Network Working Group J. Schaad
Request for Comments: 3565 Soaring Hawk Consulting
Category: Standards Track July 2003
Use of the Advanced Encryption Standard (AES) Encryption
Algorithm in Cryptographic Message Syntax (CMS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies the conventions for using the Advanced
Encryption Standard (AES) algorithm for encryption with the
Cryptographic Message Syntax (CMS).
Conventions used in this document
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 BCP 14, RFC 2119
[MUSTSHOULD].
1. Overview
This document specifies the conventions for using Advanced Encryption
Standard (AES) content encryption algorithm with the Cryptographic
Message Syntax [CMS] enveloped-data and encrypted-data content types.
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].
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RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003
1.1. AES
The Advanced Encryption Standard (AES) [AES] was developed to replace
DES [DES]. The AES Federal Information Processing Standard (FIPS)
Publication specifies a cryptographic algorithm for use by U.S.
Government organizations. However, the AES will also be widely used
by organizations, institutions, and individuals outside of the U.S.
Government.
Two researchers who developed and submitted the Rijndael algorithm
for consideration are both cryptographers from Belgium: Dr. Joan
Daemen of Proton World International and Dr. Vincent Rijmen, a
postdoctoral researcher in the Electrical Engineering Department of
Katholieke Universiteit Leuven.
The National Institute of Standards and technology (NIST) selected
the Rijndael algorithm for AES because it offers a combination of
security, performance, efficiency, ease of implementation, and
flexibility. Specifically, Rijndael appears to be consistently a
very good performer in both hardware and software across a wide range
of computing environments regardless of its use in feedback or
non-feedback modes. Its key setup time is excellent, and its key
agility is good. The very low memory requirements of the Rijndael
algorithm make it very well suited for restricted-space environments,
in which it also demonstrates excellent performance. The Rijndael
algorithm operations are among the easiest to defend against power
and timing attacks. Additionally, it appears that some defense can
be provided against such attacks without significantly impacting the
algorithm's performance. Finally, the algorithm's internal round
structure appears to have good potential to benefit from
instruction-level parallelism.
The AES specifies three key sizes: 128, 192 and 256 bits.
2. Enveloped-data Conventions
The CMS enveloped-data content type consists of encrypted content and
wrapped content-encryption keys for one or more recipients. The AES
algorithm is used to encrypt the content.
Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 6,
"Enveloped-data Content Type".
The AES content-encryption key MUST be randomly generated for each
instance of an enveloped-data content type. The content-encryption
key (CEK) is used to encrypt the content.
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RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003
AES can be used with the enveloped-data content type using any of the
following key management techniques defined in [CMS] Section 6.
1) Key Transport: The AES CEK is uniquely wrapped for each recipient
using the recipient's public RSA key and other values. Section 2.2
provides additional details.
2) Key Agreement: The AES CEK is uniquely wrapped for each recipient
using a pairwise symmetric key-encryption key (KEK) generated using
an originator's randomly generated private key (ES-DH [DH]) or
previously generated private key (SS-DH [DH]), the recipient's public
DH key, and other values. Section 2.3 provides additional details.
3) Previously Distributed Symmetric KEK: The AES CEK is wrapped
using a previously distributed symmetric KEK (such as a Mail List
Key). The methods by which the symmetric KEK is generated and
distributed are beyond the scope of this document. Section 2.4
provides additional details.
4) Password Encryption: The AES CEK is wrapped using a KEK derived
from a password or other shared secret. Section 2.5 provides
additional details.
Documents defining the use of the Other Recipient Info structure will
need to define the proper use for the AES algorithm if desired.
2.1. EnvelopedData Fields
The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows:
The EnvelopedData version is determined based on a number of factors.
See [CMS] section 6.1 for the algorithm to determine this value.
The EnvelopedData recipientInfos CHOICE is dependent on the key
management technique used. Section 2.2, 2.3, 2.4 and 2.5 provide
additional information.
