





Network Working Group                                          J. Schaad
Request for Comments: 3537                       Soaring Hawk Consulting
Category: Standards Track                                     R. Housley
                                                          Vigil Security
                                                                May 2003


       Wrapping a Hashed Message Authentication Code (HMAC) key
           with a Triple-Data Encryption Standard (DES) Key
             or an Advanced Encryption Standard (AES) Key

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 defines two methods for wrapping an HMAC (Hashed
   Message Authentication Code) key.  The first method defined uses a
   Triple DES (Data Encryption Standard) key to encrypt the HMAC key.
   The second method defined uses an AES (Advanced Encryption Standard)
   key to encrypt the HMAC key.  One place that such an algorithm is
   used is for the Authenticated Data type in CMS (Cryptographic Message
   Syntax).

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 BCP 14, 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 of variable length and the HMAC key has no
   particular parity.

   In the algorithm description, "a || b" is used to represent 'a'
   concatenated with 'b'.

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.



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   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.

   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 (step 6).

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 composed of 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) 8 octets.  LKEYPAD is composed of 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.




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   8.  Decompose the LKEYPAD into LENGTH, KEY, and PAD.  LENGTH is the
       most significant (first) octet.  KEY is the following LENGTH of
       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

   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




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   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.

   In the algorithm description, "a || b" is used to represent 'a'
   concatenated with 'b'.

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 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.









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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 of
       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



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   Wrapped Key  :  9fa0c146 5291ea6d
                :  b55360c6 cb95123c
                :  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 have not been reviewed for use
   with other encryption algorithms.

6. References

6.1  Normative 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. and R. Housley, "Advanced Encryption Standard
               (AES) Key Wrap Algorithm", RFC 3394, September 2002.



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

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

6.2 Informative References

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

   [RANDOM]    Eastlake 3rd, D., Crocker, S. and J. Schiller,
               "Randomness Recommendations for Security", RFC 1750,
               December 1994.

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

   [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. Authors' Addresses

   Jim Schaad
   Soaring Hawk Consulting

   EMail: jimsch@exmsft.com


   Russell Housley
   Vigil Security
   918 Spring Knoll Drive
   Herndon, VA 20170
   USA

   EMail: housley@vigilsec.com










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8. Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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