S/MIME Working Group R. Housley
Internet Draft RSA Laboratories
expires in six months September 2001
Triple-DES and RC2 Key Wrapping
<draft-ietf-smime-key-wrap-00.txt>
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Abstract
This document specifies the algorithm for wrapping one Triple-DES
[3DES] key with another Triple-DES key and the algorithm for wrapping
one RC2 [RC2] key with another RC2 key. These key wrap algorithms
were originally published in section 12.6 of RFC 2630 [CMS]. They
are republished since these key wrap algorithms have been found to be
useful in contexts beyond those supported by RFC 2630.
This draft is being discussed on the "ietf-smime" mailing list. To
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1 Introduction
Management of symmetric cryptographic keys often leads to situations
where one symmetric key is used to encrypt (or wrap) another. Key
wrap algorithms are commonly used in two situations. First, key
agreement algorithms (such as Diffie-Hellman [DH-X9.42]) generate a
pairwise key-encryption key, and a key wrap algorithm is used to
encrypt the content-encryption key or a multicast key with the
pairwise key-encryption key. Second, a key wrap algorithm is used to
encrypt the content-encryption key, multicast key, or session key in
a locally generated storage key-encryption key or a key-encryption
key that was distributed out-of-band.
This document specifies the algorithm for wrapping one Triple-DES key
with another Triple-DES key [3DES] and specifies the algorithm for
wrapping one RC2 key with another RC2 key [RC2]. Encryption of a
Triple-DES key with another Triple-DES key uses the algorithm
specified in section 3. Encryption of a RC2 key with another RC2 key
uses the algorithm specified in section 4. Both of these algorithms
rely on the key checksum algorithm specified in section 2. Triple-
DES and RC2 content-encryption keys are encrypted in Cipher Block
Chaining (CBC) mode [MODES].
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, and MAY are to be interpreted as described
by Scott Bradner in [STDWORDS].
The same key wrap algorithm is used for both Two-key Triple-DES and
Three-key Triple-DES keys. When a Two-key Triple-DES key is to be
wrapped, a third DES key with the same value as the first DES key is
created. Thus, all wrapped Triple-DES keys include three DES keys.
However, a Two-key Triple-DES key MUST NOT be used to wrap a Three-
key Triple-DES key that is comprised of three unique DES keys.
RC2 supports variable length keys. RC2 128-bit keys MUST be used as
key-encryption keys; however, the wrapped RC2 key MAY be of any size.
2 Key Checksum
The key checksum algorithm is used to provide a key integrity check
value. The algorithm is:
1. Compute a 20 octet SHA-1 [SHA1] message digest on the key
that is to be wrapped.
2. Use the most significant (first) eight octets of the message
digest value as the checksum value.
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3 Triple-DES Key Wrapping and Unwrapping
This section specifies the algorithms for wrapping and unwrapping one
Triple-DES key with another Triple-DES key [3DES].
3.1 Triple-DES Key Wrap
The Triple-DES key wrap algorithm encrypts a Triple-DES key with a
Triple-DES key-encryption key. The Triple-DES key wrap algorithm is:
1. Set odd parity for each of the DES key octets comprising the
Three-Key Triple-DES key that is to be wrapped, call the result
CEK.
2. Compute an 8 octet key checksum value on CEK as described above
in Section 2, call the result ICV.
3. Let CEKICV = CEK || ICV.
4. Generate 8 octets at random, call the result IV.
5. Encrypt CEKICV in CBC mode using the key-encryption key. Use
the random value generated in the previous step as the
initialization vector (IV). Call the ciphertext TEMP1.
6. Let TEMP2 = IV || TEMP1.
7. 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.
8. Encrypt TEMP3 in CBC mode using the key-encryption key. Use
an initialization vector (IV) of 0x4adda22c79e82105.
The ciphertext is 40 octets long.
Note: When the same Three-Key Triple-DES key is wrapped in different
key-encryption keys, a fresh initialization vector (IV) must be
generated for each invocation of the key wrap algorithm.
3.2 Triple-DES Key Unwrap
The Triple-DES key unwrap algorithm decrypts a Triple-DES key using a
Triple-DES key-encryption key. The Triple-DES key unwrap algorithm
is:
1. If the wrapped key is not 40 octets, then error.
2. Decrypt the wrapped key in CBC mode using the key-encryption
key. 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 TEMP2 into IV and TEMP1. IV is the most significant
(first) 8 octets, and TEMP1 is the least significant (last)
32 octets.
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5. Decrypt TEMP1 in CBC mode using the key-encryption key. Use
the IV value from the previous step as the initialization vector.
Call the ciphertext CEKICV.
6. Decompose CEKICV into CEK and ICV. CEK is the most significant
(first) 24 octets, and ICV is the least significant (last)
8 octets.
7. Compute an 8 octet key checksum value on CEK as described above
in Section 2. If the computed key checksum value does not
match the decrypted key checksum value, ICV, then error.
8. Check for odd parity each of the DES key octets comprising CEK.
If parity is incorrect, then error.
