a face-to-face meeting, with the key in 1281 either printed or electronic form, such as on a flash drive) or using automated 1282 protocols that incorporate a key-transport scheme (see Sections 5.3.4 and 5.3.5). 1283 5.3.1 Key Generation 1284 Cryptographic keys are required by most cryptographic algorithms, the exception being 1285 hash functions when not used as a component of another cryptographic process (e.g.,

72 NISTIR 7924: Reference Certificate Policy (Second Draft).

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1286 HMAC). SP 800-133^73 discusses the generation of the keys to be used with the approved 1287 cryptographic algorithms. 1288 All keys must be based directly or indirectly on the output of an approved Random Bit 1289 Generator (RBG) and must be generated within FIPS 140-compliant cryptographic 1290 modules (see FIPS 140). Any random value required by the module must be generated 1291 within a cryptographic module. 1292 SP 800-133 provides guidance on generating a key directly from an RBG, and references 1293 other publications for additional information required for the generation of keys for specific 1294 algorithms: 1295 * FIPS 186 provides rules for the generation of the key pairs to be used for the 1296 generation of digital signatures, 1297 * SP 800-108 provides methods for the generation of keys from an already-shared 1298 key (see Section 5.3.2), 1299 * SP 800-56A specifies the rules for the generation of key pairs for Diffie-Hellman 1300 and MQV key-agreement schemes (see Section 5.3.3), 1301 * SP 800-56B specifies the rules for the generation of key pairs for RSA key- 1302 agreement and key-transport schemes (see Sections 5.3.3 and 5.3.4, respectively), 1303 and 1304 * SP 800-132 specifies the rules for the generation of keys from passwords (see 1305 Section 5.3.6).

1306 5.3.2 Key Derivation 1307 Key derivation is concerned with the generation of a key from secret information, although 1308 non-secret information may also be used in the generation process in addition to the secret 1309 information. Typically, the secret information is shared among entities that need to derive 1310 the same key for subsequent interactions. The secret information could be a key that is 1311 already shared between the entities (i.e., a pre-shared key), or could be a shared secret that 1312 is derived during a key-agreement scheme (see Section 5.3.3). 1313 SP 800-108^74 specifies several key-derivation functions that use pre-shared keys. A pre- 1314 shared key could have been 1315 * Generated by one entity and provided to one or more other entities by some manual 1316 means (e.g., a courier or face-to-face meeting), 1317 * Agreed upon by the entities using an automated key-agreement scheme (see Section 1318 5.3.3), or 1319 * Generated by one entity and provided to another entity using an automated key- 1320 transport scheme (see Section 5.3.4).

^73 SP 800-133: Recommendation for Cryptographic Key Generation. ^74 SP 800-108: Recommendation for Key Derivation Using Pseudorandom Functions.

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1321 SP 800-56A, SP 800-56B and SP 800-56C⁵ provide methods for deriving keys from the 1322 shared secrets generated during key agreement (see Section 5.3.3). SP 800-56A and SP 1323 800-56 B specify two key-derivation methods for this purpose, and refer to SP 800-56C 1324 and SP 800-135⁶ for additional approved methods.

1325 5.3.3 Key Agreement 1326 Key agreement is a key-establishment procedure in which the resultant keying material is 1327 a function of information contributed by all participants in the key-agreement process so 1328 that no participant can predetermine the value of the resulting keying material 1329 independently of the contributions of the other participants. Key agreement is usually 1330 performed using automated protocols. 1331 SP 800-56A and SP 800-56B provide several automated pair-wise key-agreement schemes, 1332 i.e., key-agreement schemes involving two parties. For each scheme, a shared secret is 1333 generated, and keying material is derived from the shared secret using a key-derivation 1334 method specified or approved by reference in SP 800-56A, SP 800-56B or SP 800-56C. 1335 SP 800-56A and SP 800-56B include variations of key-agreement schemes, differing in 1336 the number of keys used and whether the keys are long term (i.e., static) or an ephemeral 1337 value (e.g., a nonce or a short-term key pair). The key-agreement schemes have two 1338 participating entities: an initiator and a responder.

