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Security Authentication Message Authentication Code This document specifies the conventions for using the AES-GMAC Message Authentication Code algorithm with the Cryptographic Message Syntax (CMS) as specified in RFC 5652.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
• .  Terminology
• .  Message Authentication Code Algorithms
  • .  AES-GMAC
• .  Implementation Considerations
• .  ASN.1 Module
• .  IANA Considerations
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction This document specifies the conventions for using the AES-GMAC Message Authentication Code (MAC) algorithm with the Cryptographic Message Syntax (CMS) .

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Message Authentication Code Algorithms This section specifies the conventions employed by CMS implementations that support the AES-GMAC Message Authentication Code (MAC) algorithm. MAC algorithm identifiers are located in the AuthenticatedData macAlgorithm field. MAC values are located in the AuthenticatedData mac field.

AES-GMAC The AES-GMAC Message Authentication Code (MAC) algorithm uses one of the following algorithm identifiers in the AuthenticatedData macAlgorithm field; the choice depends on the size of the AES key, which is either 128 bits, 192 bits, or 256 bits: aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101) csor(3) nistAlgorithm(4) 1 } id-aes128-GMAC OBJECT IDENTIFIER ::= { aes 9 } id-aes192-GMAC OBJECT IDENTIFIER ::= { aes 29 } id-aes256-GMAC OBJECT IDENTIFIER ::= { aes 49 } For all three of these algorithm identifier values, the AlgorithmIdentifier parameters field MUST be present, and the parameters MUST contain GMACParameters: GMACParameters ::= SEQUENCE { nonce OCTET STRING, -- recommended size is 12 octets length MACLength DEFAULT 12 } MACLength ::= INTEGER (12 | 13 | 14 | 15 | 16) The GMACParameters nonce field is the GMAC initialization vector. The nonce may have any number of bits between 8 and (2^64)-1, but it MUST be a multiple of 8 bits. Within the scope of any content-authentication key, the nonce value MUST be unique. A nonce value of 12 octets can be processed more efficiently, so that length for the nonce value is RECOMMENDED. The GMACParameters length field tells the size of the message authentication code. It MUST match the size in octets of the value in the AuthenticatedData mac field. A length of 12 octets is RECOMMENDED.

Implementation Considerations An implementation of the Advanced Encryption Standard (AES) Galois/Counter Mode (GCM) authenticated encryption algorithm is specified in . An implementation of AES-GCM can be used to compute the GMAC message authentication code by providing the content-authentication key as the AES key, the nonce as the initialization vector, a zero-length plaintext content, and the content to be authenticated as the additional authenticated data (AAD). The result of the AES-GCM invocation is the AES-GMAC authentication code, which is called the "authentication tag" in some implementations. In AES-GCM, the encryption step is skipped when no input plaintext is provided; therefore, no ciphertext is produced. The DEFAULT and RECOMMENDED values in GMACParameters were selected to align with the parameters defined for AES-GCM in .

ASN.1 Module The following ASN.1 module uses the definition for MAC-ALGORITHM from . CryptographicMessageSyntaxGMACAlgorithms { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-aes-gmac-alg-2020(72) } DEFINITIONS IMPLICIT TAGS ::= BEGIN -- EXPORTS All IMPORTS AlgorithmIdentifier{}, MAC-ALGORITHM FROM AlgorithmInformation-2009 -- from [RFC5912] { iso(1) identified-organization(3) dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-algorithmInformation-02(58)} ; -- Object Identifiers aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101) csor(3) nistAlgorithm(4) 1 } id-aes128-GMAC OBJECT IDENTIFIER ::= { aes 9 } id-aes192-GMAC OBJECT IDENTIFIER ::= { aes 29 } id-aes256-GMAC OBJECT IDENTIFIER ::= { aes 49 } -- GMAC Parameters GMACParameters ::= SEQUENCE { nonce OCTET STRING, -- recommended size is 12 octets length MACLength DEFAULT 12 } MACLength ::= INTEGER (12 | 13 | 14 | 15 | 16) -- Algorithm Identifiers maca-aes128-GMAC MAC-ALGORITHM ::= { IDENTIFIER id-aes128-GMAC PARAMS TYPE GMACParameters ARE required IS-KEYED-MAC TRUE } maca-aes192-GMAC MAC-ALGORITHM ::= { IDENTIFIER id-aes192-GMAC PARAMS TYPE GMACParameters ARE required IS-KEYED-MAC TRUE } maca-aes256-GMAC MAC-ALGORITHM ::= { IDENTIFIER id-aes256-GMAC PARAMS TYPE GMACParameters ARE required IS-KEYED-MAC TRUE } END -- of CryptographicMessageSyntaxGMACAlgorithms

