draft-ietf-smime-x942-05.txt   draft-ietf-smime-x942-06.txt 
E. Rescorla E. Rescorla
INTERNET-DRAFT RTFM Inc. INTERNET-DRAFT RTFM Inc.
<draft-ietf-smime-x942-05.txt> January 1999 (Expires July 1999) <draft-ietf-smime-x942-06.txt> March 1999 (Expires September 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 and is in full conformance with
all provisions of Section 10 of RFC2026. 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.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.'' material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the To learn the current status of any Internet-Draft, please check the
skipping to change at line 104 skipping to change at line 105
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
q is a large prime q is a large prime
g = h^{(p-1)/q} mod p, where g = h^{(p-1)/q} mod p, where
h is any integer with 1 < h < p-1 such that h(p-1)/q mod p > 1 h is any integer with 1 < h < p-1 such that h{(p-1)/q} mod p > 1
(g has order q mod p; i.e. g^q mod p = 1 if g!=1) (g has order q mod p; i.e. g^q mod p = 1 if g!=1)
j a large integer such that p=qj + 1 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.
skipping to change at line 139 skipping to change at line 140
OtherInfo ::= SEQUENCE { OtherInfo ::= SEQUENCE {
keyInfo KeySpecificInfo, keyInfo KeySpecificInfo,
partyAInfo [0] OCTET STRING OPTIONAL, partyAInfo [0] OCTET STRING OPTIONAL,
suppPubInfo [2] OCTET STRING suppPubInfo [2] OCTET STRING
} }
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 MEK wrapping algorithm with which
this KEK will be used. Note that this is NOT an AlgorithmIdentifier, this KEK will be used. Note that this is NOT an AlgorithmIdentifier,
but simply the OBJECT IDENTIFIER. No parameters are used. but simply the OBJECT IDENTIFIER. No parameters are used.
counter is a 32 bit number, represented in network byte order. Its counter is a 32 bit number, represented in network byte order. Its
initial value is 1 for any ZZ, i.e. the byte sequence 00 00 00 01 initial value is 1 for any ZZ, i.e. the byte sequence 00 00 00 01
(hex), and it is incremented by one every time the above key generation (hex), and it is incremented by one every time the above key generation
function is run for a given KEK. function is run for a given KEK.
partyAInfo is a random string provided by the sender. In CMS, it is provided as partyAInfo 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 a parameter in the UserKeyingMaterial field (encoded as an OCTET STRING). If
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Note that these ASN.1 definitions use EXPLICIT tagging. (In ASN.1, Note that these ASN.1 definitions use EXPLICIT tagging. (In ASN.1,
EXPLICIT tagging is implicit unless IMPLICIT is explicitly speci- EXPLICIT tagging is implicit unless IMPLICIT is explicitly speci-
fied.) fied.)
To generate a KEK, one generates one or more KM blocks (incrementing To generate a KEK, one generates one or more KM blocks (incrementing
counter appropriately) until enough material has been generated. The counter appropriately) until enough material has been generated. The
KM blocks are concatenated left to right in the obvious order. I.e. KM blocks are concatenated left to right in the obvious order. I.e.
KM(counter=1) || KM(counter=2)... KM(counter=1) || KM(counter=2)...
Note that the only source of secret entropy in this computation is Note that the only source of secret entropy in this computation is
ZZ, so the security of this data is limited to the size of p and q, ZZ. Even if a string longer than ZZ is generated, the effective key
even if a string longer than ZZ is generated. However, if partyAInfo space of the KEK is limited by the size of ZZ, in addition to any
is different for each message, a different KEK will be generated for security level considerations imposed by the parameters p and
each message. Note that partyAInfo MUST be used in Static-Static q.However, if partyAInfo is different for each message, a different
mode, but MAY appear in Ephemeral-Static mode. KEK will be generated for each message. Note that partyAInfo MUST be
used in Static-Static mode, but MAY appear in Ephemeral-Static mode.
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 RC2-128, 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
a counter value of 1, and the left-most 128 bits are directly con- a counter value of 1, and the left-most 128 bits are directly con-
verted to an RC2 key. verted to an RC2 key. Similarly, for RC2-40, which requires 40 bits
of keying material, the algorithm is run once, with a counter value
of 1, and the leftmost 40 bits are used as the key.
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.
3DES-EDE-ECB 192 bits 3DES-EDE-ECB 192 bits
RC2 (all) 128 bits RC2-128 128 bits
RC2-40 40 bits
RC2 effective key lengths are equal to RC2 real key lengths.
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2.1.5. Public Key Validation 2.1.5. Public Key Validation
The following algorithm MAY be used to validate a received public key The following algorithm MAY be used to validate a received public key
y. y.
