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Network Working Group Request for Comments: 3537 Category: Standards Track |
J. Schaad Soaring Hawk Consulting R. Housley Vigil Security May 2003 |
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 © The Internet Society (2003). All Rights Reserved.
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).
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.
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].
[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.
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'.
This algorithm encrypts an HMAC key with a 3DES KEK. The algorithm is:
2. Let LKEY = LENGTH || KEY.
5. Let LKEYPADICV = LKEYPAD || ICV.
8. Let TEMP2 = IV || TEMP1.
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).
This algorithm decrypts an HMAC key using a 3DES KEK. The algorithm is:
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.
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
TEMP3 : c12c0b50 00bd4656
: b145a4df 9c961266
: 4b865c47 96b018a6
: 6b8332f4 10827a15
: 756bd5e0 798c0d05
Wrapped Key : 0f1d715d 75a0aaf6
: 6f02e371 c08b79e2
: a1253dc4 3040136b
: dc161118 601f2863
: e2929b3b dd17697c
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'.
This algorithm encrypts an HMAC key with an AES KEK. The algorithm is:
2. Let LKEY = LENGTH || KEY.
The AES key unwrap algorithm decrypts an HMAC key using an AES KEK. The AES key unwrap algorithm is:
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.
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
: d47b38cc e84dd804
: fbcec5e3 75c3cb13
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.
[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.
[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.
[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.
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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