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Versions: 00 01 02 03 04 05 06 RFC 3565

S/MIME Working Group                                         J. Schaad
Internet Draft                                 Soaring Hawk Consulting
Document: draft-ietf-smime-aes-alg-04.txt                   R. Housley
Expires: July 2002                                    RSA Laboratories
                                                          January 2002


 Use of the AES Encryption Algorithm and RSA-OAEP Key Transport in CMS


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   Comments or suggestions for improvement may be made on the "ietf-
   smime" mailing list, or directly to the author.

Abstract

   This document specifies the conventions for using the Advanced
   Encryption Standard (AES) algorithm [AES] for encryption and the
   RSAES-OAEP key transport algorithm [PKCS#1v2.0] for key management
   with the Cryptographic Message Syntax (CMS) [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 RFC 2119
   [MUSTSHOULD].

1  Overview

   This document specifies the conventions for using the RSAES-OAEP key
   transport algorithm and Advanced Encryption Standard (AES) content
   encryption algorithm with the Cryptographic Message Syntax [CMS]
   enveloped-data and encrypted-data content types.


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                   Use of the AES Algorithm in CMS      February 2002


   This document presents the use of the two algorithms together, since
   we anticipate that they will be used together.  However, the two
   algorithms can be used independently.  For example, RSA-OAEP could be

   used to transport Triple-DES keys, and AES keys could be distributed
   out-of-band for use with mail lists.

   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.

1.2  RSA-OAEP

   When the variant of the RSA key transport algorithm specified in PKCS

   #1 Version 1.5 [PKCS#1v1.5] is used for key management, it is
   vulnerable to adaptive chosen ciphertext attacks.  This attack is
   described in [RSALAB] and [CRYPTO98].  The use of PKCS #1 Version 1.5

   key transport in interactive applications is especially vulnerable,
   but countermeasures are described in [MMA].  Exploitation of this
   identified vulnerability, revealing the result of a particular RSA
   decryption, requires access to an oracle which will respond to
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                   Use of the AES Algorithm in CMS      February 2002


   hundreds of thousands of ciphertexts, which are constructed
   adaptively in response to previously-received replies providing
   information on the successes or failures of attempted decryption
   operations.

   The attack appears significantly less feasible in store-and-forward
   environments, such as S/MIME.  When PKCS #1 Version 1.5 key transport

   is applied as an intermediate encryption layer within an interactive
   request-response communications environment, exploitation could be
   more feasible.  However, Secure Sockets Layer (SSL) [SSL] and
   Transport Layer Security (TLS) [TLS] protocol implementations could
   include countermeasures that detect and prevent Bleichenbacher's and
   other chosen-ciphertext attacks, without changing the way the RSA key

   transport algorithm is used.  These countermeasures are performed
   within the protocol level.  In the interest of long-term security
   assurance, it is prudent to adopt an improved cryptographic technique

   rather than embedding countermeasures in protocols.

   An updated version of PKCS #1 has been published: PKCS #1 Version 2.0

   [PKCS#1v2.0].  This new document supersedes RFC 2313 [PKCS#1v1.5].
   PKCS #1 Version 2.0 preserves support for the encryption padding
   format defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also
   defines a new alternative.  To resolve the adaptive chosen ciphertext

   vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
   of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
   is used to provide confidentiality, such as key transport.

   This document specifies the use of RSAES-OAEP key transport algorithm

   in the Cryptographic Message Syntax (CMS) [CMS].  CMS can be used in
   either a store-and-forward or an interactive request-response
   environment.

   CMS supports variety of architectures for certificate-based key
   management, particularly the one defined by the PKIX working group
   [PROFILE].  PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
   same RSA public key information.  Thus, a certified RSA public key
   may be used with either RSA key transport technique.

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
   RSAES-OAEP key transport algorithm is used to wrap the content-
   encryption key for one recipient.  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".

   An 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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   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
   DH-ES [DH] using the originator's randomly generated private key, 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.

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 originatorInfo field is not used for the RSAES-OAEP

   key transport algorithm.  However, this field MAY be present to
   support recipients using other key management algorithms.

