Using the AES-GMAC Algorithm with the Cryptographic Message Syntax (CMS)
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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.
This document specifies the conventions for using the AES-GMAC
Message Authentication Code (MAC) algorithm with the
Cryptographic Message Syntax (CMS) .
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.
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.
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:
For all three of these algorithm identifier values, the
AlgorithmIdentifier parameters field MUST be present, and the parameters
MUST contain GMACParameters:
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.
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 .
The following ASN.1 module uses the definition for MAC-ALGORITHM
from .
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 |
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
Advanced Encryption Standard (AES)
National Institute of Standards and Technology
Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC
Informative References
Authentication weaknesses in GCM
GCM Update
Many thanks to
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for their careful review and thoughtful improvements.