1339 1340 Figure 6: Key Agreement Example

⁷⁵ SP 800-56C: Recommendation for Key Derivation through Extraction-then-Expansion. ⁷⁶ SP 800-135: Recommendation for Existing Application-Specific Key Derivation Functions.

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Figure 6 provides an example of a scheme where the responder uses a static key pair during the scheme, and the initiator uses an ephemeral key pair. Note that other key-agreement schemes may use other arrangements of key pairs (e.g., each party could use a static key pair or each party could use an ephemeral key pair). In the example provided in the figure above, the responder's private key is retained by the responder (who is the owner of the key pair), but the responder's public key may be provided to anyone. In this example, the public key is provided to the initiator:

1. The initiator obtains the responder's public key (e.g., from a CA or directly from the responder); this public key is the responder's contribution to the key-agreement process.
2. The initiator then generates a short-term key pair (i.e., an ephemeral key pair), and sends the ephemeral public key to the responder, retaining the ephemeral private key. The ephemeral public key is the initiator's contribution to the key-agreement process.
3. Both parties use their own key pair and the other party's public key to generate a shared secret.
4. Both parties then use their copy of the shared secret to derive one or more keys that are (hopefully) identical.

Key confirmation is an optional, but highly recommended, step that provides assurance that both parties now have the same (identical) key(s), and is shown in Figure 6 for the case that the initiator receives key confirmation from the responder. See SP 800-56A and SP 800-56B for further information.

SP 800-56A specifies Diffie-Hellman (DH) and MQV key-agreement schemes using finite field or elliptic curve mathematics and asymmetric key pairs to generate the shared secret, and SP 800-56B specifies two RSA key-agreement schemes. SP 800-56A and SP 800-56B also provide an analysis of the merits of each key-agreement scheme.

5.3.4 Key Transport

Key transport is a method whereby one party (the sender) generates a key and distributes it to one or more other parties (the receiver(s)). Key transport could be accomplished using manual methods (e.g., using a courier) or performed using automated protocols. SP 800-56A and SP 800-56B provide automated pair-wise key-transport schemes, and an analysis of the merits of each key-transport scheme.

5.3.4.1 SP 800-56A Key Transport

SP 800-56A specifies a key-transport method whereby a key-establishment transaction includes both a key-agreement process and a key-wrapping process. Key wrapping is a process that provides both confidentiality and integrity protection for keying material using a symmetric-key algorithm (see Section 5.3.5 for further information about key wrapping).

During the transaction, the key generated during the key-agreement part of the transaction is used as a key-wrapping key with a symmetric-key algorithm (e.g., AES) by the sending

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1380 party to wrap a key to be sent to the other party (the receiver). Note that the sender can be 1381 either the initiator or the responder in the key-agreement process. 1382 Figure 7 illustrates the key transport process that follows the key-agreement discussed in 1383 Section 5.3.3 and shown in Figure 6. After the key-agreement part of the transaction, the 1384 initiator and responder share a symmetric key-wrapping key, which is then used as follows:

Sender (Communications) Receiver

Derived Key Generate Symmetric Key

Wrap Symmetric Key

Ciphertext Key

Derived Key

Unwrap Ciphertext Key

Key Confirmation

Original Plaintext Symmetric Key

1385 1386 Figure 7: SP 800-56A Key Transport Example 1387 The sender: 1388 1. Generates (or otherwise obtains) a symmetric key to be transported (note that 1389 the sender could have been either the initiator or the responder in the key- 1390 agreement part of the transaction), 1391 2. Wraps the symmetric key from step 1 using the key-wrapping key, and 1392 3. Sends the resulting ciphertext (i.e., the wrapped key) to the intended receiver. 1393 The receiver: 1394 4. Unwraps the ciphertext using his copy of the key-wrapping key to obtain the 1395 original plaintext symmetric key, and 1396 5. Optionally performs key confirmation; although this step is optional, it is highly 1397 recommended to provide assurance that both parties now have the same 1398 symmetric key. 1399 5.3.4.2 SP 800-56B Key Transport 1400 SP 800-56B specifies two very different methods for transporting keys whereby the sender 1401 uses the receiver's public key to securely transport keying material to the receiver.

