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Versions: 00 01 02 rfc3537                                              
S/MIME Working Group                                          J. Schaad
Internet Draft                                  Soaring Hawk Consulting
draft-ietf-smime-hmac-key-wrap-00.txt                        R. Housley
Category: Informational                                RSA Laboratories
                                                           January 2002


        Wrapping an HMAC key with a Triple-DES Key or an AES Key


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
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   http://www.ietf.org/shadow.html.

Abstract

   The key wrap algorithms defined in [3DES-WRAP] and [AES-WRAP] cover
   the of wrapping a Triple-DES key with another Triple-DES key and
   wrapping an AES key with another AES key, respectively.  This
   document specifies two similar mechanisms.  One specifies the
   mechanism for wrapping an HMAC key with a Triple-DES key, and the
   other specifies the mechanism for wrapping an HMAC key with an AES
   key.


1. Introduction

   Standard methods exist for encrypting a Triple-DES (3DES) content-
   encryption key (CEK) with a 3DES key-encryption key (KEK) [3DES-
   WRAP] and for encrypting an AES CEK with an AES KEK [AES-WRAP].
   Triple-DES key wrap imposes parity restrictions, and in both
   instances there are restrictions on the size of the key being
   wrapped that make the encryption of HMAC [HMAC] keying material
   difficult.

   This document specifies a mechanism for the encryption of an HMAC
   key of arbitrary length by a 3DES KEK or an AES KEK.




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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [STDWORDS].


2. HMAC Key Guidelines

   [HMAC] suggests that the key be at least as long as the output (L)
   of the hash function being used.  When keys longer than the block
   size of the hash algorithm are used, they are hashed and the
   resulting hash value is used.  Using keys much longer than L
   provides no security benefit, unless the random function used to
   create the key has low entropy output.


3. HMAC Key Wrapping and Unwrapping with Triple-DES

   This section specifies the algorithms for wrapping and unwrapping an
   HMAC key with a 3DES KEK [3DES].

   The 3DES wrapping of HMAC keys is based on the algorithm defined in
   Section 3 of [3DES-WRAP].  The major differences are due to the fact
   that an HMAC key is variable length and the HMAC key has no
   particular parity.

3.1 Wrapping an HMAC Key with a Triple-DES Key-Encryption Key

   This algorithm encrypts an HMAC key with a 3DES KEK.  The algorithm
   is:

   1.  Let the HMAC key be called KEY, and let the length of KEY in
       octets be called LENGTH.  LENGTH is a single octet.
   2.  Let LKEY = LENGTH || KEY.
   3.  Let LKEYPAD = LKEY || PAD.  If the length of LKEY is a multiple
       of 8, the PAD has a length of zero.  If the length of LKEY is
       not a multiple of 8, then PAD contains the fewest number of
       random octets to make the length of LKEYPAD a multiple of 8.
   4.  Compute an 8 octet key checksum value on LKEYPAD as described
       in Section 2 of [3DES-WRAP], call the result ICV.
   5.  Let LKEYPADICV = LKEYPAD || ICV.
   6.  Generate 8 octets at random, call the result IV.
   7.  Encrypt LKEYPADICV in CBC mode using the 3DES KEK.
       Use the random value generated in the previous step as the
       initialization vector (IV).  Call the ciphertext TEMP1.
   8.  Let TEMP2 = IV || TEMP1.
   9.  Reverse the order of the octets in TEMP2.  That is, the most
       significant (first) octet is swapped with the least significant
       (last) octet, and so on.  Call the result TEMP3.
   10. Encrypt TEMP3 in CBC mode using the 3DES KEK.  Use
       an initialization vector (IV) of 0x4adda22c79e82105.




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   Note:  When the same HMAC key is wrapped in different 3DES KEKs, a
   fresh initialization vector (IV) must be generated for each
   invocation of the HMAC key wrap algorithm.

