draft-ietf-smime-x942-03.txt   draft-ietf-smime-x942-04.txt 
E. Rescorla E. Rescorla
INTERNET-DRAFT RTFM Inc. INTERNET-DRAFT RTFM Inc.
<draft-ietf-smime-x942-03.txt> November 1998 (Expires May 1999) <draft-ietf-smime-x942-04.txt> December 1998 (Expires June 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,
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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. Diffie-Hellam is a key agreement algorithm used by two parties group. Diffie-Hellman is a key agreement algorithm used by two par-
to agree on a shared secret. An algorithm for converting the shared ties to agree on a shared secret. An algorithm for converting the
secret into an arbitrary amount of keying material is provided. The shared secret into an arbitrary amount of keying material is pro-
resulting keying material is used as a symmetric encryption key. The vided. The resulting keying material is used as a symmetric encryp-
D-H variant described requires the recipient to have a certificate, tion key. The D-H variant described requires the recipient to have a
but the originator may have a static key pair (with the public key certificate, but the originator may have a static key pair (with the
placed in a certificate) or an ephemeral key pair. public key placed in a certificate) or an ephemeral key pair.
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.
skipping to change at line 70 skipping to change at line 69
"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 content encryption key key-encryption key (KEK) to encrypt (wrap) a content-encryption key
(CEK) which is in turn used to encrypt the message data. (CEK) 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, called ZZ. When the same originator and recipient secret number, called ZZ. When the same originator and recipient
public/private key pairs are used, the same ZZ value will result. public/private key pairs are used, the same ZZ value will result.
The ZZ value is then converted into a shared symmetric cryptographic The ZZ value is then converted into a shared symmetric cryptographic
key. When the originator employs a static private/public key pair, key. When the originator employs a static private/public key pair,
the introduction of public random values are used to ensure that the the introduction of public random values are used to ensure that the
resulting symmetric key will be different for each key agreement. resulting symmetric key will be different for each key agreement.
2.1.1. Generation of ZZ 2.1.1. Generation of ZZ
X9.42 defines that the shared secret ZZ is generated as follows: X9.42 defines that the shared secret ZZ is generated as follows:
ZZ = g ^ (xb * xa) (mod p) ZZ = g ^ (xb * xa) mod p
Note that the individual parties actually perform the computations: Note that the individual parties actually perform the computations:
ZZ = yb ^ xa (mod p) = ya ^ xb (mod p) ZZ = yb ^ xa (mod p) = ya ^ xb mod p
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
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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 = h(p-1)/q mod p, where
j a large integer such that h is any integer with 1 < h < p-1 such that h(p-1)/q mod p > 1
q is a large prime and p=qj + 1 (g has order q mod p)
q is a large prime
j a large integer such that p=qj + 1
(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 RecipientIden- In [CMS], the recipient's key is identified by the CMS RecipientIden-
tifier, which points to the recipient's certificate. The sender's tifier, which points to the recipient's certificate. The sender's
key is identified using the OriginatorIdentifierOrKey field, either key is identified using the OriginatorIdentifierOrKey field, either
by reference to the sender's certificate or by inline inclusion of a by reference to the sender's certificate or by inline inclusion of a
key. 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.
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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) }
algorithm is the ASN.1 algorithm OID of the symmetric algorithm with which algorithm is the ASN.1 algorithm OID of the symmetric algorithm with which
this KEK will be used. this KEK will be used. Note that this is NOT an AlgorithmIdentifier,
counter is a 32 bit number, represented in network byte order. but simply the OBJECT IDENTIFIER. No paramters are used.
Its initial value is 1, i.e. the byte sequence 00 00 00 01 (hex) counter is a 32 bit number, represented in network byte order. Its
pubInfo is a random string provided by the sender. In CMS, it is provided initial value is 1 for any ZZ, i.e. the byte sequence 00 00 00 01
as a parameter in the UserKeyingMaterial field (a 512 bit value represented (hex), and it is incremented by one every time the above key generation
as an OCTET STRING). function is run for a given KEK.
pubInfo is a random string provided by the sender. In CMS, it is provided as
a parameter in the UserKeyingMaterial field (encoded as an OCTET STRING). If
provided this pubInfo MUST contain 512 bits.