   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:


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   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
   RSAES-OAEP algorithm, and the associated parameters used to encrypt
   the CEK for the recipient.  The key encryption process is described
   in [PKCS#1v2.0].  See section 4.2 of this document for the algorithm
   identifier and the parameter syntax.

   The KeyTransRecipientInfo encryptedKey is the result of encrypting
   the CEK with the recipient's RSA public key using the RSAES-OAEP
   algorithm.

   Note: When using a Triple-DES CEK, implementations MUST adjust the
   parity bits for each DES key comprising the Triple-DES key prior to
   RSAES-OAEP encryption.

   Note: The same key wrap algorithm is used for both Two-key Triple-DES

   and Three-key Triple-DES CEK keys.  When a Two-key Triple-DES key is
   to be wrapped, a third DES key with the same value as the first DES
   key is created.  Thus, all wrapped Triple-DES keys include three DES
   keys.

2.3  KeyAgreeRecipientInfo Fields

   This section describes the conventions for using ES-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 originatoryKey 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

   Consenquently,

   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
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   30 5f
      30 11
         06 09 60 86 48 01 65 03 04 01 2c            ; 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:

   6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a

   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 2c            ; 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:

   73 36 a5 ae 90 33 31 39 cb 3f 0e 90 cd d8 03 96 66 36 61 b0

   Consequently,

   K = 6f da b9 fa 67 09 30 3e 7e 2f 68 50 29 6f 28 fb 1b a6 4e 2a
       73 36 a5 ae 90 33 31 39 cb 3f 0e 90


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

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   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.

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3  Encrypted-data Conventions

   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 and the RSAES-OAEP key transport algorithm.

4.1  AES Algorithm Identifiers and Parameters

   The AES algorithm is defined in [AES].  RSA #1 v1.5 [PKCS#1v1.5]
   MUST NOT be used to transport AES keys.  RSAES-OAEP [PKCS#1v2.0] 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.

4.2  RSAES-OAEP Algorithm Identifiers and Parameters

   The RSAES-OAEP key transport algorithm is the RSA encryption scheme
   defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted
   is the content-encryption key.
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   The RSA key is identified in a certificate using the rsaEncryption
   object identifier:

      pkcs-1  OBJECT IDENTIFIER ::= {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) }


      rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1 }

   Note: This is the same algorithm identifier used by RSAES-PKCS1-v1_5.

   This means that the existence of an RSA key in a certificate cannot
   be used to infer that a recipient can decrypt an RSAES-OAEP encrypted

   content-encryption key.

   The object identifier for RSAES-OAEP is:

      id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  { pkcs-1 7 }

   The AlgorithmIdentifier parameters field MUST be present, and the
   parameters field MUST contain RSAES-OAEP-params.  RSAES-OAEP-params
   have the following syntax:

      RSAES-OAEP-params  ::=  SEQUENCE  {
         hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
         maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,

         pSourceFunc [2] AlgorithmIdentifier
            DEFAULT pSpecifiedEmptyIdentifier  }

      sha1Identifier  ::=  AlgorithmIdentifier {
         id-sha1, NULL }

      mgf1SHA1Identifier  ::=  AlgorithmIdentifier  {
         id-mgf1, sha1Identifier }

      pSpecifiedEmptyIdentifier  ::=  AlgorithmIdentifier  {
         id-pSpecified, nullOctetString }

      id-sha1  OBJECT IDENTIFIER ::=  {
         iso(1) identified-organization(3) oiw(14) secsig(3)
         algorithms(2) 26 }

      id-mgf1  OBJECT IDENTIFIER  ::=  { pkcs-1 8 }

      id-pSpecified  OBJECT IDENTIFIER  ::=  { pkcs-1 9 }

      nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }

   The fields of type RSAES-OAEP-params have the following meanings:

   hashFunc identifies the one-way hash function.  Implementations MUST
   support SHA-1 [SHA1].  The SHA-1 algorithm identifier is comprised of

   the id-sha1 object identifier and a parameter of NULL.
   Implementations that perform key encryption MUST omit the hashFunc
   field when SHA-1 is used, indicating that the default algorithm was
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                   Use of the AES Algorithm in CMS      February 2002


   used.  Implementations that perform key decryption MUST recognize
   both the id-sha1 object identifier and an absent hashFunc field as an

   indication that SHA-1 was used.