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1402 Figure 8 provides a simplified example of one of the key-transport methods in SP 800-56B. 1403 The receiver must have a key pair that is used during a key-transport transaction. Key 1404 transport is accomplished as follows. 1405 The sender: 1406 1. Obtains the public key of the intended receiver, 1407 2. Generates a symmetric key to be transported, 1408 3. Encrypts the symmetric key using the receiver's public key, and 1409 4. Sends the resulting ciphertext key to the receiver. 1410 The receiver: 1411 5. Uses his private key to decrypt the ciphertext key, thus obtaining the original 1412 plaintext key. 1413 6. Optionally performs key confirmation; although this step is optional, it is 1414 highly recommended to provide assurance that both parties now have the same 1415 symmetric key.

Sender (Communications) Receiver Obtain Receiver's Public Key Responder's Private Key Generate Symmetric Key Encrypt Symmetric Key Ciphertext Key Decrypt Symmetric Key Key Confirmation Original Plaintext Symmetric Key

1416 1417 Figure 8: SP 800-56B Key Transport Example 1418 5.3.5 Key Wrapping 1419 Key wrapping is a method used to provide confidentiality and integrity protection to keys 1420 (and possibly other information) using a symmetric key-wrapping key and a symmetric- 1421 key block cipher algorithm. The wrapped keying material can then be stored or transmitted 1422 securely. Unwrapping the keying material requires the use of the same same algorithm and 1423 key-wrapping key that was used during the original wrapping process.

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1424 Key wrapping differs from simple encryption in that the wrapping process includes an 1425 integrity feature. During the unwrapping process, this integrity feature is used to detect 1426 accidental or intentional modifications to the wrapped keying material. 1427 Three methods have been specified in SP 800-38F^77 for key wrapping, and other SP 800- 1428 38 modes (or combination of modes) that that can also be used for key wrapping are also 1429 approved in SP 800-38F. Depending on the method or mode, either AES or TDEA can be 1430 used. 1431 5.3.6 Derivation of a Key from a Password 1432 Keys can be derived from passwords. Due to the ease of guessing most passwords, keys 1433 derived in this manner are not suitable to be used for most applications. However, SP 800- 1434 132^78 specifies a family of functions that can be used to derive keying material from a 1435 password^79 for electronic storage applications (e.g., when encrypting an entire disk drive). 1436 5.4 Key Management Issues 1437 A number of issues need to be addressed for selecting and using a CKMS. 1438 5.4.1 Manual vs. Automated Key Establishment 1439 As discussed in Sections 5.3 and 5.3.4, keys can be established between entities either 1440 manually or using automated methods. In many cases, a hybrid approach is used in which 1441 an entity generates and manually distributes one or more keys to other entities, and 1442 thereafter these keys are used to establish other keys (see SP 800-56A and SP 800-56B). 1443 The number of keys to be manually distributed depends on the type of cryptography to be 1444 used (i.e., symmetric or asymmetric methods) and must be considered when selecting the 1445 capabilities required of a CKMS. 1446 5.4.2 Selecting and Operating a CKMS 1447 A CKMS could be designed, implemented and operated by the organization that will use 1448 it. Or, the organization could operate a CKMS procured from a vendor. Or, an organization 1449 could procure the services of a third party that procures a CKMS from a vendor. Whichever 1450 choice is made, the organization needs to make sure that the CKMS that is used provides 1451 the protections that are required for the organization’s information. SP 800-130 and SP 1452 800-152 discuss the considerations that need to be addressed by the federal organization, 1453 including the scalability of the CKMS, and the metadata to be associated with the keys. 1454 5.4.3 Storing and Protecting Keys 1455 Keys can be stored in a number of places and protected in a variety of ways. They could 1456 be stored in a safe. They could be present only in a validated cryptographic module where

77 SP 800-38F: Recommendation for Block Cipher Modes of Operation: Methods for Key Wrapping. 78 SP 800-132: Recommendation for Password-Based Key Derivation Part 1: Storage Applications. 79 Note that this publication considers a passphrase to be a password.