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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. See also [P1363] for mode is used, this check may not be necessary. See also [P1363] for
more information on Public Key validation. more information on Public Key validation.
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 20 bytes 00 01 02 03 04 05 06 07 08 09 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13 0a 0b 0c 0d 0e 0f 10 11 12 13
The key encryption algorithm is 3DES-EDE. The key wrap algorithm is 3DES-EDE wrap.
No partyAInfo is used No partyAInfo is used
Consequently, the input to the first invocation of SHA-1 is: Consequently, the input to the first invocation of SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1a 30 1d
30 10 30 13
06 08 2a 86 48 86 f7 0d 03 07 ; 3DES OID 06 0b 2a 86 48 86 f7 0d 01 09 10 03 06 ; 3DES wrap OID
04 04 04 04
00 00 00 01 ; Counter 00 00 00 01 ; Counter
a2 06 a2 06
04 04 04 04
00 00 00 c0 ; key length 00 00 00 c0 ; key length
And the output is the 20 bytes: And the output is the 20 bytes:
b4 85 32 07 a9 da b2 9a 23 5a a8 a5 3f ed cd 65 92 26 0a 4a a0 96 61 39 23 76 f7 04 4d 90 52 a3 97 88 32 46 b6 7f 5f 1e
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 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1a 30 1d
30 10
06 08 2a 86 48 86 f7 0d 03 07 ; 3DES OID
04 04
00 00 00 02 ; Counter
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30 13
06 0b 2a 86 48 86 f7 0d 01 09 10 03 06 ; 3DES wrap OID
04 04
00 00 00 02 ; Counter
a2 06 a2 06
04 04 04 04
00 00 00 c0 ; key length 00 00 00 c0 ; key length
And the output is the 20 bytes: And the output is the 20 bytes:
9d 95 43 57 15 e9 c8 31 ce 8a 64 fe e4 c8 d6 0c bf 50 a6 e1 f6 3e b5 fb 5f 56 d9 b6 a8 34 03 91 c2 d3 45 34 93 2e 11 30
Consequently, Consequently,
K1'=b4 85 32 07 a9 da b2 9a K1'=a0 96 61 39 23 76 f7 04
K2'=23 5a a8 a5 3f ed cd 65 K2'=4d 90 52 a3 97 88 32 46
K3'=92 26 0a 4a 9d 95 43 57 K3'=b6 7f 5f 1e f6 3e b5 fb
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 20 bytes 00 01 02 03 04 05 06 07 08 09 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
0a 0b 0c 0d 0e 0f 10 11 12 13 0a 0b 0c 0d 0e 0f 10 11 12 13
The key encryption algorithm is RC2 The key wrap algorithm is RC2-128 key wrap, so we need 128 bits (16
bytes) of keying material.
The partyAInfo used is the 64 bytes The partyAInfo used is the 64 bytes
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
Consequently, the input to SHA-1 is: Consequently, the input to SHA-1 is:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 5e 30 61
30 10 30 13
06 08 2a 86 48 86 f7 0d 03 02 ; RC2 OID 06 0b 2a 86 48 86 f7 0d 01 09 10 03 07 ; RC2 wrap OID
04 04 04 04
00 00 00 01 ; Counter 00 00 00 01 ; Counter
a0 42 a0 42
04 40 04 40
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
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a2 06 a2 06
04 04 04 04
00 00 00 80 ; key length 00 00 00 80 ; key length
And the output is the 20 bytes: And the output is the 20 bytes:
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52 45 e1 6d 27 57 be d6 8e 20 53 6b 38 b7 63 47 63 13 1d fd
Consequently, Consequently,
K=52 45 e1 6d 27 57 be d6 8e 20 53 6b 38 b7 63 47 K=48 95 0c 46 e0 53 00 75 40 3c ce 72 88 96 04 e0
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 (derived from the algorithms in FIPS PUB erating primes of this form (derived from the algorithms in FIPS PUB
186-1[DSS], and [X942]can be found in appendix A. 186-1[FIPS-186] 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 be puted by calculating g^x mod p. To comply with this draft, m MUST be
>=160, (consequently, q MUST each be at least 160 bits long). When >=160 bits in length, (consequently, q MUST be at least 160 bits
symmetric ciphers stronger than DES are to be used, a larger m may be long). When symmetric ciphers stronger than DES are to be used, a
advisable. p must be a minimum of 160 bits long. larger m may be advisable. p must be a minimum of 512 bits long.
2.2.1. Group Parameter Generation 2.2.1. Group Parameter Generation
Agents SHOULD generate domain parameters (g,p,q) using the following Agents SHOULD generate domain parameters (g,p,q) using the following
algorithm, derived from [FIPS-186] and [X942]. When this algorithm is algorithm, derived from [FIPS-186] and [X942]. When this algorithm is
used, the correctness of the generation procedure can be verified by used, the correctness of the generation procedure can be verified by
a third party by the algorithm of 2.2.2. a third party by the algorithm of 2.2.2.