Note that the only source of secret entropy in this computation is Note that the only source of secret entropy in this computation is
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ZZ, so the security of this data is limited to the size of ZZ, even ZZ, so the security of this data is limited to the size of ZZ, even
if more data than ZZ is generated. However, if pubInfo is different if more data than ZZ is generated. However, if pubInfo is different
for each message, a different KEK will be generated for each message. for each message, a different KEK will be generated for each message.
Note that pubInfo is required in Static-Static mode, but MAY appear Note that pubInfo MUST be used in Static-Static mode, but MAY appear
in Ephemeral-Static mode. in Ephemeral-Static mode.
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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.
For 3DES, which requires 192 bits of keying material, the algorithm For 3DES, which requires 192 bits of keying material, the algorithm
must be run twice, once with a counter value of 1 (to generate K1', must be run twice, once with a counter value of 1 (to generate K1',
K2', and the first 32 bits of K3') and once with a counter value of 2 K2', and the first 32 bits of K3') and once with a counter value of 2
(to generate the last 32 bits of K3). K1',K2' and K3' are then parity (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. For RC2, which adjusted to generate the 3 DES keys K1,K2 and K3. For RC2, which
requires 128 bits of keying material, the algorithm is run once, with requires 128 bits of keying material, the algorithm is run once, with
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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 [LAW98] on the sender's key pair. If Ephemeral-Static subgroup attack [LAW98] on the sender's key pair. If Ephemeral-Static
mode is used, this check may not be necessary. mode is used, this check may not be necessary.
Note that this procedure may be subject to pending patents. Note that this procedure may be subject to pending patents.
2.1.6. Example 1 2.1.6. Example 1
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
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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: Consequently, the input to the first invocation of SHA-1 is:
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00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ
30 13 30 12
30 11 30 10
06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 06 08 2a 86 48 86 f7 0d 03 07 ; 3DES-EDE OID
04 04 00 00 00 01 ; Counter 04 04 00 00 00 01 ; Counter
And the output is the 20 bytes: And the output is the 20 bytes:
a8 c6 4e 46 1a aa c2 36 45 c9 2e c6 0e 8a c1 96 8f fb 94 b3 20 be 23 e3 3b 72 ef 16 8e e3 ae 18 5a 00 93 b0 d6 49 56 22
The input to the second invocation of SHA-1 is: The input to the second invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ
30 13 30 12
30 11 30 10
06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 06 08 2a 86 48 86 f7 0d 03 07 ; 3DES-EDE OID
04 04 00 00 00 01 ; Counter 04 04 00 00 00 02 ; Counter
And the output is the 20 bytes: And the output is the 20 bytes:
49 eb c8 09 27 77 19 c1 a3 0c cc 49 bd 0c 12 5e e0 f9 1a cc 3b 4e fd d4 6e ff 6b 6d 35 a9 cd e3 e3 e7 05 39 e0 31 53 de
Consequently, Consequently,
K1'=a8 c6 4e 46 1a aa c2 36 K1'=20 be 23 e3 3b 72 ef 16
K2'=45 c9 2e c6 0e 8a c1 96 K2'=8e e3 ae 18 5a 00 93 b0
K3'=8f fb 94 b3 49 eb c8 09 K3'=d6 49 56 22 3b 4e fd d4
Note: These keys are not parity adjusted Note: These keys are not parity adjusted
2.1.7. Example 2 2.1.7. Example 2
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 RC2 The key encryption algorithm is RC2
No pubInfo is used The pubInfo used is the 64 bytes 01 23 45 67 89 ab cd ef 01 23 45 67
89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45
