AES-GMAC in CMS: Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)
The identifiers and parameters for using the AES-GMAC authentication code inside CMS messages.
Housley, R., "Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)", RFC 9044, DOI 10.17487/RFC9044, June 2021, <https://www.rfc-editor.org/info/rfc9044>.
@misc{rfc9044,
series = {Request for Comments},
number = 9044,
howpublished = {RFC 9044},
publisher = {RFC Editor},
doi = {10.17487/RFC9044},
url = {https://www.rfc-editor.org/info/rfc9044},
author = {R. Housley},
title = {{Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)}},
pagetotal = 9,
year = 2021,
month = jun,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Housley Request for Comments: 9044 Vigil Security Category: Standards Track June 2021 ISSN: 2070-1721 Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS) Abstract 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 https://www.rfc-editor.org/info/rfc9044. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction 2. Terminology 3. Message Authentication Code Algorithms 3.1. AES-GMAC 4. Implementation Considerations 5. ASN.1 Module 6. IANA Considerations 7. Security Considerations 8. References 8.1. Normative References 8.2. Informative References Acknowledgements Author's Address 1. Introduction This document specifies the conventions for using the AES-GMAC [AES] [GCM] Message Authentication Code (MAC) algorithm with the Cryptographic Message Syntax (CMS) [RFC5652]. 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Message Authentication Code Algorithms This section specifies the conventions employed by CMS [RFC5652] implementations that support the AES-GMAC [AES] [GCM] Message Authentication Code (MAC) algorithm. MAC algorithm identifiers are located in the AuthenticatedData macAlgorithm field. MAC values are located in the AuthenticatedData mac field. 3.1. AES-GMAC The AES-GMAC [AES] [GCM] 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. 4. Implementation Considerations An implementation of the Advanced Encryption Standard (AES) Galois/ Counter Mode (GCM) authenticated encryption algorithm is specified in [GCM]. 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 Section 3.2 of [RFC5084]. 5. ASN.1 Module The following ASN.1 module uses the definition for MAC-ALGORITHM from [RFC5912]. 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 6. IANA Considerations IANA has registered the object identifier shown in Table 1 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 | +---------+--------------------------+------------+ Table 1 7. 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 [GCM]. 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 [F2005] [MV2005]. 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. [RFC4086] 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 [RFC7696]. 8. References 8.1. Normative References [AES] National Institute of Standards and Technology, "Advanced Encryption Standard (AES)", FIPS PUB 197, DOI 10.6028/NIST.FIPS.197, November 2001, <https://doi.org/10.6028/NIST.FIPS.197>. [GCM] Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", NIST Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D, November 2007, <https://doi.org/10.6028/NIST.SP.800-38D>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <https://www.rfc-editor.org/info/rfc5652>. [RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, June 2010, <https://www.rfc-editor.org/info/rfc5912>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. 8.2. Informative References [F2005] Ferguson, N., "Authentication weaknesses in GCM", May 2005, <https://csrc.nist.gov/csrc/media/projects/block- cipher-techniques/documents/bcm/comments/cwc-gcm/ ferguson2.pdf>. [MV2005] McGrew, D. and J. Viega, "GCM Update", May 2005, <https://csrc.nist.gov/CSRC/media/Projects/Block-Cipher- Techniques/documents/BCM/Comments/CWC-GCM/gcm-update.pdf>. [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, <https://www.rfc-editor.org/info/rfc4086>. [RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated Encryption in the Cryptographic Message Syntax (CMS)", RFC 5084, DOI 10.17487/RFC5084, November 2007, <https://www.rfc-editor.org/info/rfc5084>. [RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to-Implement Algorithms", BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015, <https://www.rfc-editor.org/info/rfc7696>. Acknowledgements Many thanks to Hans Aschauer, Hendrik Brockhaus, Quynh Dang, Roman Danyliw, Tim Hollebeek, Ben Kaduk, Mike Ounsworth, and Magnus Westerlund for their careful review and thoughtful improvements. Author's Address Russ Housley Vigil Security, LLC 516 Dranesville Road Herndon, VA 20170 United States of America Email: housley@vigilsec.com
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§1What the RFC Specifies
RFC 9044 specifies the conventions for using the AES-GMAC Message Authentication Code (MAC) algorithm with the Cryptographic Message Syntax (CMS) defined in RFC 5652. A MAC lets a receiver who holds the shared key check that content was not changed. The document is short: it is mostly identifiers, a parameter structure and an ASN.1 module.