9. Use CEK as a Triple-DES key.
3.3 Triple-DES Key Wrap 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 Triple-DES key wrap
algorithm has been assigned the following algorithm identifier:
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
The AlgorithmIdentifier parameter field MUST be NULL.
4 RC2 Key Wrapping and Unwrapping
This section specifies the algorithms for wrapping and unwrapping one
RC2 key with another RC2 key [RC2].
4.1 RC2 Key Wrap
The RC2 key wrap algorithm encrypts a RC2 key with a RC2 key-
encryption key. The RC2 key wrap algorithm is:
1. Let the RC2 key be called CEK, and let the length of CEK in
octets be called LENGTH. LENGTH is a single octet.
2. Let LCEK = LENGTH || CEK.
3. Let LCEKPAD = LCEK || PAD. If the length of LCEK is a multiple
of 8, the PAD has a length of zero. If the length of LCEK is
not a multiple of 8, then PAD contains the fewest number of
random octets to make the length of LCEKPAD a multiple of 8.
4. Compute an 8 octet key checksum value on LCEKPAD as described
above in Section 2, call the result ICV.
5. Let LCEKPADICV = LCEKPAD || ICV.
6. Generate 8 octets at random, call the result IV.
7. Encrypt LCEKPADICV in CBC mode using the key-encryption key.
Use the random value generated in the previous step as the
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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 key-encryption key. Use
an initialization vector (IV) of 0x4adda22c79e82105.
Note: When the same RC2 key is wrapped in different key-encryption
keys, a fresh initialization vector (IV) must be generated for each
invocation of the key wrap algorithm.
4.2 RC2 Key Unwrap
The RC2 key unwrap algorithm decrypts a RC2 key using a RC2 key-
encryption key. The RC2 key unwrap 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 key-encryption key.
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 key-encryption key. Use
the IV value from the previous step as the initialization vector.
Call the plaintext LCEKPADICV.
6. Decompose the LCEKPADICV into LCEKPAD, and ICV. ICV is the
least significant (last) octet 8 octets. LCEKPAD is the
remaining octets.
7. Compute an 8 octet key checksum value on LCEKPAD as described
above in Section 2. If the computed key checksum value does
not match the decrypted key checksum value, ICV, then error.
8. Decompose the LCEKPAD into LENGTH, CEK, and PAD. LENGTH is the
most significant (first) octet. CEK 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 CEK as an RC2 key.
4.3 RC2 Key Wrap 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 RC2 key wrap algorithm
has been assigned the following algorithm identifier:
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id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
The AlgorithmIdentifier parameter field MUST be RC2wrapParameter:
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the RC2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0),
because the one octet (A0) encoding represents a negative number.
References
3DES American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation. 1998.
CMS Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
DES American National Standards Institute. ANSI X3.106,
"American National Standard for Information Systems - Data
Link Encryption". 1983.
DH-X9.42 Rescorla, E. Diffie-Hellman Key Agreement Method.
RFC 2631. June 1999.
DSS National Institute of Standards and Technology.
FIPS Pub 186: Digital Signature Standard. 19 May 1994.
MODES National Institute of Standards and Technology.
FIPS Pub 81: DES Modes of Operation. 2 December 1980.
RANDOM Eastlake, D., S. Crocker, and J. Schiller. Randomness
Recommendations for Security. RFC 1750. December 1994.
RC2 Rivest, R. A Description of the RC2 (r) Encryption Algorithm.
RFC 2268. March 1998.
SHA1 National Institute of Standards and Technology.
FIPS Pub 180-1: Secure Hash Standard. 17 April 1995.
STDWORDS Bradner, S. Key Words for Use in RFCs to Indicate
Requirement Levels. RFC2119. March 1997.
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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.
Security Considerations
Implementations must protect the key-encryption key. Compromise of
the key-encryption key may result in the disclosure of all keys that
have been wrapped with the key-encryption key, which may lead to the
disclosure of all traffic protected with those wrapped key.
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.
If the key-encryption key and wrapped key are associated with
different symmetric encryption algorithms, the effective security
provided to data encrypted with the wrapped key is determined by the
weaker of the two algorithms. If, for example, data is encrypted
with 168-bit Triple-DES and that Triple-DES key is wrapped with a
40-bit RC2 key, then at most 40 bits of protection is provided. A
trivial search to determine the value of the 40-bit RC2 key can
recover Triple-DES key, and then the Triple-DES key can be used to
decrypt the content. Therefore, implementers must ensure that key-
encryption algorithms are as strong or stronger than content-
encryption algorithms.
These key wrap algorithms specified in this document have been
reviewed for use with Triple-DES and RC2, and they have not been
reviewed for use with other encryption algorithms. Similarly, the
key wrap algorithms make use of CBC mode [MODES], and they have not
been reviewed for use with other cryptographic modes.
Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Carl Ellison, Peter Gutmann, Bob
Jueneman, Don Johnson, and Burt Kaliski for their support in defining
these algorithms.
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Author Address
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
rhousley@rsasecurity.com
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