3.2  Unwrapping an HMAC Key with a Triple-DES Key-Encryption Key

   This algorithm decrypts an HMAC key using a 3DES KEK.  The algorithm
   is:

   1.  If the wrapped key is not a multiple of 8 octets, then error.
   2.  Decrypt the wrapped key in CBC mode using the 3DES KEK.
       Use an initialization vector (IV) of 0x4adda22c79e82105.  Call
       the output TEMP3.
   3.  Reverse the order of the octets in TEMP3.  That is, the most
       significant (first) octet is swapped with the least significant
       (last) octet, and so on.  Call the result TEMP2.
   4.  Decompose the TEMP2 into IV and TEMP1.  IV is the most
       significant (first) 8 octets, and TEMP1 is the remaining octets.
   5.  Decrypt TEMP1 in CBC mode using the 3DES KEK.  Use
       the IV value from the previous step as the initialization
       vector.  Call the plaintext LKEYPADICV.
   6.  Decompose the LKEYPADICV into LKEYPAD, and ICV.  ICV is the
       least significant (last) octet 8 octets.  LKEYPAD is the
       remaining octets.
   7.  Compute an 8 octet key checksum value on LKEYPAD as described
       in Section 2 of [3DES-WRAP].  If the computed key checksum value
       does not match the decrypted key checksum value, ICV, then
       error.
   8.  Decompose the LKEYPAD into LENGTH, KEY, and PAD.  LENGTH is the
       most significant (first) octet.  KEY is the following LENGTH
       octets.  PAD is the remaining octets, if any.
   9.  If the length of PAD is more than 7 octets, then error.
   10. Use KEY as an HMAC key.

3.3 HMAC Key Wrap with Triple-DES Algorithm Identifier

   Some security protocols employ ASN.1 [X.208-88, X.209-88], and these
   protocols employ algorithm identifiers to name cryptographic
   algorithms.  To support these protocols, the HMAC Key Wrap with
   Triple-DES algorithm has been assigned the following algorithm
   identifier:

      id-alg-HMACwith3DESwrap OBJECT IDENTIFIER ::= { iso(1)
          member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
          smime(16) alg(3) 11 }

   The AlgorithmIdentifier parameter field MUST be NULL.

3.4 HMAC Key Wrap with Triple-DES Test Vector

   KEK          :  5840df6e 29b02af1
                :  ab493b70 5bf16ea1
                :  ae8338f4 dcc176a8

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   HMAC_KEY     :  c37b7e64 92584340
                :  bed12207 80894115
                :  5068f738

   IV           :  050d8c79 e0d56b75

   PAD          :  38be62

   ICV          :  1f363a31 cdaa9037

   LKEYPADICV   :  14c37b7e 64925843
                :  40bed122 07808941
                :  155068f7 38be62fe
                :  1f363a31 cdaa9037

   TEMP1        :  157a8210 f432836b
                :  a618b096 475c864b
                :  6612969c dfa445b1
                :  5646bd00 500b2cc1

   TEMP3        :  c12c0b50 00bd4656
                :  b145a4df 9c961266
                :  4b865c47 96b018a6
                :  6b8332f4 10827a15
                :  756bd5e0 798c0d05

   Wrapped Key  :  0f1d715d 75a0aaf6
                :  6f02e371 c08b79e2
                :  a1253dc4 3040136b
                :  dc161118 601f2863
                :  e2929b3b dd17697c

4. HMAC Key Wrapping and Unwrapping with AES

   This section specifies the algorithms for wrapping and unwrapping an
   HMAC key with an AES KEK [AES-WRAP].

   The AES wrapping of HMAC keys is based on the algorithm defined in
   [AES-WRAP].  The major difference is inclusion of padding due to the
   fact that the length of an HMAC key may not be a multiple of 64
   bits.

4.1 Wrapping an HMAC Key with an AES Key-Encryption Key

   This algorithm encrypts an HMAC key with an AES KEK.  The algorithm
   is:

   1.  Let the HMAC key be called KEY, and let the length of KEY in
       octets be called LENGTH.  LENGTH is a single octet.
   2.  Let LKEY = LENGTH || KEY.
   3.  Let LKEYPAD = LKEY || PAD.  If the length of LKEY is a multiple
       of 8, the PAD has a length of zero.  If the length of LKEY is

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       not a multiple of 8, then PAD contains the fewest number of
       random octets to make the length of LKEYPAD a multiple of 8.
   4.  Encrypt LKEYPAD using the AES key wrap algorithm specified in
       section 2.2.1 of [AES-WRAP], using the AES KEK as the encryption
       key.  The result is 8 octets longer than LKEYPAD.