   maskGenFunc identifies the mask generation function. Implementations
   MUST support MFG1 [PKCS#1v2.0].  MFG1 requires a one-way hash
   function, and it is identified in the parameter field of the MFG1
   algorithm identifier.  Implementations MUST support SHA-1 [SHA1].
   The MFG1 algorithm identifier is comprised of the id-mgf1 object
   identifier and a parameter that contains the algorithm identifier of
   the one-way hash function employed with MFG1.  The SHA-1 algorithm
   identifier is comprised of the id-sha1 object identifier and a
   parameter of NULL.  Implementations that perform key encryption MUST
   omit the maskGenFunc field when MFG1 with SHA-1 is used, indicating
   that the default algorithm was used.  Implementations that perform
   key decryption MUST recognize both the id-mgf1 and id-sha1 object
   identifiers as well as an absent maskGenFunc field as an indication
   that MFG1 with SHA-1 was used.

   pSourceFunc identifies the source (and possibly the value) of the
   encoding parameters, commonly called P.  Implementations MUST
   represent P by an algorithm identifier, id-pSpecified, indicating
   that P is explicitly provided as an OCTET STRING in the parameters.
   The default value for P is an empty string.  In this case, pHash in
   EME-OAEP contains the hash of a zero length string.  Implementations
   MUST support a zero length P value.  Implementations that perform key

   encryption MUST omit the pSourceFunc field when a zero length P value

   is used, indicating that the default value was used.  Implementations

   that perform key decryption MUST recognize both the id-pSpecified
   object identifier and an absent pSourceFunc field as an indication
   that a zero length P value was used.

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  RSAES-OEAP SMIMECapability Attribute

   When constructing a signedData object, compliant software MAY include

   the SMIMECapabilities signed attribute announcing that it supports
   the RSAES-OAEP algorithm.

   The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the

   id-RSAES-OAEP object identifier in the capabilityID field and MUST
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   include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
   field.

      RSAES-OAEP-Default-Identifier  ::=  AlgorithmIdentifier  {
        id-RSAES-OAEP,  { sha1Identifier, mgf1SHA1Identifier,
                          pSpecifiedEmptyIdentifier  }  }

   When all of the default settings are selected, the SMIMECapability
   SEQUENCE representing RSAES-OAEP MUST be DER-encoded as:

         30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00

5.2  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 associated with this object identifier MUST is
   AESSMimeCapability.

       AESSMimeCapabilty ::= NULL

   The encodings for the mandatory key sizes are:

         Key Size                   Capability
          128          30 0D 06 09 60 86 48 01 65 03 04 01 02 30 00
          196          30 0D 06 09 60 86 48 01 65 03 04 01 16 30 00
          256          30 0D 06 09 60 86 48 01 65 03 04 01 2A 30 00

   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 and RSA #1 v1.5 are both used to transport the same CEK,
   then an attacker can still use the Bleichenbacher attack against the
   RSA #1 v1.5 encrypted key.  It is generally unadvisable to mix both
   RSA-OAEP and RSA #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, RSA public/private key pairs, and MGF
   seeds rely 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
 Schaad, Housley                12 
                   Use of the AES Algorithm in CMS      February 2002


   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.


References

AES         National Institute of Standards.
            FIPS Pub 197: Advanced Encryption Standard (AES).
            26 November 2001.

AES-WRAP    Schaad, J., R. Housley, "AES Key Wrap Algorithm",
            Draft-ietf-smime-aes-key-wrap-00.txt

CMS         Housley, R., Cryptographic Message Syntax.
            draft-ietf-smime-rfc2630bis-06.txt.

CMSALG      Housley, R., Cryptographic Message Syntax (CMS) Algorithms,
            draft-ietf-smime-cmsalg-07.txt.

