draft-ietf-smime-x942-01.txt   draft-ietf-smime-x942-02.txt 
E. Rescorla E. Rescorla
INTERNET-DRAFT RTFM Inc. INTERNET-DRAFT RTFM Inc.
<draft-ietf-smime-x942-01.txt> October 1998 (Expires April 1999) <draft-ietf-smime-x942-02.txt> November 1998 (Expires May 1999)
Diffie-Hellman Key Agreement Method Diffie-Hellman Key Agreement Method
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
skipping to change at line 33 skipping to change at line 33
munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
ftp.isi.edu (US West Coast). ftp.isi.edu (US West Coast).
Abstract Abstract
This document standardizes one particular Diffie-Hellman variant, This document standardizes one particular Diffie-Hellman variant,
based on the ANSI X9.42 standard, developed by the ANSI X9F1 working based on the ANSI X9.42 standard, developed by the ANSI X9F1 working
group. An algorithm for converting the shared secret into an arbi- group. An algorithm for converting the shared secret into an arbi-
trary amount of keying material is provided. trary amount of keying material is provided.
TODO
Redo the examples to match the new algorithm for computing K.
1. Introduction 1. Introduction
In [DH76] Diffie and Hellman describe a means for two parties to In [DH76] Diffie and Hellman describe a means for two parties to
agree upon a shared secret in such a way that the secret will be una- agree upon a shared secret in such a way that the secret will be una-
vailable to eavesdroppers. This secret may then be converted into vailable to eavesdroppers. This secret may then be converted into
cryptographic keying material for other (symmetric) algorithms. A cryptographic keying material for other (symmetric) algorithms. A
large number of minor variants of this process exist. This document large number of minor variants of this process exist. This document
describes one such variant, based on the ANSI X9.42 specification. describes one such variant, based on the ANSI X9.42 specification.
1.1. Discussion of this Draft 1.1. Discussion of this Draft
This draft is being discussed on the "ietf-smime" mailing list. To This draft is being discussed on the "ietf-smime" mailing list. To
join the list, send a message to <ietf-smime-request@imc.org> with join the list, send a message to <ietf-smime-request@imc.org> with
Rescorla [Page 1] Internet-Draft Diffie-Hellman Key Agreement Method
the single word "subscribe" in the body of the message. Also, there the single word "subscribe" in the body of the message. Also, there
is a Web site for the mailing list at <http://www.imc.org/ietf- is a Web site for the mailing list at <http://www.imc.org/ietf-
smime/>. smime/>.
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1.2. Requirements Terminology 1.2. Requirements Terminology
Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
"MAY" that appear in this document are to be interpreted as described "MAY" that appear in this document are to be interpreted as described
in [RFC2119]. in [RFC2119].
2. Overview Of Method 2. Overview Of Method
Diffie-Hellman key agreement requires that both the sender and reci- Diffie-Hellman key agreement requires that both the sender and reci-
pient of a message have key pairs. By combining one's private key and pient of a message have key pairs. By combining one's private key and
the other party's public key, both parties can compute the same the other party's public key, both parties can compute the same
shared secret number. This number can then be converted into crypto- shared secret number. This number can then be converted into crypto-
graphic keying material. That keying material is typically used as a graphic keying material. That keying material is typically used as a
key encryption key (KEK) to encrypt (wrap) a key (a message encryp- key encryption key (KEK) to encrypt (wrap) a message encrytion key
tion key -- MEK) which is in turn used to encrypt the message data. (MEK) which is in turn used to encrypt the message data.
2.1. Key Agreement 2.1. Key Agreement
The first stage of the key agreement process is to compute a shared The first stage of the key agreement process is to compute a shared
secret number ZZ (which will be constant for any pair of Diffie- secret number ZZ (which will be constant for any pair of Diffie-
Hellman keys). ZZ is then converted into a shared symmetric key. Note Hellman keys). ZZ is then converted into a shared symmetric key. Note
that the symmetric key will be different for each key agreement, due that the symmetric key will be different for each key agreement, due
to the introduction of public random components. to the introduction of public random components.