67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23
45 67 89 ab cd ef
Consequently, the input to the first invocation of SHA-1 is: Consequently, the input to SHA-1 is:
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00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ
30 56 30 56
30 10 30 10
06 08 2a 86 48 86 f7 0d 03 02 ; RC2 OID 06 08 2a 86 48 86 f7 0d 03 02 ; RC2 OID
04 04 00 00 00 02 ; Counter 04 04 00 00 00 01 ; Counter
a2 42 04 40 a2 42
04 40
01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef ; PubInfo 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef ; PubInfo
01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef
01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef
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01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef 01 23 45 67 89 ab cd ef
And the output is the 20 bytes: And the output is the 20 bytes:
f3 91 81 0b 33 34 3e c5 48 e5 a5 49 51 83 c0 0a 99 7e b4 1e c7 4e c5 80 68 7d 70 aa 06 38 d3 e3 c7 2a da 5a 67 4e cf 06
Consequently, Consequently,
K=f3 91 81 0b 33 34 3e c5 48 e5 a5 49 51 83 c0 0a K=c7 4e c5 80 68 7d 70 aa 06 38 d3 e3 c7 2a da 5a
2.2. Key and Parameter Requirements 2.2. Key and Parameter Requirements
X9.42 requires that the group parameters be of the form p=jq + 1 X9.42 requires that the group parameters be of the form p=jq + 1
where q is a large prime of length m and j>=2. An algorithm for gen- where q is a large prime of length m and j>=2. An algorithm for gen-
erating primes of this form can be found in FIPS PUB 186-1[DSS] and erating primes of this form (derived from the algorithms in FIPS PUB
ANSI, X9.30-1 1996 [X930], as well as in [X942]. 186-1[DSS], and [X942]can be found in appendix A.
X9.42 requires that the private key x be in the interval [2, (q - X9.42 requires that the private key x be in the interval [2, (q -
2)]. x should be randomly generated in this interval. y is then com- 2)]. x should be randomly generated in this interval. y is then com-
puted by calculating g^x (mod p). To comply with this draft, m MUST puted by calculating g^x mod p. To comply with this draft, m MUST be
be >=128, (consequently, q and x MUST each be at least 128 bits >=160, (consequently, q and x MUST each be at least 128 bits long).
long). When symmetric ciphers stronger than DES are to be used, a When symmetric ciphers stronger than DES are to be used, a larger m
larger m may be advisable. may be advisable.
2.2.1. Group Parameter Generation 2.2.1. Group Parameter Generation
Agents SHOULD generate domain parameters (g,p,q) using the algorithms Agents SHOULD generate domain parameters (g,p,q) using the following
specified in Appendixes 2 and 3 of [FIPS-186]. algorithm, derived from [FIPS-186] and [X942]. When this algorithm is
used, the correctness of the generation procedure can be verified by
a third party by the algorithm of 2.2.2.
2.2.1.1. Generation of p, q
This algorithm generates a p, q pair where q is of length m and
p is of length L.
Let L - 1 = n*m + b where both b and n are integers and
0 <= b < 160.
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1. Choose an arbitrary sequence of at least m bits and call it SEED.
Let g be the length of SEED in bits.
2. Set U = 0
3. Set m' = m / 160, where / represents integer division,
consequently, if m = 200, m' = 1.
4. for i = 0 to m' - 1
U = U + SHA[SEED + i] XOR SHA[(SEED + m' + i) mod 2^160] * 2^(160 * i)
Note that for m=160, this reduces to the algorithm of [FIPS186]
U = SHA[SEED] XOR SHA[(SEED+1) mod 2^160 ].
5. Form q from U by setting the most significant bit (the 2^(m-1) bit)
and the least significant bit to 1. In terms of boolean operations,
q = U OR 2^(m-1) OR 1. Note that 2^(m-1) < q < 2^m
6. Use a robust primality algorithm to test whether q is prime.
7. If q is not prime then go to 1.
8. Let counter = 0 and offset = 2
9. For k = 0 to n let
Vk = SHA[(SEED + offset + k) mod 2^160 ].