4.2  Unwrapping an HMAC Key with an AES Key

   The AES key unwrap algorithm decrypts an HMAC key using an AES KEK.
   The AES key unwrap algorithm is:

   1.  If the wrapped key is not a multiple of 8 octets, then error.
   2.  Decrypt the wrapped key using the AES key unwrap algorithm
       specified in section 2.2.2 of [AES-WRAP], using the AES KEK as
       the decryption key.  If the unwrap algorithm internal integrity
       check fails, then error, otherwise  call the result LKEYPAD.
   3.  Decompose the LKEYPAD into LENGTH, KEY, and PAD.  LENGTH is the
       most significant (first) octet.  KEY is the following LENGTH
       octets.  PAD is the remaining octets, if any.
   4.  If the length of PAD is more than 7 octets, then error.
   5.  Use KEY as an HMAC key.

4.3 HMAC Key Wrap with AES Algorithm Identifier

   Some security protocols employ ASN.1 [X.208-88, X.209-88], and these
   protocols employ algorithm identifiers to name cryptographic
   algorithms.  To support these protocols, the HMAC Key Wrap with AES
   algorithm has been assigned the following algorithm identifier:

      id-alg-HMACwithAESwrap OBJECT IDENTIFIER ::= { iso(1)
          member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
          smime(16) alg(3) 12 }

   The AlgorithmIdentifier parameter field MUST be NULL.

4.4 HMAC Key Wrap with AES Test Vector

   KEK          :  5840df6e 29b02af1
                :  ab493b70 5bf16ea1
                :  ae8338f4 dcc176a8

   HMAC_KEY     :  c37b7e64 92584340
                :  bed12207 80894115
                :  5068f738

   PAD          :  050d8c

   LKEYPAD      :  14c37b7e 64925843
                :  40bed122 07808941
                :  155068f7 38050d8c

   Wrapped Key  :  9fa0c146 5291ea6d
                :  b55360c6 cb95123c

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                :  d47b38cc e84dd804
                :  fbcec5e3 75c3cb13

5. Security Considerations

   Implementations must protect the key-encryption key (KEK).
   Compromise of the KEK may result in the disclosure of all HMAC keys
   that have been wrapped with the KEK, which may lead to loss of data
   integrity protection.

   The use of these key wrap functions provide confidentiality and data
   integrity, but they do not necessarily provide data origination
   authentication.  Anyone possessing the KEK can create a message that
   passes the integrity check.  If data origination authentication is
   also desired, then the KEK distribution mechanism must provide data
   origin authentication of the KEK.  Alternatively, a digital
   signature may be used.

   Implementations must randomly generate initialization vectors (IVs)
   and padding.  The generation of quality random numbers is difficult.

   RFC 1750 [RANDOM] offers important guidance in this area, and
   Appendix 3 of FIPS Pub 186 [DSS] provides one quality PRNG
   technique.

   The key wrap algorithms specified in this document have been
   reviewed for use with Triple-DES and AES, and they have not been
   reviewed for use with other encryption algorithms.

6. References

   3DES      American National Standards Institute.  ANSI X9.52-1998,
             Triple Data Encryption Algorithm Modes of Operation.
             1998.

   3DES-WRAP Housley, R.,  Triple-DES and RC2 Key Wrapping.  RFC 3217.
             December 2001.

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

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

   DSS       National Institute of Standards and Technology.
             FIPS Pub 186: Digital Signature Standard.  19 May 1994.

   HMAC      Krawczyk, H., M. Bellare, and R. Canetti. HMAC: Keyed-
             Hashing for Message Authentication. RFC 2104.
             February 1997.

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

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   RFC2026   Bradner, S., "The Internet Standards Process - Revision
             3", BCP 9, RFC 2026, October 1996.

   STDWORDS  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997

   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.

7. Author's Addresses

   Jim Schaad
   Soaring Hawk Consulting

   Email: jimsch@exmsft.com

   Russell Housley
   RSA Laboratories
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   Email: rhousley@rsasecurity.com


























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