IANA Considerations IANA has registered the object identifier shown in in the "SMI Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)" registry.

Decimal	Description	References
72	id-mod-aes-gmac-alg-2020	RFC 9044

Security Considerations The CMS provides a method for authenticating data. This document identifies the conventions for using the AES-GMAC algorithm with the CMS. The key management technique employed to distribute message-authentication keys must itself provide authentication; otherwise, the content is delivered with integrity from an unknown source. When more than two parties share the same message-authentication key, data origin authentication is not provided. Any party that knows the message-authentication key can compute a valid MAC; therefore, the content could originate from any one of the parties. Within the scope of any content-authentication key, the AES-GMAC nonce value MUST be unique. Use of a nonce value more than once allows an attacker to generate valid AES-GMAC authentication codes for arbitrary messages, resulting in the loss of authentication as described in Appendix A of . Within the scope of any content-authentication key, the authentication tag length (MACLength) MUST be fixed. If AES-GMAC is used as a building block in another algorithm (e.g., as a pseudorandom function), AES-GMAC MUST be used only one time by that algorithm. For instance, AES-GMAC MUST NOT be used as the pseudorandom function for PBKDF2. When initialization vector (IV) lengths other than 96 bits are used, the GHASH function is used to process the provided IV, which introduces a potential for IV collisions. However, IV collisions are not a concern with CMS AuthenticatedData because a fresh content-authentication key is usually generated for each message. The probability of a successful forgery is close to 2^(-t), where t is the number of bits in the authentication tag length (MACLength*8). This nearly ideal authentication protection is achieved for CMS AuthenticatedData when a fresh content-authentication key is generated for each message. However, the strength of GMAC degrades slightly as a function of the length of the message being authenticated . Implementations SHOULD use 16-octet authentication tags for messages over 2^64 octets. Implementations must randomly generate message-authentication keys. The use of inadequate pseudorandom number generators (PRNGs) to generate keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys, searching the resulting small set of possibilities, rather than brute-force searching the whole key space. The generation of quality random numbers is difficult. offers important guidance in this area. Implementers should be aware that cryptographic algorithms become weaker with time. As new cryptanalysis techniques are developed and computing performance improves, the work factor to break a particular cryptographic algorithm will reduce. Therefore, cryptographic algorithm implementations should be modular, allowing new algorithms to be readily inserted. That is, implementers should be prepared to regularly update the set of algorithms in their implementations. More information is available in BCP 201 .

References Normative References National Institute of Standards and Technology In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the conventions for using the AES-CCM and the AES-GCM authenticated encryption algorithms with the Cryptographic Message Syntax (CMS) authenticated-enveloped-data content type. [STANDARDS-TRACK] Many IETF protocols use cryptographic algorithms to provide confidentiality, integrity, authentication, or digital signature. Communicating peers must support a common set of cryptographic algorithms for these mechanisms to work properly. This memo provides guidelines to ensure that protocols have the ability to migrate from one mandatory-to-implement algorithm suite to another over time.

Acknowledgements Many thanks to , , , , , , , and for their careful review and thoughtful improvements.

Author's Address Vigil Security, LLC

516 Dranesville Road Herndon VA 20170 United States of America housley@vigilsec.com