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1457 the module itself might adequately protect the keys, depending on its design. Keys could 1458 also be stored on electronic media, such as a flash drive; in this case, a key may need to be 1459 encrypted or split into key components so that no single person can determine what the key 1460 is. These issues need to be addressed for operational keys. 1461 Certain keys may need to be backed up so that if an operational key is inadvertently lost or 1462 modified, it can be recovered and operations resumed. Some keys may also need to be 1463 archived for long-term storage (e.g., because of legal requirements or to decrypt archived 1464 data). A key-recovery capability is needed whenever keys are backed up or archived. This 1465 capability needs to be designed so that the keys can be recovered in an acceptable amount 1466 of time and only by those entities authorized to do so; see Part 1 of SP 800-57 for more 1467 information about key backup, key archiving and key recovery. 1468 5.4.4 Cryptoperiods 1469 A cryptoperiod is the time span during which a specific key is authorized for use. A 1470 cryptoperiod for a key is assigned for a number of reasons, including limiting the amount 1471 of exposure of encrypted data if a single key is compromised. Cryptoperiods are usually 1472 assigned for a carefully considered period of time or by the maximum amount of data 1473 protected by the key. Tradeoffs associated with the determination of a cryptoperiod involve 1474 the risks and consequences of exposure. Section 5.3 of SP 800-57, Part 1 provides a more 1475 detailed discussion of the need for establishing cryptoperiods, the factors to be considered 1476 when deciding on a suitable cryptoperiod and some suggestions for the length of 1477 cryptoperiods. 1478 5.4.5 Use Validated Algorithms and Cryptographic Modules 1479 Cryptographic algorithms must be validated and implemented in FIPS 140-validated 1480 cryptographic modules. Every IT product available makes a claim as to functionality and/or 1481 offered security. When protecting sensitive data, a minimum level of assurance is needed 1482 that a product's stated security claim is valid. There are also legislative restrictions 1483 regarding certain types of technology, such as cryptography, that require federal agencies 1484 to use only tested and validated products. 1485 Federal agencies, private industry, and the public rely on cryptography for the protection 1486 of information and communications used in electronic commerce, critical infrastructure, 1487 and other application areas. At the core of all products offering cryptographic services is 1488 the cryptographic module. Cryptographic modules, which contain cryptographic 1489 algorithms, are used in products and systems to provide security services such as 1490 confidentiality, integrity, and authentication. Although cryptography is used to provide 1491 security, weaknesses such as poor design or weak algorithms can render the product 1492 insecure and place highly sensitive information at risk. Adequate testing and validation of 1493 the cryptographic module and its underlying cryptographic algorithms against established 1494 standards is essential to provide security assurance. 1495 NIST has established programs to validate the implementation of the approved 1496 cryptographic algorithms and the cryptographic modules in which they are used: the

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1497 Cryptographic Algorithm Validation Program (CAVP) and the Cryptographic Module 1498 Validation Program (CMVP). Information about the CAVP is available at 1499 http://csrc.nist.gov/groups/STM/cavp/index.html, while information about the CMVP is 1500 available at http://csrc.nist.gov/groups/STM/cmvp/index.html.

1501 Also, see Section 5.1.2 in this document for a discussion of the security requirements for 1502 cryptographic modules.