2.1.1. Generation of ZZ 2.1.1. Generation of ZZ
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where ^ denotes exponentiation where ^ denotes exponentiation
ya is party a's public key; ya = g ^ xa (mod p) ya is party a's public key; ya = g ^ xa (mod p)
yb is party b's public key; yb = g ^ xb (mod p) yb is party b's public key; yb = g ^ xb (mod p)
xa is party a's private key xa is party a's private key
xb is party b's private key xb is party b's private key
p is a large prime p is a large prime
g is a generator for the integer group specified by p. g is a generator for the integer group specified by p.
(See Section 2.2 for criteria for keys and parameters) (See Section 2.2 for criteria for keys and parameters)
In CMS, the recipient's key is identified by the CMS In CMS, the recipient's key is identified by the CMS RecipientIden-
tifier, which points to the recipient's certificate. The sender's
key is identified using the OriginatorIdentifierOrKey field, either
by reference to the sender's certificate or by inline inclusion of a
key.
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RecipientIdentifier, which points to the recipient's certificate.
The sender's key is identified using the OriginatorIdentifierOrKey
field, either by reference to the sender's certificate or by inline
inclusion of a key.
2.1.2. Generation of Keying Material 2.1.2. Generation of Keying Material
X9.42 provides an algorithm for generating an essentially arbitrary X9.42 provides an algorithm for generating an essentially arbitrary
amount of keying material from ZZ. Our algorithm is derived from that amount of keying material from ZZ. Our algorithm is derived from that
algorithm by mandating some optional fields and omitting others. algorithm by mandating some optional fields and omitting others.
KM = H ( ZZ || OtherInfo) KM = H ( ZZ || OtherInfo)
H is the message digest function SHA-1 [FIPS-180] ZZ is the shared H is the message digest function SHA-1 [FIPS-180]
key computed in Section 2.1.1 OtherInfo is the DER encoding of the ZZ is the shared key computed in Section 2.1.1
following structure: OtherInfo is the DER encoding of the following structure:
OtherInfo ::= SEQUENCE { OtherInfo ::= SEQUENCE {
keyInfo KeySpecificInfo, keyInfo KeySpecificInfo,
pubInfo [2] OCTET STRING OPTIONAL, pubInfo [2] OCTET STRING OPTIONAL,
} }
KeySpecificInfo ::= SEQUENCE { KeySpecificInfo ::= SEQUENCE {
algorithm OBJECT IDENTIFIER, algorithm OBJECT IDENTIFIER,
counter OCTET STRING SIZE (4..4) } counter OCTET STRING SIZE (4..4) }
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message. message.
2.1.3. KEK Computation 2.1.3. KEK Computation
Each key encryption algorithm requires a specific size key (n). The Each key encryption algorithm requires a specific size key (n). The
KEK is generated by mapping the left n-most bytes of KM onto the key. KEK is generated by mapping the left n-most bytes of KM onto the key.
Consequently, for a DES [FIPS-46-1] key, which requires 64 bits of Consequently, for a DES [FIPS-46-1] key, which requires 64 bits of
keying material, the algorithm is only run once, with a counter value keying material, the algorithm is only run once, with a counter value
of 1. The first 64 bits of the output are parity adjusted and con- of 1. The first 64 bits of the output are parity adjusted and con-
verted into a DES key. For 3DES, which requires 192 bits of keying verted into a DES key. For 3DES, which requires 192 bits of keying
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material, the algorithm must be run twice, once with a counter value material, the algorithm must be run twice, once with a counter value
of 1 (to generate K1', K2', and the first 32 bits of K3') and once of 1 (to generate K1', K2', and the first 32 bits of K3') and once
with a counter value of 2 (to generate the last 32 bits of K3). with a counter value of 2 (to generate the last 32 bits of K3).
K1',K2' and K3' are then parity adjusted to generate the 3 DES keys K1',K2' and K3' are then parity adjusted to generate the 3 DES keys
K1,K2 and K3. K1,K2 and K3.
