AES in CMS: Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)

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].

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.

   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.

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].

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

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
   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
   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.

   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.

   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.

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.

   [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.

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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