S/MIME Working Group J. Schaad
Internet Draft Soaring Hawk Consulting
draft-ietf-smime-hmac-key-wrap-02.txt R. Housley
Category: Standards Vigil Security
February 2003
Wrapping an HMAC key with a Triple-DES Key or an AES Key
Status of this Memo
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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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HMAC Key Wrap February 2002
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.
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.
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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HMAC Key Wrap February 2002
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
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HMAC Key Wrap February 2002
: 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
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
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HMAC Key Wrap February 2002
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.
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
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: 155068f7 38050d8c
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 they have not been
reviewed for use with other encryption algorithms.
6. References
This section provides normative and informative 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., R. Housley, Advanced Encryption Standard (AES)
Key Wrap Algorithm, RFC 3394, September 2002.
HMAC Krawczyk, H., M. Bellare, and R. Canetti. HMAC: Keyed-
Schaad & Housley Standards - July 2002 6
HMAC Key Wrap February 2002
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, D., S. Crocker, 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. Author's 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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