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2.1.4. Keylengths for common algorithms 2.1.4. Keylengths for common algorithms
Some common key encryption algorithms have KEKs of the following Some common key encryption algorithms have KEKs of the following
lengths. lengths.
DES-ECB 64 bits DES-ECB 64 bits
3DES-EDE-ECB 192 bits 3DES-EDE-ECB 192 bits
RC2 (all) 128 bits RC2 (all) 128 bits
2.1.5. Public Key Validation 2.1.5. Public Key Validation
The following algorithm MAY be used to validate received public keys. The following algorithm MAY be used to validate received public keys.
1. Verify that y lies within the interval [2,p-1]. If it does not, 1. Verify that y lies within the interval [2,p-1]. If it does not,
the key is invalid. the key is invalid.
2. Compute y^q (mod p). If the result == 1, the key is valid. 2. Compute y^q (mod p). If the result == 1, the key is valid.
Otherwise the key is invalid. Otherwise the key is invalid.
The primary purpose of public key validation is to prevent a small The primary purpose of public key validation is to prevent a small
subgroup attack [REFERENCE?] on the sender's key pair. If Ephemeral- subgroup attack [SUBGROUP] on the sender's key pair. If Ephemeral-
Static mode is used, this check is unnecessary. Note that this pro- Static mode is used, this check is unnecessary. Note that this pro-
cedure may be subject to pending patents. cedure may be subject to pending patents.
2.1.6. Example 2.1.6. Example
ZZ is the 16 bytes 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ZZ is the 16 bytes 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
The key encryption algorithm is 3DES-EDE. The key encryption algorithm is 3DES-EDE.
No pubInfo is used No pubInfo is used
Consequently, the input to the first invocation of SHA-1 is: 00 01 02 Consequently, the input to the first invocation of SHA-1 is:
03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 30 13
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ
30 13
30 11 30 11
06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID
04 04 00 00 00 01 ; Counter 04 04 00 00 00 01 ; Counter
And the output is the 20 bytes: a8 c6 4e 46 1a aa c2 36 45 c9 2e c6 And the output is the 20 bytes:
0e 8a c1 96 8f fb 94 b3
a8 c6 4e 46 1a aa c2 36 45 c9 2e c6 0e 8a c1 96 8f fb 94 b3
The input to the second invocation of SHA-1 is:
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The input to the second invocation of SHA-1 is: 00 01 02 03 04 05 06 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ
07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 30 13 30 13
30 11 30 11
06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID
04 04 00 00 00 01 ; Counter 04 04 00 00 00 01 ; Counter
And the output is the 20 bytes: 49 eb c8 09 27 77 19 c1 a3 0c cc 49 And the output is the 20 bytes:
bd 0c 12 5e e0 f9 1a cc
49 eb c8 09 27 77 19 c1 a3 0c cc 49 bd 0c 12 5e e0 f9 1a cc
Consequently, Consequently,
K1=a8 c6 4e 46 1a aa c2 36 K1=a8 c6 4e 46 1a aa c2 36
K2=45 c9 2e c6 0e 8a c1 96 K2=45 c9 2e c6 0e 8a c1 96
K3=8f fb 94 b3 49 eb c8 09 K3=8f fb 94 b3 49 eb c8 09
Note: These keys are not parity adjusted Note: These keys are not parity adjusted
2.2. Key and Parameter Requirements 2.2. Key and Parameter Requirements
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Agents SHOULD generate domain parameters (g,p,q) using the algorithms Agents SHOULD generate domain parameters (g,p,q) using the algorithms
specified in Appendixes 2 and 3 of [FIPS-186]. specified in Appendixes 2 and 3 of [FIPS-186].