2.2.1.1. Generation of p, q 2.2.1.1. Generation of p, q
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0 <= b < 160. 0 <= b < 160.
1. Choose an arbitrary sequence of at least m bits and call it SEED. 1. Choose an arbitrary sequence of at least m bits and call it SEED.
Let g be the length of SEED in bits. Let g be the length of SEED in bits.
2. Set U = 0 2. Set U = 0
3. Set m' = m / 160, where / represents integer division, 3. Set m' = m / 160, where / represents integer division,
consequently, if m = 200, m' = 1. consequently, if m = 200, m' = 1.
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4. for i = 0 to 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) 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] Note that for m=160, this reduces to the algorithm of [FIPS-186]
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U = SHA[SEED] XOR SHA[(SEED+1) mod 2^160 ]. 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) 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, 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 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. 6. Use a robust primality algorithm to test whether q is prime.
7. If q is not prime then go to 1. 7. If q is not prime then go to 1.
skipping to change at line 391 skipping to change at line 400
14. If p passes the test performed in step 13 go to step 17. 14. If p passes the test performed in step 13 go to step 17.
15. Let counter = counter + 1 and offset = offset + n + 1. 15. Let counter = counter + 1 and offset = offset + n + 1.
16. If counter >= 4096 go to step 1. Otherwise go to step 9. 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 17. Save the value of SEED and the value of counter for use
in certifying the proper generation of p and q. in certifying the proper generation of p and q.
Note: A robust primality test is one where the probability of Note: A robust primality test is one where the probability of
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a non-prime number passing the test is at most 2^-80. [FIPS-186] a non-prime number passing the test is at most 2^-80. [FIPS-186]
provides a suitable algorithm, as does [X9.42]. provides a suitable algorithm, as does [X9.42].
2.2.1.2. Generation of g 2.2.1.2. Generation of g
This section gives an algorithm (derived from [FIPS186]) for This section gives an algorithm (derived from [FIPS-186]) for gen-
erating g.
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generating g.
1. Let j = (p - 1)/q. 1. Let j = (p - 1)/q.
2. Set h = any integer, where 1 < h < p - 1 and h differs 2. Set h = any integer, where 1 < h < p - 1 and h differs
from any value previously tried. from any value previously tried.
3. Set g = h^j mod p 3. Set g = h^j mod p
4. If g = 1 go to step 2 4. If g = 1 go to step 2
2.2.2. Group Parameter Validation 2.2.2. Group Parameter Validation
skipping to change at line 440 skipping to change at line 449
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 partyAInfo MAY be omitted, similarly different for each message and partyAInfo 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
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 partyAInfo MUST be used for each mes- optimization) then a separate partyAInfo MUST be used for each mes-
sage. All implementations of this standard MUST implement Ephemeral- sage. All implementations of this standard MUST implement Ephemeral-
Static mode. Static mode.
In order to resist small subgroup attacks, the recipient SHOULD per- In order to resist small subgroup attacks, the recipient SHOULD
form the check described in 2.1.5. If an opponent cannot determine
success or failure of a decryption operation by the recipient, the
recipient MAY choose to omit this check.
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perform the check described in 2.1.5. If an opponent cannot determine
success or failure of a decryption operation by the recipient, the
recipient MAY choose to omit this check. See also [LL97] for a method
of generating keys which are not subject to small subgroup attack.
2.4. Static-Static Mode 2.4. Static-Static Mode
In Static-Static mode, both the sender and the recipient have a In Static-Static mode, both the sender and the recipient have a
static (and certified) key pair. Since the sender's and recipient's 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 keys are therefore the same for each message, ZZ will be the same for
each message. Thus, partyAInfo MUST be used (and different for each each message. Thus, partyAInfo MUST be used (and different for each
message) in order to ensure that different messages use different message) in order to ensure that different messages use different
KEKs. Implementations MAY implement Static-Static mode. KEKs. Implementations MAY implement Static-Static mode.
In order to prevent small subgroup attacks, both originator and reci- In order to prevent small subgroup attacks, both originator and reci-
pient SHOULD either perform the validation step described in S 2.1.5 pient SHOULD either perform the validation step described in Section
or verify that the CA has properly verified the validity of the key. 2.1.5 or verify that the CA has properly verified the validity of the
key. See also [LL97] for a method of generating keys which are not
subject to small subgroup attack.
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 Burt Kaliski, Paul Hoffman, Stephen
Housley, Brian Korver, Jim Schaad, Mark Schertler, Peter Yee, and Henson, Russ Housley, Brian Korver, John Linn, Jim Schaad, Mark
Robert Zuccherato for their expert advice and review. Schertler, Peter Yee, and Robert Zuccherato for 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)
186, "Digital Signature Standard", 1994 May 19. 186, "Digital Signature Standard", 1994 May 19.
