INTERNET DRAFT R. Housley
S/MIME Working Group Vigil Security
Expires September 2007 March 2007
Using AES-CCM and AES-GCM Authenticated Encryption
in the Cryptographic Message Syntax (CMS)
<draft-ietf-smime-cms-aes-ccm-and-gcm-01.txt>
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Abstract
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.
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1. Introduction
This document specifies the conventions for using AES-CCM and AES-GCM
authenticated encryption algorithms as the content-authenticated-
encryption algorithm with the Cryptographic Message Syntax [CMS]
authenticated-enveloped-data content type [AuthEnv].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [STDWORDS].
1.2. ASN.1
CMS values are generated using ASN.1 [X.208-88], using the Basic
Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
(DER) [X.509-88].
1.3. AES
Dr. Joan Daemen and Dr. Vincent Rijmen, both from Belgium, developed
the Rijndael block cipher algorithm, and they submitted it for
consideration as the Advanced Encryption Standard (AES). Rijndael
was selected by the National Institute for Standards and Technology
(NIST), and it is specified in a U.S. Federal Information Processing
Standard (FIPS) Publication [AES]. NIST selected the Rijndael
algorithm for AES because it offers a combination of security,
performance, efficiency, ease of implementation, and flexibility.
Specifically, the algorithm performs well in both hardware and
software across a wide range of computing environments. Also, the
very low memory requirements of the algorithm make it very well
suited for restricted-space environments. The AES is widely used by
organizations, institutions, and individuals outside of the U.S.
Government.
The AES specifies three key sizes: 128, 192, and 256 bits.
1.4. AES-CCM
The Counter with CBC-MAC (CCM) mode of operation is specified in
[CCM]. CCM is a generic authenticated encryption block cipher mode.
CCM is defined for use with any 128-bit block cipher, but in this
document, CCM is used with the AES block cipher.
AES-CCM has four inputs: an AES key, a nonce, a plaintext, and
optional additional authenticated data (AAD). AES-CCM generates two
outputs: a ciphertext and a message authentication code (also called
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an authentication tag).
The nonce is generated by the party performing the authenticated
encryption operation. Within the scope of any authenticated-
encryption key, the nonce value MUST be unique. That is, the set of
nonce values used with any given key MUST NOT contain any duplicate
values. Using the same nonce for two different messages encrypted
with the same key destroys the security properties.
AAD is authenticated but not encrypted. Thus, the AAD is not
included in the AES-CCM output. It can be used to authenticate
plaintext packet headers. In the CMS authenticated-enveloped-data
content type, authenticated attributes comprise the AAD.
1.5. AES-GCM
The Galois/Counter Mode (GCM) is specified in [GCM]. GCM is a
generic authenticated encryption block cipher mode. GCM is defined
for use with any 128-bit block cipher, but in this document, GCM is
used with the AES block cipher.
AES-GCM has four inputs: an AES key, an initialization vector (IV), a
plaintext content, and optional additional authenticated data (AAD).
AES-GCM generates two outputs: a ciphertext and message
authentication code (also called an authentication tag). To have a
common set of terms for AES-CCM and AES-GCM, the AES-GCM IV is
referred to as a nonce in the remainder of this document.
The nonce is generated by the party performing the authenticated
encryption operation. Within the scope of any authenticated-
encryption key, the nonce value MUST be unique. That is, the set of
nonce values used with any given key MUST NOT contain any duplicate
values. Using the same nonce for two different messages encrypted
with the same key destroys the security properties.
AAD is authenticated but not encrypted. Thus, the AAD is not
included in the AES-GCM output. It can be used to authenticate
plaintext packet headers. In the CMS authenticated-enveloped-data
content type, authenticated attributes comprise the AAD.
2. Automatic Key Management
The reuse of an AES-CCM or AES-GCM nonce/key combination destroys the
security guarantees. As a result, it can be extremely difficult to
use AES-CCM or AES-GCM securely when using statically configured
keys. For safety's sake, implementations MUST use an automated key
management system.
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The CMS authenticated-enveloped-data content type supports four
general key management techniques:
Key Transport: the content-authenticated-encryption key is
encrypted in the recipient's public key;
Key Agreement: the recipient's public key and the sender's
private key are used to generate a pairwise symmetric key, then
the content-authenticated-encryption key is encrypted in the
pairwise symmetric key;
Symmetric Key-Encryption Keys: the content-authenticated-
encryption key is encrypted in a previously distributed
symmetric key-encryption key; and
Passwords: the content-authenticated-encryption key is encrypted
in a key-encryption key that is derived from a password or
other shared secret value.