1503 5.4.6 Control of Keying Material 1504 The access to keys needs to be controlled. A key should only be accessible by an authorized 1505 entity, and only for the purpose for which it is authorized. For example, a key designated 1506 for key transport must not be used for the generation or verification of digital signatures.

1507 The proliferation of keys also needs to be controlled. While it is often convenient to make 1508 copies of keys, these extra copies need to be accounted for. If a key is compromised, that 1509 key and all its copies may need to be destroyed to prevent subsequent unauthorized use. 1510 For example, if a private key used for the generation of a digital signature is compromised, 1511 and a copy of the key still exists after the original copy was destroyed, then there is a 1512 possibility that the copy could be used to generate unauthorized digital signatures at a later 1513 time.

1514 Users must be provided with a list of responsibilities and liabilities, and each user should 1515 sign a statement acknowledging these concerns before receiving a key. Users must be made 1516 aware of their unique responsibilities, especially regarding the significance of a key 1517 compromise or loss. Users must be able to store their secret and private keys securely, so 1518 that no intruder can access them, yet the keys must be readily accessible for legitimate use.

1519 5.4.7 Compromises 1520 It is imperative to have a plan for handling the compromise or suspected compromise of 1521 keys, particularly those used and managed at a central site (e.g., the keys used by a CA to 1522 sign certificates); this should be established before the system becomes operational. A 1523 compromise-recovery plan should address what actions will be taken with compromised 1524 system software and hardware, CA keys, user keys, previously generated signatures, 1525 encrypted data, etc. SP 800-57, Part 1 includes discussions of the effects of a key 1526 compromise, measures for minimizing the likelihood or consequences of a key 1527 compromise, and what should be considered in developing a compromise-recovery plan.

1528 If someone's private or secret key is lost or compromised, other users must be made aware 1529 of this, so that they will no longer initiate the protection of data using a compromised key, 1530 or accept data protected with a compromised key without assessing and accepting the risk 1531 of doing so. This notification is often accomplished using CRLs or Compromised Key Lists 1532 (CKLs); see SP 800-57, Part 1 for discussions.

1533 In some cases, a key and all copies of the key should be destroyed immediately upon the 1534 detection of a key compromise. For example, a private key used for the generation of digital 1535 signatures should be immediately destroyed. However, the corresponding public key may 1536 need to remain available for verifying the signatures that were previously generated using

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1537 the compromised private key. Note that there is a risk associated with accepting these 1538 signatures. 1539 5.4.8 Accountability and Auditing 1540 Accountability involves the identification of those entities that have access to or control of 1541 cryptographic keys throughout their lifecycles. Accountability can be an effective tool to 1542 help prevent key compromises and to reduce the impact of compromises when they are 1543 detected. Accountability 1) aids in the determination of when a compromise could have 1544 occurred and what individuals could have been involved, 2) discourages key compromise 1545 because users know their access to the key is known, and 3) is useful in determining where 1546 the key was used and what data or other keys were protected by a compromised key, and 1547 therefore, may also be compromised. 1548 Auditing is another mechanism used for the detection and recovery from key compromises. 1549 Auditing includes reviewing the actions of humans that use, operate and maintain systems, 1550 looking for unusual events that may indicate inappropriate actions by the humans or 1551 processes using a key management system. 1552

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1553 1554 SECTION 6: OTHER ISSUES

1555 The use of cryptography should not be undertaken without a thorough risk analysis, and a 1556 determination of the sensitivity of the information to be protected and the security controls 1557 to be used (see SP 800-175A and SP 800-53). After performing a risk assessment and 1558 determining the sensitivity level of the information to be protected (Low, Moderate or 1559 High) and the security controls to be used, a number of issues need to be addressed to 1560 ensure that cryptography is used properly.