It is a small instruction sheet that tells everyone how to stick a particular tamper-proof seal on messages, so all programs do it the same way.
§3Algorithm Identifiers and Parameters
In CMS, MAC algorithm identifiers go in the AuthenticatedData macAlgorithm field and MAC values go in the mac field. AES-GMAC uses one of three identifiers, chosen by the AES key size of 128, 192 or 256 bits: id-aes128-GMAC, id-aes192-GMAC or id-aes256-GMAC.
For all three, the AlgorithmIdentifier parameters field MUST be present and MUST contain GMACParameters, a SEQUENCE of a nonce and a length. The nonce is the GMAC initialization vector. It may have any number of bits between 8 and (2^64)-1 but MUST be a multiple of 8 bits, and it MUST be unique within the scope of any content-authentication key. A 12-octet nonce is RECOMMENDED because it can be processed more efficiently.
The length field is the MAC size in octets (12 to 16, default 12). It MUST match the size of the value in the mac field, and 12 octets is RECOMMENDED.
There are three versions of the stamp, one for each size of secret key. Each stamp comes with a never-repeated number and a note saying how long the stamp is.
§4Computing GMAC from AES-GCM
An existing AES-GCM implementation can produce AES-GMAC. Provide the content-authentication key as the AES key, the nonce as the initialization vector, a zero-length plaintext, and the content to be authenticated as the additional authenticated data (AAD). The result, which some implementations call the authentication tag, is the AES-GMAC authentication code. Because there is no plaintext, the encryption step is skipped and no ciphertext is produced.
The DEFAULT and RECOMMENDED values in GMACParameters were chosen to align with the parameters for AES-GCM in Section 3.2 of RFC 5084.
If you already own a machine that scrambles and stamps, you can feed it nothing to scramble, and just keep the stamp it makes.
§5The ASN.1 Module and IANA Registration
Section 5 gives an ASN.1 module, CryptographicMessageSyntaxGMACAlgorithms, that uses the MAC-ALGORITHM definition from RFC 5912. It defines the three object identifiers under the NIST aes arc: { aes 9 } for 128-bit keys, { aes 29 } for 192-bit keys and { aes 49 } for 256-bit keys. It also defines GMACParameters and MACLength, an INTEGER restricted to 12, 13, 14, 15 or 16, and marks each algorithm as a keyed MAC whose parameters are required.
IANA registered one object identifier for the module itself: decimal 72, id-mod-aes-gmac-alg-2020, in the "SMI Security for S/MIME Module Identifier" registry.
The formal rulebook writes the three stamp types and their options in a precise machine-readable language, and the official registry now lists this rulebook under number 72.
§7Security Considerations
Reusing a nonce under the same key lets an attacker produce valid AES-GMAC codes for arbitrary messages, so the nonce MUST be unique per content-authentication key, and the tag length MUST be fixed within that scope. If AES-GMAC is a building block in another algorithm, it MUST be used only once by that algorithm; for example, it MUST NOT be the pseudorandom function for PBKDF2.
The key distribution method must itself provide authentication, and when more than two parties share a key, data origin authentication is not provided, since any holder can compute a valid MAC. The forgery probability is close to 2^(-t) for a tag of t bits, though GMAC strength degrades slightly with message length, so implementations SHOULD use 16-octet tags for messages over 2^64 octets. Keys must be randomly generated with a good source of randomness, and implementations should be modular to allow algorithm replacement over time.
Never use the same one-time number twice with the same secret, or a cheater can forge stamps. Pick secrets truly at random, and everyone who knows the secret can make stamps, so you cannot tell which of them made one.