The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations
MUST support content encryption with AES, but implementations MAY
support other algorithms as well.
The EnvelopedData unprotectedAttrs MAY be present.
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2.2. KeyTransRecipientInfo Fields
The enveloped-data content type is ASN.1 encoded using the
EnvelopedData syntax. The fields of the EnvelopedData syntax MUST be
populated as follows:
The KeyTransRecipientInfo version MUST be either 0 or 2. If the
RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
version MUST be 0. If the RecipientIdentifier is
subjectKeyIdentifier, then the version MUST be 2.
The KeyTransRecipientInfo RecipientIdentifier provides two
alternatives for specifying the recipient's certificate, and thereby
the recipient's public key. The recipient's certificate MUST contain
a RSA public key. The CEK is encrypted with the recipient's RSA
public key. The issuerAndSerialNumber alternative identifies the
recipient's certificate by the issuer's distinguished name and the
certificate serial number; the subjectKeyIdentifier identifies the
recipient's certificate by the X.509 subjectKeyIdentifier extension
value.
The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the
key transport algorithm (i.e., RSAES-OAEP [RSA-OAEP]), and the
associated parameters used to encrypt the CEK for the recipient.
The KeyTransRecipientInfo encryptedKey is the result of encrypting
the CEK with the recipient's RSA public key.
2.3. KeyAgreeRecipientInfo Fields
This section describes the conventions for using ES-DH or SS-DH and
AES with the CMS enveloped-data content type to support key
agreement. When key agreement is used, then the RecipientInfo
keyAgreeRecipientInfo CHOICE MUST be used.
The KeyAgreeRecipient version MUST be 3.
The EnvelopedData originatorInfo field MUST be the originatorKey
alternative. The originatorKey algorithm fields MUST contain the
dh-public-number object identifier with absent parameters. The
originatorKey publicKey MUST contain the originator's ephemeral
public key.
The EnvelopedData ukm MAY be present.
The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH
algorithm identifier [CMSALG].
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2.3.1. ES-DH/AES Key Derivation
Generation of the AES KEK to be used with the AES-key wrap algorithm
is done using the method described in [DH].
2.3.1.1. Example 1
ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13
The key wrap algorithm is AES-128 wrap, so we need 128 bits (16
bytes) of keying material.
No partyAInfo is used.
Consequently, the input to SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1b
30 11
06 09 60 86 48 01 65 03 04 01 05 ; AES-128 wrap OID
04 04
00 00 00 01 ; Counter
a2 06
04 04
00 00 00 80 ; key length
And the output is the 32 bytes:
d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9
Consequently,
K= d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64
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2.3.1.2. Example 2
ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13
The key wrap algorithm is AES-256 key wrap, so we need 256 bits (32
bytes) of keying material.
The partyAInfo used is the 64 bytes
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
Consequently, the input to first invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 5f
30 11
06 09 60 86 48 01 65 03 04 01 2d ; AES-256 wrap OID
04 04
00 00 00 01 ; Counter
a0 42
04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
a2 06
04 04
00 00 01 00 ; key length
And the output is the 20 bytes:
88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B 32 30 D8 93
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The input to second invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 5f
30 11
06 09 60 86 48 01 65 03 04 01 2d ; AES-256 wrap OID
04 04
00 00 00 02 ; Counter
a0 42
04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
a2 06
04 04
00 00 01 00 ; key length
And the output is the 20 bytes:
CB A8 F9 22 BD 1B 56 A0 71 C9 6F 90 36 C6 04 2C AA 20 94 37
Consequently,
K = 88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B
32 30 D8 93 CB A8 F9 22 BD 1B 56 A0
2.3.2. AES CEK Wrap Process
The AES key wrap algorithm encrypts one AES key in another AES key.
The algorithm produces an output 64-bits longer than the input AES
CEK, the additional bits are a checksum. The algorithm uses 6*n AES
encryption/decryption operations where n is number of 64-bit blocks
in the AES CEK. Full details of the AES key wrap algorithm are
available at [AES-WRAP].