1561 This section identifies issues to be addressed after determining that cryptography is 1562 required.

1563 6.1 Required Security Strength

1564 The minimum security strength is determined by the sensitivity level of the information 1565 (see SP 800-175A). SP 800-152 requires a security strength of at least 112 bits for the 1566 protection of Low-impact information, 128 bits for Moderate-impact information, and 192 1567 bits for High-impact information. The required security strength can then be used to 1568 determine the algorithm and key size to be used. Section 6 of SP 800-57, Part 1 provides 1569 tables for selecting appropriate algorithms and key sizes.

1570 6.2 Interoperability

1571 Interoperability is the ability of one entity to communicate with another entity, whether the 1572 entities are people, devices or processes. In order to communicate, the entities must have: 1573 * A communications channel (e.g., the Internet) and the same communications 1574 protocol (e.g., TLS), and 1575 * Policies that allow the entities to communicate.

1576 In order to communicate securely, the entities must also have: 1577 * Trust that each entity will enforce its own policies. 1578 * Interoperable cryptographic capabilities as discussed in Section 4, and 1579 * Share appropriate keying material that has been established securely (see Section 1580 5.3).

1581 For example, if entities A and B are in two different organizations, and 1582 * Each organization has a policy that allows the entities to communicate, 1583 * Each entity trusts that the other entity will enforce its own policies, 1584 * There is a TLS capability that can be used for communication, 1585 * Each entity can encrypt and decrypt information using AES with a 128-bit key and 1586 establish keys using 2048-bit RSA key transport (see Section 5.3.4), and 1587 * One of the entities can generate a 128-bit AES key and act as the sender in the key- 1588 transport scheme, and the other entity has a 2048-bit RSA key pair and can act as 1589 the receiver (see Section 5.3.4.2 for a discussion on key transport),

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then the two entities have a secure and interoperable communication channel that can be used to establish a 128-bit key for encrypting information using AES.

6.3 When Algorithms are no Longer Approved In the case that an algorithm is no longer approved for providing adequate protection (e.g., the algorithm may have been “broken”), any information protected by the algorithm could be re-protected using an approved algorithm that is expected to protect the information for the remainder of its security life. However, if the information protected using the no-longer-approved algorithm was already collected by an adversary, the security of the re-protected information may not be as desired (see Section 5.6.4 for SP 800-57, Part 1 for additional discussion).

6.4 Registration Authorities (RAs) As discussed in Section 5.2.3.1, an RA verifies the identity of users applying for a certificate and authenticates other information to be included in a certificate generated by a Certification Authority (CA). The correctness of this information is the linchpin on which the security of using certificates is based. Once this information is verified, the appropriate information is submitted to a CA for certificate generation using a signed certification request. The CA must deem the RA as trustworthy, e.g.,

• Appropriate identification is provided by an entity requesting a certificate and is fully checked by the RA;
• Information submitted for inclusion in the certificate is checked for validity (e.g., that the public key is valid, and the private key is in the possession of the claimed owner); and
• The RA provides adequate protection for the private key used to sign the certification request.

6.5 Cross Certification Cross certification is the establishment of a trust relationship between two Certification Authorities (CAs) through the signing of each other's public key in a certificate referred to as a "cross-certificate." Cross-certificates provide a means to create a chain of trust from a single, trusted, root CA to multiple other CAs so that subscribers in one CA domain can interact safely with subscribers in other CA domains (e.g., the subscriber in one CA domain has assurance of the identity of the subscriber in the other domain and assurance of the accurateness of the other information provided by his certificate). Cross certification should only be performed when each CA examines the other CA's policies, finds them acceptable and trusts that CA to operate in accordance with those policies.