All of these key management techniques meet the automated key
management system requirement as long as a fresh content-
authenticated-encryption key is generated for the protection of each
content. Note that some of these key management techniques use one
key-encryption key to encrypt more than one content-authenticated-
encryption key during the system life cycle. As long as fresh
content-authenticated-encryption key is used each time, AES-CCM and
AES-GCM can be used safely with the CMS authenticated-enveloped-data
content type.
In addition to these four general key management techniques, CMS
supports other key management techniques. See Section 6.2.5 of
[CMS]. Since the properties of these key management techniques are
unknown, no statement can be made about whether these key management
techniques meet the automated key management system requirement.
Designers and implementers must perform their own analysis if one of
these other key management techniques is supported.
3. Content Authenticated Encryption Algorithms
This section specifies the conventions employed by CMS
implementations that support content authenticated encryption using
AES-CCM or AES-GCM.
Content authenticated encryption algorithm identifiers are located in
the AuthEnvelopedData EncryptedContentInfo contentEncryptionAlgorithm
field.
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Content authenticated encryption algorithms are used to encipher the
content located in the AuthEnvelopedData EncryptedContentInfo
encryptedContent field and to provide the message authentication code
for the AuthEnvelopedData mac field. Note that the message
authentication code provides integrity protection for both the
AuthEnvelopedData authAttrs and the AuthEnvelopedData
EncryptedContentInfo encryptedContent.
3.1. AES-CCM
The AES-CCM authenticated encryption algorithm is described in [CCM].
A brief summary of the properties of AES-CCM is provided in Section
1.4.
Neither the plaintext content nor the optional AAD inputs need to be
padded prior to invoking AES-CCM.
There are three algorithm identifiers for AES-CCM, one for each AES
key size:
aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
organization(1) gov(101) csor(3) nistAlgorithm(4) 1 }
id-aes128-CCM OBJECT IDENTIFIER ::= { aes 7 }
id-aes192-CCM OBJECT IDENTIFIER ::= { aes 27 }
id-aes256-CCM OBJECT IDENTIFIER ::= { aes 47 }
With all three AES-CCM algorithm identifiers, the AlgorithmIdentifier
parameters field MUST be present, and the parameters field must
contain a CCMParameter:
CCMParameters ::= SEQUENCE {
aes-nonce OCTET STRING (SIZE(7..13)),
aes-ICVlen AES-CCM-ICVlen DEFAULT 12 }
AES-CCM-ICVlen ::= INTEGER (4 | 6 | 8 | 10 | 12 | 14 | 16)
The aes-nonce parameter field contains 15-L octets, where L is the
size of the length field. With the CMS, the normal situation is for
the content-authenticated-encryption key to be used for a single
content, therefore L=8 is RECOMMENDED. See [CCM] for a discussion of
the trade-off between the maximum content size and the size of the
Nonce. Within the scope of any content-authenticated-encryption key,
the nonce value MUST be unique. That is, the set of nonce values
used with any given key MUST NOT contain any duplicate values.
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The aes-ICVlen parameter field tells the size of the message
authentication code. It MUST match the size in octets of the value
in the AuthEnvelopedData mac field. A length of 12 octets is
RECOMMENDED.
3.2. AES-GCM
The AES-GCM authenticated encryption algorithm is described in [GCM].
A brief summary of the properties of AES-CCM is provided in Section
1.5.
Neither the plaintext content nor the optional AAD inputs need to be
padded prior to invoking AES-GCM.
There are three algorithm identifiers for AES-GCM, one for each AES
key size:
aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
organization(1) gov(101) csor(3) nistAlgorithm(4) 1 }
id-aes128-GCM OBJECT IDENTIFIER ::= { aes 6 }
id-aes192-GCM OBJECT IDENTIFIER ::= { aes 26 }
id-aes256-GCM OBJECT IDENTIFIER ::= { aes 46 }
With all three AES-GCM algorithm identifiers, the AlgorithmIdentifier
parameters field MUST be present, and the parameters field must
contain a GCMParameter:
GCMParameters ::= SEQUENCE {
aes-nonce OCTET STRING, -- recommended size is 12 octets
aes-ICVlen AES-GCM-ICVlen DEFAULT 12 }
AES-GCM-ICVlen ::= INTEGER (12 | 13 | 14 | 15 | 16)
The aes-nonce is the AES-GCM initialization vector. The algorithm
specification permits the nonce to have any number of bits between 1
and 2^64. However, the use of OCTET STRING within GCMParameters
requires the nonce to be a multiple of 8 bits. Within the scope of
any content-authenticated-encryption key, the nonce value MUST be
unique, but need not have equal lengths. A nonce value of 12 octets
can be processed more efficiently, so that length is RECOMMENDED.