CRYPTO98    Bleichenbacher, D., "Chosen Ciphertext Attacks Against
            Protocols Based on the RSA Encryption Standard PKCS #1,"
            in H. Krawczyk (editor), Advances in Cryptology - CRYPTO'98
            Proceedings, Lecture Notes in Computer Science 1462 (1998),
            Springer-Verlag, pp. 1-12.

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.

MMA         Rescorla, E., Preventing the Million Message Attack
            on CMS, RFC 3218, January 2002.

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.

 Schaad, Housley                13 
                   Use of the AES Algorithm in CMS      February 2002


PROFILE     Housley, R., W. Ford, W. Polk, and D. Solo.  Internet
            X.509 Public Key Infrastructure: Certificate and CRL
            Profile.  <draft-ietf-smime-new-part1.txt>.

RANDOM      Eastlake, D., S. Crocker, and J. Schiller.  Randomness
            Recommendations for Security.  RFC 1750.  December 1994.

RSALABS     Bleichenbacher, D., B. Kaliski, and J. Staddon.
            Recent Results on PKCS #1: RSA Encryption Standard.
            RSA Laboratories' Bulletin No. 7, June 26, 1998.
            [At http://www.rsasecurity.com/rsalabs/bulletins]

SHA1        National Institute of Standards and Technology.
            FIPS Pub 180-1: Secure Hash Standard.  17 April 1995.

SSL         Freier, A., P. Karlton, and P. Kocher.  The SSL Protocol,
            Version 3.0.  Netscape Communications.  November 1996.
            [At http://www.netscape.com/eng/ssl3/draft302.txt]

SYMKEYDIST  Turner, S.  CMS Symmetric Key Management and Distribution.
            RFC TDB. Date TBD.
            <draft-ietf-smime-symkeydist-06.txt>

TLS         Dierks, T. and C. Allen.  The TLS Protocol Version 1.0.
            RFC 2246.  January 1999.

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.


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.  We wish to extend a special thanks to
   Burt Kaliski.

Author's Addresses

   Jim Schaad
   Soaring Hawk Consulting

   Email: jimsch@exmsft.com

   Russell Housley
   RSA Laboratories
   918 Spring Knoll Drive
 Schaad, Housley                14 
                   Use of the AES Algorithm in CMS      February 2002


   Herndon, VA 20170
   USA

   Email: rhousley@rsasecurity.com

Appendix A  ASN.1 Module

CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549)
      pkcs(1) pkcs-9(9) smime(16) modules(0) aes-rsaes-oaep(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 S/MIME Capabilty parameter for all the above object identifiers

AESSMimeCapability ::= NULL

--  Definitions for RSA-OAEP

pkcs-1  OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
                   rsadsi(113549) pkcs(1) pkcs-1(1) }
id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  { pkcs-1 7 }

RSAES-OAEP-params  ::=  SEQUENCE  {
    hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
    maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
    pSourceFunc [2] AlgorithmIdentifier
           DEFAULT pSpecifiedEmptyIdentifier }

sha1Identifier AlgorithmIdentifier ::= { id-sha1, NULL }
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                   Use of the AES Algorithm in CMS      February 2002



mgf1SHA1Identifier AlgorithmIdentifier ::=  {id-mgf1, sha1Identifier }

nullOctetString OCTET STRING (SIZE (0)) ::= { ''H }

pSpecifiedEmptyIdentifier AlgorithmIdentifier ::=  { id-pSpecified,
                    nullOctetString }

id-sha1  OBJECT IDENTIFIER ::=  { iso(1) identified-organization(3)
              oiw(14) secsig(3) algorithms(2) 26 }

id-mgf1  OBJECT IDENTIFIER  ::=  { pkcs-1 8 }

id-pSpecified  OBJECT IDENTIFIER  ::=  { pkcs-1 9 }

rSAES-OAEP-Default-Identifier  AlgorithmIdentifier ::=  {
        id-RSAES-OAEP,  { sha1Identifier, mgf1SHA1Identifier,
                          pSpecifiedEmptyIdentifier  }  }

END


































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