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Appendix A: References

1625 1626 The following FIPS and NIST Special Publications (SP) apply to the use of cryptography 1627 in the Federal Government. 1628 All publications are available at http://csrc.nist.gov/publications. 1629

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| FIPS 197 | Federal Information Processing Standard 197, Advanced Encryption Standard (AES), November 2001. FIPS 197 specifies a symmetric key block cipher algorithm. The Standard supports key sizes of 128, 192, and 256 bits and a block size of 128 bits. | | FIPS 198 | Federal Information Processing Standard 198-1, Keyed-Hash Message Authentication Code (HMAC), published in July 2008. FIPS 198-1 defines a message authentication code (MAC) that uses a cryptographic hash function in conjunction with a secret key for the calculation and verification of the MACs. | | FIPS 199 | Federal Information Processing Standard 199, Standards for Security Categorization of Federal Information and Information Systems, February 2004. FIPS 199 establishes security categories for both information and information systems. The security categories are based on the potential impact on an organization if certain events occur that jeopardize the information and information systems needed by the organization to accomplish its assigned mission, protect its assets, fulfill its legal responsibilities, maintain its day-to-day functions, and protect individuals. | | FIPS 202 | Federal Information Processing Standard 202, SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions, August 2015. FIPS 202 specifies SHA3-224, SHA3-256, SHA3-384 and SHA3-512. This FIPS also specifies two extendable-output functions (SHAKE128 and SHAKE256), which are not, in themselves, considered to be hash functions. | | SP 800-22 | Special Publication 800-22, A Statistical Test Suite for Random and Pseudorandom Number Generators for Cryptographic Applications, April 2010. SP 800-22 discusses some aspects of selecting and testing random and pseudorandom number generators for providing random numbers that are indistinguishable from truly random output. | | SP 800-32 | Special Publication 800-32, Federal Agency Use of Public Key Technology for Digital Signatures and Authentication, February 2001. |

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| | publication also identifies TLS extensions for which mandatory support must be provided and identifies other recommended extensions. |
| :--- | :--- |
| SP 800-53 | Special Publication 800-53, Rev. 4, *Security and Privacy Controls for Federal Information Systems and Organizations*, April 2013.<br><br>SP 800-53 provides a catalog of security and privacy controls for federal information systems and organizations, and a process for selecting controls to protect organizational operations (including mission, functions, image, and reputation), organizational assets, individuals, other organizations, and the Nation from a diverse set of threats, including hostile cyber attacks, natural disasters, structural failures, and human errors. |
| SP 800-56A | Special Publication 800-56A, *Recommendation for Pair-Wise Key-Establishment Schemes Using Discrete Logarithm Cryptography*, May 2013.<br><br>SP 800-56A specifies key-establishment schemes based on the discrete logarithm problem over finite fields and elliptic curves, including several variations of Diffie-Hellman and Menezes-Qu-Vanstone (MQV) key establishment schemes. |
| SP 800-56B | Special Publication 800-56B, *Recommendation for Pair-Wise Key-Establishment Schemes Using Integer Factorization Cryptography*, September 2014.<br><br>SP 800-56B specifies key-establishment schemes using integer-factorization cryptography (RSA). Both key transport and key agreement schemes are specified. |
| SP 800-56C | Special Publication 800-56C, *Recommendation for Key Derivation through Extraction-then-Expansion*, November 2011.<br><br>SP 800-56C specifies techniques for the derivation of keying material from a shared secret established during a key-establishment scheme defined in SP 800-56A or SP 800-56B through an extraction-then-expansion procedure. |
| SP 800-57, Part 1 | Special Publication 800-57, Part 1, *Recommendation for Key Management: Part 1: General (Revision 3)*, January 2016.<br><br>Part 1 of SP 800-57 provides general guidance and best practices for the management of cryptographic keying material. It focuses on issues involving the management of cryptographic keys: their generation, use, and eventual destruction. Related topics, such as algorithm selection and appropriate key size, cryptographic policy, and cryptographic module selection, are also included. |

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80 The algorithm and key length may be used to process already-protected information, but there may be a risk in doing so.

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1630 1631 Non-NIST Publications:

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NATIONAL SECURITY ARCHIVE

National Security Archive, Suite 701, Gelman Library, The George Washington University, 2130 H Street, NW, Washington, D.C., 20037, Phone: 202/994-7000, Fax: 202/994-7005, nsarchiv@gwu.edu