[P1363] "Standard Specifications for Public Key Cryptography", IEEE P1363 [P1363] "Standard Specifications for Public Key Cryptography", IEEE P1363
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working group draft, 1998, Annex D. working group draft, 1998, Annex D.
[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.
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[LL97] C.H. Lim and P.J. Lee, "A key recovery attack on discrete log-based [LL97] C.H. Lim and P.J. Lee, "A key recovery attack on discrete log-based
schemes using a prime order subgroup", B.S. Kaliski, Jr., editor, schemes using a prime order subgroup", B.S. Kaliski, Jr., editor,
Advances in Cryptology - Crypto '97, Lecture Notes in Computer Science, Advances in Cryptology - Crypto '97, Lecture Notes in Computer Science,
vol. 1295, 1997, Springer-Verlag, pp. 249-263. vol. 1295, 1997, Springer-Verlag, pp. 249-263.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels." RFC 2119. March 1997. Levels." RFC 2119. March 1997.
[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.
skipping to change at line 519 skipping to change at line 534
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. S 2.3 Static mode and for the receiver during Ephemeral-Static mode. Sec-
and 2.4 describe appropriate practices to protect against this tions 2.3 and 2.4 describe appropriate practices to protect against
attack. Alternatively, it is possible to generate keys in such a this attack. Alternatively, it is possible to generate keys in such a
fashion that they are resistant to this attack. See [LL97] fashion that they are resistant to this attack. See [LL97]
In order to securely generate a symmetric key of length l, m (the The security level provided by these methods depends on several fac-
length in bits of q, and hence x) should be at least 2l. m MUST tors. It depends on the length of the symmetric key (typically, a 2^l
always be >= 160. Consequently, for RC2, m should be >=256 and for security level if the length is l bits); the size of the prime q (a
3DES, >=224 (the effective length of 3DES keys is 112 bits). 2^{m/2} security level); and the size of the prime p (where the secu-
rity level grows as a subexponential function of the size in bits).
A good design principle is to have a balanced system, where all three
security levels are approximately the same. If many keys are derived
from a given pair of primes p and q, it may be prudent to have higher
levels for the primes. In any case, the overall security is limited
by the lowest of the three levels.
Author's Address Author's Address
Rescorla [Page 11] Internet-Draft Diffie-Hellman Key Agreement Method
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 11] Internet-Draft Diffie-Hellman Key Agreement Method Rescorla [Page 12] 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 558 skipping to change at line 581
2.1. Key Agreement ................................................ 2 2.1. Key Agreement ................................................ 2
2.1.1. Generation of ZZ ........................................... 2 2.1.1. Generation of ZZ ........................................... 2
2.1.2. Generation of Keying Material .............................. 3 2.1.2. Generation of Keying Material .............................. 3
2.1.3. KEK Computation ............................................ 4 2.1.3. KEK Computation ............................................ 4
2.1.4. Keylengths for common algorithms ........................... 4 2.1.4. Keylengths for common algorithms ........................... 4
2.1.5. Public Key Validation ...................................... 4 2.1.5. Public Key Validation ...................................... 5
2.1.6. Example 1 .................................................. 5 2.1.6. Example 1 .................................................. 5
2.1.7. Example 2 .................................................. 6 2.1.7. Example 2 .................................................. 6
2.2. Key and Parameter Requirements ............................... 7 2.2. Key and Parameter Requirements ............................... 7
2.2.1. Group Parameter Generation ................................. 7 2.2.1. Group Parameter Generation ................................. 7
2.2.1.1. Generation of p, q ....................................... 7 2.2.1.1. Generation of p, q ....................................... 7
2.2.1.2. Generation of g .......................................... 8 2.2.1.2. Generation of g .......................................... 9
2.2.2. Group Parameter Validation ................................. 9 2.2.2. Group Parameter Validation ................................. 9
2.3. Ephemeral-Static Mode ........................................ 9 2.3. Ephemeral-Static Mode ........................................ 9
2.4. Static-Static Mode ........................................... 10 2.4. Static-Static Mode ........................................... 10
2.4. Acknowledgements ............................................. 10 2.4. Acknowledgements ............................................. 10
Rescorla [Page 12] Internet-Draft Diffie-Hellman Key Agreement Method Rescorla [Page 13] Internet-Draft Diffie-Hellman Key Agreement Method
2.4. References ................................................... 10 2.4. References ................................................... 10
Security Considerations ........................................... 11 Security Considerations ........................................... 11
Author's Address .................................................. 11 Author's Address .................................................. 11
 End of changes. 

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