NIST has assigned the following OIDs to define the AES key wrap
algorithm.
id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }
In all cases the parameters field MUST be absent. The OID gives the
KEK key size, but does not make any statements as to the size of the
wrapped AES CEK. Implementations MAY use different KEK and CEK
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sizes. Implements MUST support the CEK and the KEK having the same
length. If different lengths are supported, the KEK MUST be of equal
or greater length than the CEK.
2.4. KEKRecipientInfo Fields
This section describes the conventions for using AES with the CMS
enveloped-data content type to support previously distributed
symmetric KEKs. When a previously distributed symmetric KEK is used
to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE
MUST be used. The methods used to generate and distribute the
symmetric KEK are beyond the scope of this document. One possible
method of distributing keys is documented in [SYMKEYDIST].
The KEKRecipientInfo fields MUST be populated as specified in [CMS]
Section 6.2.3, KEKRecipientInfo Type.
The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be
one of the OIDs defined in section 2.3.2 indicating that the AES wrap
function is used to wrap the AES CEK. The KEKRecipientInfo
keyEncryptionAlgorithm parameters field MUST be absent.
The KEKRecipientInfo encryptedKey field MUST include the AES CEK
wrapped using the previously distributed symmetric KEK as input to
the AES wrap function.
2.5. PasswordRecipientInfo Fields
This section describes the conventions for using AES with the CMS
enveloped-data content type to support password-based key management.
When a password derived KEK is used to wrap the AES CEK, then the
RecipientInfo PasswordRecipientInfo CHOICE MUST be used.
The keyEncryptionAlgorithm algorithm field MUST be one of the OIDs
defined in section 2.3.2 indicating the AES wrap function is used to
wrap the AES CEK. The keyEncryptionAlgorithm parameters field MUST
be absent.
The encryptedKey field MUST be the result of the AES key wrap
algorithm applied to the AES CEK value.
3. Encrypted-data Conventions
The CMS encrypted-data content type consists of encrypted content
with implicit key management. The AES algorithm is used to encrypt
the content.
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RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003
Compliant software MUST meet the requirements for constructing an
enveloped-data content type stated in [CMS] Section 8,
"Encrypted-data Content Type".
The encrypted-data content type is ASN.1 encoded using the
EncryptededData syntax. The fields of the EncryptedData syntax MUST
be populated as follows:
The EncryptedData version is determined based on a number of factors.
See [CMS] section 9.1 for the algorithm to determine this value.
The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
field MUST specify a symmetric encryption algorithm. Implementations
MUST support encryption using AES, but implementations MAY support
other algorithms as well.
The EncryptedData unprotectedAttrs MAY be present.
4. Algorithm Identifiers and Parameters
This section specified algorithm identifiers for the AES encryption
algorithm.
4.1. AES Algorithm Identifiers and Parameters
The AES algorithm is defined in [AES]. RSAES-OAEP [RSA-OAEP] MAY be
used to transport AES keys.
AES is added to the set of symmetric content encryption algorithms
defined in [CMSALG]. The AES content-encryption algorithm, in Cipher
Block Chaining (CBC) mode, for the three different key sizes are
identified by the following object identifiers:
id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain a AES-IV:
AES-IV ::= OCTET STRING (SIZE(16))
Content encryption algorithm identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.
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Content encryption algorithms are used to encrypt the content located
in the EnvelopedData EncryptedContentInfo encryptedContent and the
EncryptedData EncryptedContentInfo encryptedContent fields.
5. SMIMECapabilities Attribute Conventions
An S/MIME client SHOULD announce the set of cryptographic functions
it supports by using the S/MIME capabilities attribute. This
attribute provides a partial list of object identifiers of
cryptographic functions and MUST be signed by the client. The
algorithm OIDs SHOULD be logically separated in functional categories
and MUST be ordered with respect to their preference.
RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support.
5.1. AES S/MIME Capability Attributes
If an S/MIME client is required to support symmetric encryption with
AES, the capabilities attribute MUST contain the AES object
identifier specified above in the category of symmetric algorithms.
The parameter with this encoding MUST be absent.
The encodings for the mandatory key sizes are:
Key Size Capability
128 30 0B 06 09 60 86 48 01 65 03 04 01 02
196 30 0B 06 09 60 86 48 01 65 03 04 01 16
256 30 0B 06 09 60 86 48 01 65 03 04 01 2A
When a sending agent creates an encrypted message, it has to decide
which type of encryption algorithm to use. In general the decision
process involves information obtained from the capabilities lists
included in messages received from the recipient, as well as other
information such as private agreements, user preferences, legal
restrictions, and so on. If users require AES for symmetric
encryption, the S/MIME clients on both the sending and receiving side
MUST support it, and it MUST be set in the user preferences.
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6. Security Considerations
If RSA-OAEP [PKCS#1v2.0] and RSA PKCS #1 v1.5 [PKCS#1v1.5] are both
used to transport the same CEK, then an attacker can still use the
Bleichenbacher attack against the RSA PKCS #1 v1.5 encrypted key. It
is generally unadvisable to mix both RSA-OAEP and RSA PKCS#1 v1.5 in
the same set of recipients.
Implementations must protect the RSA private key and the CEK.
Compromise of the RSA private key may result in the disclosure of all
messages protected with that key. Compromise of the CEK may result
in disclosure of the associated encrypted content.
The generation of AES CEKs relies on random numbers. The use of
inadequate pseudo-random number generators (PRNGs) to generate these
values 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 1750 [RANDOM] offers important
guidance in this area.
When wrapping a CEK with a KEK, the KEK MUST always be at least the
same length as the CEK. An attacker will generally work at the
weakest point in an encryption system. This would be the smaller of
the two key sizes for a brute force attack.
Normative References
[AES] National Institute of Standards. FIPS Pub 197:
Advanced Encryption Standard (AES). 26 November 2001.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3369, August 2002.
[AES-WRAP] Schaad, J. and R. Housley, "Advanced Encryption
Standard (AES) Key Wrap Algorithm", RFC 3394, September
2002.
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms, RFC 3370, August 2002.
[DES] National Institute of Standards and Technology. FIPS
Pub 46: Data Encryption Standard. 15 January 1977.
[DH] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, June 1999.
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RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003
[MUSTSHOULD] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RSA-OAEP] Housley, R. "Use of the RSAES-OAEP Key Transport
Algorithm in the Cryptographic Message Syntax (CMS)",
RFC 3560, July 2003.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One
(ASN.1). 1988.
[X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988.
Informational References
[MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message
Specification", RFC 2633, June 1999.
[PKCS#1v1.5] Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5",
RFC 2313, March 1998.
[PKCS#1v2.0] Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0",
RFC 2437, October 1998.
[RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[SYMKEYDIST] Turner, S., "CMS Symmetric Key Management and
Distribution", Work in Progress, January 2003.
Acknowledgements
This document is the result of contributions from many professionals.
We appreciate the hard work of all members of the IETF S/MIME Working
Group.
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RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003
Appendix A ASN.1 Module
CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-aes(19) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- PKIX
AlgorithmIdentifier
FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit(18)};
-- AES information object identifiers --
aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
organization(1) gov(101) csor(3)_ nistAlgorithms(4) 1 }
-- AES using CBC-chaining mode for key sizes of 128, 192, 256
id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
-- AES-IV is a the parameter for all the above object identifiers.
AES-IV ::= OCTET STRING (SIZE(16))
-- AES Key Wrap Algorithm Identifiers - Parameter is absent
id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }
END
Author's Address
Jim Schaad
Soaring Hawk Consulting
EMail: jimsch@exmsft.com
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