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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-03.txt                   R. Housley
Expires: May 2001                                     RSA Laboratories
                                                         November 2001


 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 method [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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   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.

1.1  AES

   The Advanced Encryption Standard (AES) is being developed to replace
   DES [DES].  The AES will be a new Federal Information Processing
   Standard (FIPS) Publication that will specify 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.

   NIST has posted the Draft FIPS for the AES (see
   http://csrc.nist.gov/encryption/aes).  The AES will become official
   after a 90-day public comment period, NIST makes appropriate changes
   to the Draft FIPS, and the Secretary of Commerce approves the FIPS.
   Current estimates place this sometime in late 2001.  In other words,
   any day now.

   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.

   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
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   identified vulnerability, revealing the result of a particular RSA
   decryption, requires access to an oracle which will respond to
   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 within 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.

   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].

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.



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   Compliant software MUST meet the requirements for constructing an
   enveloped-data content type stated in [CMS] Section 6, "Enveloped-
   data Content Type".

   A content-encryption key MUST be randomly generated for each instance

   of an enveloped-data content type.  The content-encryption key 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
   DH-ES using the a randomly generated private key value for the
   originator, 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 value.  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 and 2.4 provide
   additional information.

   The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
   field MUST specify a symmetric encryption algorithm.  Implementations

   MUST support the encryption of AES keys, 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 content-encryption key 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 content-encryption key for the recipient.  The key-encryption
   process is described in [PKCS#1v2.0].  See section 4.1 of this
   document for the algorithm identifier and the parameter syntax.

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

   Note: When using a Triple-DES content-encryption key, implementations

   MUST adjust the parity bits for each DES key comprising the Triple-
   DES key prior to RSAES-OAEP encryption.

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 sender's ephemeral public
   key.

   The EnvelopedData ukm MAY be absent.


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   The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH
   algorithm identifier.

2.3.1  ES-DH/AES Key Derivation

   Generation of the an AES key used in doing AES-KeyWrap is done using
   the method in [DH] with the following modifications:

   The Hash function H will be [SHA-256] rather than SHA-1.

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 (20
   bytes) of keying material.

   No partyAInfo is used.

   Consequently, the input to the first invocation of SHA-256 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:

   79 66 a0 38 22 28 1e a3 eb 08 d9 bc 69 5b d8 ff
   89 23 26 4d 2b ef ee 73 99 c0 a7 91 18 60 44 c1

   Consenquently,

   K=79 66 a0 38 22 28 1e a3 eb 08 d9 bc 69 5b d8 ff 89 23 26 4d

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
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   01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01

   Consequently, the input to SHA-256 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 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 32 bytes:

   4f cd e4 58 60 0b 85 fb 47 f4 5a c8 1c 23 a9 4a
   3e 64 4b 79 82 9d 98 66 df a5 ee 80 2c 80 99 bb

   Consequently,

   K=4f cd e4 58 60 0b 85 fb 47 f4 5a c8 1c 23 a9 4a
     3e 64 4b 79 82 9d 98 66 df a5 ee 80 2c 80 99 bb

2.3.2  AES CEK Wrap Process


   The AES key-wrap algorithm encrypts one AES key in another AES key.
   The algorithm inputs are a value in a multiple of 64-bits (in this
   case, the AES CEK) and an AES KEK of standard size.  The algorithm
   produces an output 64-bits longer than the input, the additional bits

   acting as a checksum for the original data.  The algorithm uses 6*n
   AES encryption/decryption operations where n is number of 64-bit
   blocks.  Full details of the AES key-wrap algorithm are available at
   [AES-KEYWRAP].

   NIST has assigned the following OIDs to define the 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 CEK.  Implementations MAY use different size KEK and CEK
   values.  Implements MUST support the CEK and the KEK having the same

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   length.  If different lengths are supported, the KEK MUST be of equal

   or greater length then 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 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.
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   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 in

   CMS.  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.

   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.
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   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, OCTET STRING SIZE (0) }

      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 }

   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
   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
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                   Use of the AES Algorithm in CMS      November 2000


   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
   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


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   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
   content-encryption key, 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 content-
   encryption key.  Compromise of the RSA private key may result in the
   disclosure of all messages protected with that key.  Compromise of
   the content-encryption key may result in disclosure of the associated

   encrypted content.

   The generation of 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 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 content-encryption key with a key-encryption key, the

   key-encryption key should always be at least the same length as the
   content -encryption key.  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.

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                   Use of the AES Algorithm in CMS      November 2000


7  Open Issues

   - References to each algorithm that would be acceptable to the RFC
   editor.   We have a FIPS number for AES (but it is not yet officially

   published).  We have no document for the key wrap algorithm yet and
   need to work out the details with NIST for publishing.
   - Does the oid for key derivation need to be changed since we are
   using SHA-256 not SHA-1?
   - Need to provide an ASN.1 module as [PKCS#1v2.0] is not 1988 syntax.


References

AES         J. Daemen, V. Rijmen, "The Rijndael Block Cipher", FIPS
            197, <To Be Published>.


AES-KEYWRAP NIST, "AES Key-Wrap Algorithm", TBD.
            < http://www.nist.gov/kms/key-wrap.pdf>

CMS         Housley, R.  Cryptographic Message Syntax.  RFC 2630.
            June 1999.

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 TBD, Date TBD.
            <draft-ietf-smime-pkcs1-01.txt>

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.

PROFILE     Housley, R., W. Ford, W. Polk, and D. Solo.  Internet
            X.509 Public Key Infrastructure: Certificate and CRL
            Profile.  RFC 2459.  January 1999.
 Schaad, Housley                13 
                   Use of the AES Algorithm in CMS      November 2000



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.
            [Available 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.
            [Available at http://draft-freier-ssl-version3-02.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
   Herndon, VA 20170
   USA

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                   Use of the AES Algorithm in CMS      November 2000


   Email: rhousley@rsasecurity.com





















































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