10. Let W be the integer
W = V0 + V1*2^160 + ... + Vn-1*2(n-1)*160 + (Vn mod 2^b)
* 2n*160
and let
X = W + 2^(L-1)
Note that 0 <= W < 2^(L-1) and hence 2^(L-1)
11. Let c = X mod (2 * q) and set p = X - (c-1). Note that p is congruent
to 1 mod (2 * q).
12. If p < 2^(L -1) then go to step 15.
13. Perform a robust primality test on p.
14. If p passes the test performed in step 13 go to step 17.
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15. Let counter = counter + 1 and offset = offset + n + 1.
16. If counter >= 4096 go to step 1. Otherwise go to step 9.
17. Save the value of SEED and the value of counter for use
in certifying the proper generation of p and q.
Note: A robust primality test is one where the probability of
a non-prime number passing the test is at most 2^-80.
2.2.1.2. Generation of g
This section gives an algorithm (derived from [FIPS186]) for generat-
ing g.
1. Let j = (p - 1)/q.
2. Set h = any integer, where 1 < h < p - 1 and h differs
from any value previously tried.
3. Set g = h^j mod p
4. If g = 1 go to step 2
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:
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] 2. Verify that when the p,q generation procedure of [FIPS-186]
Appendix 2 is followed with seed 'seed', that p is found when Appendix 2 is followed with seed 'seed', that p is found when
This demonstrates that the parameters were randomly chosen and This demonstrates that the parameters were randomly chosen and
do not have a special form. do not have a special form.
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.
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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,
since it serves merely to decouple multiple KEKs generated by the since it serves merely to decouple multiple KEKs generated by the
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same set of pairwise keys. If, however, the same ephemeral sender key same set of pairwise keys. If, however, the same ephemeral sender key
is used for multiple messages (e.g. it is cached as a performance is used for multiple messages (e.g. it is cached as a performance
optimization) then a separate pubInfo MUST be used for each message. optimization) then a separate pubInfo MUST be used for each message.
All implementations of this standard MUST implement Ephemeral-Static All implementations of this standard MUST implement Ephemeral-Static
mode. mode.
In order to resist small subgroup attacks, the recipient SHOULD per-
form the check described in 2.1.5. If the sender cannot determine
success or failure of decryption, the recipient MAY choose to omit
this check.
2.4. Static-Static Mode
In Static-Static mode, both the sender and the recipient have a
static (and certified) key pair. Since the sender's and recipient's
keys are therefore the same for each message, ZZ will be the same for
each message. Thus, pubInfo MUST be used (and different for each mes-
sage) in order to ensure that different messages use different KEKs.
Implementations MAY implement Static-Static mode.
In order to prevent small subgroup attacks, both originator and reci-
pient SHOULD either perform the validation step described in S 2.1.5
or verify that the CA has properly verified the validity of the key.
Acknowledgements Acknowledgements
The Key Agreement method described in this document is based on work The Key Agreement method described in this document is based on work
done by the ANSI X9F1 working group. The author wishes to extend his done by the ANSI X9F1 working group. The author wishes to extend his
thanks for their assistance. thanks for their assistance.
The author also wishes to thank Paul Hoffman, Stephen Henson, Russ The author also wishes to thank Paul Hoffman, Stephen Henson, Russ
Housley, Brian Korver, Mark Schertler, and Peter Yee for their expert Housley, Brian Korver, Jim Schaad, Mark Schertler, and Peter Yee for
advice and review. their expert advice and review.
References References
[CMS] Housley, R., "Cryptographic Message Syntax", draft-ietf-smime-cms-07.txt. [CMS] Housley, R., "Cryptographic Message Syntax", draft-ietf-smime-cms-07.txt.
[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.
[FIPS-186] Federal Information Processing Standards Publication (FIPS PUB) [FIPS-186] Federal Information Processing Standards Publication (FIPS PUB)
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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.