The aes-ICVlen parameter field tells the size of the message
authentication code. It MUST match the size in octets of the value
in the AuthEnvelopedData mac field. A length of 12 octets is
RECOMMENDED.
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4. Security Considerations
AES-CCM and AES-GCM make use of the AES block cipher in counter mode
to provide encryption. When used properly, counter mode provides
strong confidentiality. Bellare, Desai, Jokipii, Rogaway show in
[BDJR] that the privacy guarantees provided by counter mode are at
least as strong as those for CBC mode when using the same block
cipher.
Unfortunately, it is easy to misuse counter mode. If counter block
values are ever used for more that one encryption operation with the
same key, then the same key stream will be used to encrypt both
plaintexts, and the confidentiality guarantees are voided.
Fortunately, the CMS AuthEnvelopedData provides all of the tools
needed to avoid misuse of counter mode. Automated key management is
discussed in Section 2.
There are fairly generic precomputation attacks against all block
cipher modes that allow a meet-in-the-middle attack against the key.
These attacks require the creation and searching of huge tables of
ciphertext associated with known plaintext and known keys. Assuming
that the memory and processor resources are available for a
precomputation attack, then the theoretical strength of any block
cipher mode is limited to 2^(n/2) bits, where n is the number of bits
in the key. The use of long keys is the best countermeasure to
precomputation attacks. Use of an unpredictable nonce value in the
counter block significantly increases the size of the table that the
attacker must compute to mount a successful precomputation attack.
Implementations must randomly generate content-authenticated-
encryption keys. The use of inadequate pseudo-random number
generators (PRNGs) to generate cryptographic 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. RFC 4086 [RANDOM] offers important guidance in
this area.
5. References
5.1. Normative References
[AES] NIST, FIPS PUB 197, "Advanced Encryption Standard (AES)",
November 2001.
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[CCM] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, September 2003.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.
[GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of
Operation (GCM)", Submission to NIST, May 2005.
http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/
gcm/gcm-revised-spec.pdf.
[STDWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
5.2. Informative References
[BDJR] Bellare, M, Desai, A., Jokipii, E., and P. Rogaway,
"A Concrete Security Treatment of Symmetric Encryption:
Analysis of the DES Modes of Operation", Proceedings
38th Annual Symposium on Foundations of Computer
Science, 1997.
[RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", RFC 4086, June 2005.
6. IANA Considerations
None.
{{{ RFC Editor: Please remove this section prior to publication. }}}
Appendix: ASN.1 Module
CMS-AES-CCM-and-AES-GCM
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-aes-ccm-and-gcm(32) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
-- EXPORTS All
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-- 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-CCM OBJECT IDENTIFIER ::= { aes 7 }
id-aes192-CCM OBJECT IDENTIFIER ::= { aes 27 }
id-aes256-CCM OBJECT IDENTIFIER ::= { aes 47 }
id-aes128-GCM OBJECT IDENTIFIER ::= { aes 6 }
id-aes192-GCM OBJECT IDENTIFIER ::= { aes 26 }
id-aes256-GCM OBJECT IDENTIFIER ::= { aes 46 }
-- Parameters for AigorithmIdentifier
CCMParameters ::= SEQUENCE {
aes-nonce OCTET STRING (SIZE(7..13)),
aes-ICVlen AES-CCM-ICVlen DEFAULT 12 }
AES-CCM-ICVlen ::= INTEGER (4 | 6 | 8 | 10 | 12 | 14 | 16)
GCMParameters ::= SEQUENCE {
aes-nonce OCTET STRING, -- recommended size is 12 octets
aes-ICVlen AES-GCM-ICVlen DEFAULT 12 }
AES-GCM-ICVlen ::= INTEGER (12 | 13 | 14 | 15 | 16)
END
Authors' Addresses
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley(at)vigilsec.com
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