2.2.2. Group Parameter Validation 2.2.2. Group Parameter Validation
The ASN.1 for DH keys in [PKIX] includes elements j and validation- The ASN.1 for DH keys in [PKIX] includes elements j and validation-
Parms which MAY be used by recipients of a key to verify that the Parms which MAY be used by recipients of a key to verify that the
group parameters were correctly generated. Two checks are possible: group parameters were correctly generated. Two checks are possible:
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1. Verify that p=qj + 1. This demonstrates that the parameters meet 1. Verify that p=qj + 1. This demonstrates that the parameters meet
the X9.42 parameter criteria. the X9.42 parameter criteria.
2. Verify that when the p,q generation procedure of [FIPS-186] Appendix 2 2. Verify that when the p,q generation procedure of [FIPS-186] Appendix 2
is followed with seed 'seed', that p is found when 'counter' = pgenCounter. is followed with seed 'seed', that p is found when 'counter' = pgenCounter.
This demonstrates that the parameters were randomly chosen and do not This demonstrates that the parameters were randomly chosen and do not
have a special form. have a special form.
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Whether agents provide validation information in their certificates Whether agents provide validation information in their certificates
is a local matter between the agents and their CA. is a local matter between the agents and their CA.
2.3. Ephemeral-Static Mode 2.3. Ephemeral-Static Mode
In Ephemeral-Static mode, the recipient has a static (and certified) In Ephemeral-Static mode, the recipient has a static (and certified)
key pair, but the sender generates a new key pair for each message key pair, but the sender generates a new key pair for each message
and sends it using the originatorKey production. If the sender's key and sends it using the originatorKey production. If the sender's key
is freshly generated for each message, the shared secret ZZ will be is freshly generated for each message, the shared secret ZZ will be
similarly different for each message and pubInfo MAY be omitted, similarly different for each message and pubInfo MAY be omitted,
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[FIPS-46-1] Federal Information Processing Standards Publication (FIPS PUB) [FIPS-46-1] Federal Information Processing Standards Publication (FIPS PUB)
46-1, Data Encryption Standard, Reaffirmed 1988 January 22 46-1, Data Encryption Standard, Reaffirmed 1988 January 22
(supersedes FIPS PUB 46, 1977 January 15). (supersedes FIPS PUB 46, 1977 January 15).
[FIPS-81] Federal Information Processing Standards Publication (FIPS PUB) [FIPS-81] Federal Information Processing Standards Publication (FIPS PUB)
81, DES Modes of Operation, 1980 December 2. 81, DES Modes of Operation, 1980 December 2.
[FIPS-180] Federal Information Processing Standards Publication (FIPS PUB) [FIPS-180] Federal Information Processing Standards Publication (FIPS PUB)
180-1, "Secure Hash Standard", 1995 April 17. 180-1, "Secure Hash Standard", 1995 April 17.
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[FIPS-186] Federal Information Processing Standards Publication (FIPS PUB) [FIPS-186] Federal Information Processing Standards Publication (FIPS PUB)
186, "Digital Signature Standard", 1994 May 19. 186, "Digital Signature Standard", 1994 May 19.
[PKIX] Housley, R., Ford, W., Polk, W., Solo, D., "Internet X.509 Public [PKIX] Housley, R., Ford, W., Polk, W., Solo, D., "Internet X.509 Public
Key Infrastructure Certificate and CRL Profile", RFC-XXXX. Key Infrastructure Certificate and CRL Profile", RFC-XXXX.
[SUBGROUP] I still need a reference for the Small Subgroup attack.
[X942] "Agreement Of Symmetric Keys Using Diffie-Hellman and MQV Algorithms", [X942] "Agreement Of Symmetric Keys Using Diffie-Hellman and MQV Algorithms",
ANSI draft, 1998. ANSI draft, 1998.
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Security Considerations Security Considerations
All the security in this system is provided by the secrecy of the All the security in this system is provided by the secrecy of the
private keying material. If either sender or recipient private keys private keying material. If either sender or recipient private keys
are disclosed, all messages sent or received using that key are are disclosed, all messages sent or received using that key are
compromised. Similarly, loss of the private key results in an inabil- compromised. Similarly, loss of the private key results in an inabil-
ity to read messages sent using that key. ity to read messages sent using that key.
Author's Address Author's Address
 End of changes. 

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