[LAW98] L. Law, A. Menezes, M. Qu, J. Solinas and S. Vanstone, [LAW98] L. Law, A. Menezes, M. Qu, J. Solinas and S. Vanstone,
"An efficient protocol for authenticated key agreement", "An efficient protocol for authenticated key agreement",
Technical report CORR 98-05, University of Waterloo, 1998. Technical report CORR 98-05, University of Waterloo, 1998.
[X942] "Agreement Of Symmetric Keys Using Diffie-Hellman and MQV Algorithms", [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels." RFC 2119. March 1997.
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[X942] "Agreement Of Symmetric Keys Using Diffie-Hellman and MQV Algorithms",
ANSI draft, 1998. ANSI draft, 1998.
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.
Static Diffie-Hellman keys are vulnerable to a small subgroup attack Static Diffie-Hellman keys are vulnerable to a small subgroup attack
[LAW98]. In practice, this issue arises for both sides in Static- [LAW98]. In practice, this issue arises for both sides in Static-
Static mode and for the receiver during Ephemeral-Static mode. In Static mode and for the receiver during Ephemeral-Static mode. S 2.3
Static-Static mode, both originator and recipient SHOULD either per- and 2.4 describe appropriate practices to protect against this
form the validation step described in S 2.1.5 or verify that the CA attack.
has properly verified the validity of the key. In Ephemeral-Static
mode, the recipient SHOULD perform the check described in 2.1.5. If In order to securely generate a symmetric key of length l, m (the
the sender cannot determine success or failure of decryption, the length in bits of q, and hence x) should be at least 2l. m MUST
recipient MAY choose to omit this check. always be >= 160. Consequently, for RC2, m should be >=256 and for
3DES, >=336 (the effective length of 3DES keys is 168 bits).
Author's Address Author's Address
Eric Rescorla <ekr@rtfm.com> Eric Rescorla <ekr@rtfm.com>
RTFM Inc. RTFM Inc.
30 Newell Road, #16 30 Newell Road, #16
East Palo Alto, CA 94303 East Palo Alto, CA 94303
Rescorla [Page 8] Internet-Draft Diffie-Hellman Key Agreement Method Rescorla [Page 10] Internet-Draft Diffie-Hellman Key Agreement Method
Table of Contents Table of Contents
1. Introduction ................................................... 1 1. Introduction ................................................... 1
1.1. Discussion of this Draft ..................................... 2 1.1. Discussion of this Draft ..................................... 2
1.2. Requirements Terminology ..................................... 2 1.2. Requirements Terminology ..................................... 2
2. Overview Of Method ............................................. 2 2. Overview Of Method ............................................. 2
skipping to change at line 406 skipping to change at line 522
2.1.5. Public Key Validation ...................................... 4 2.1.5. Public Key Validation ...................................... 4
2.1.6. Example 1 .................................................. 4 2.1.6. Example 1 .................................................. 4
2.1.7. Example 2 .................................................. 5 2.1.7. Example 2 .................................................. 5
2.2. Key and Parameter Requirements ............................... 6 2.2. Key and Parameter Requirements ............................... 6
2.2.1. Group Parameter Generation ................................. 6 2.2.1. Group Parameter Generation ................................. 6
2.2.2. Group Parameter Validation ................................. 6 2.2.1.1. Generation of p, q ....................................... 6
2.3. Ephemeral-Static Mode ........................................ 7 2.2.1.2. Generation of g .......................................... 8
2.3. Acknowledgements ............................................. 7 2.2.2. Group Parameter Validation ................................. 8
2.3. References ................................................... 7 2.3. Ephemeral-Static Mode ........................................ 8
Security Considerations ........................................... 8 2.4. Static-Static Mode ........................................... 9
Author's Address .................................................. 8 2.4. Acknowledgements ............................................. 9
Rescorla [Page 11] Internet-Draft Diffie-Hellman Key Agreement Method
2.4. References ................................................... 9
Security Considerations ........................................... 10
Author's Address .................................................. 10
 End of changes. 

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