E. Rescorla INTERNET-DRAFT RTFM Inc. <draft-ietf-smime-x942-02.txt> November 1998 (Expires May 1999) Diffie-Hellman Key Agreement Method Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document standardizes one particular Diffie-Hellman variant, based on the ANSI X9.42 standard, developed by the ANSI X9F1 working group. An algorithm for converting the shared secret into an arbi- trary amount of keying material is provided. 1. Introduction 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- vailable to eavesdroppers. This secret may then be converted into cryptographic keying material for other (symmetric) algorithms. A large number of minor variants of this process exist. This document describes one such variant, based on the ANSI X9.42 specification. 1.1. Discussion of this Draft 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 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- smime/>. Rescorla [Page 1]Internet-Draft Diffie-Hellman Key Agreement Method 1.2. Requirements Terminology Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and "MAY" that appear in this document are to be interpreted as described in [RFC2119]. 2. Overview Of Method 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 the other party's public key, both parties can compute the same shared secret number. This number can then be converted into crypto- graphic keying material. That keying material is typically used as a key encryption key (KEK) to encrypt (wrap) a message encrytion key (MEK) which is in turn used to encrypt the message data. 2.1. Key Agreement 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- Hellman keys). ZZ is then converted into a shared symmetric key. Note that the symmetric key will be different for each key agreement, due to the introduction of public random components. 2.1.1. Generation of ZZ X9.42 defines that the shared secret ZZ is generated as follows: ZZ = g ^ (xb * xa) (mod p) Note that the individual parties actually perform the computations: ZZ = yb ^ xa (mod p) = ya ^ xb (mod p) where ^ denotes exponentiation ya is party a's public key; ya = g ^ xa (mod p) yb is party b's public key; yb = g ^ xb (mod p) xa is party a's private key xb is party b's private key p is a large prime g is a generator for the integer group specified by p. (See Section 2.2 for criteria for keys and parameters) 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. Rescorla [Page 2]Internet-Draft Diffie-Hellman Key Agreement Method 2.1.2. Generation of Keying Material X9.42 provides an algorithm for generating an essentially arbitrary amount of keying material from ZZ. Our algorithm is derived from that algorithm by mandating some optional fields and omitting others. KM = H ( ZZ || OtherInfo) H is the message digest function SHA-1 [FIPS-180] ZZ is the shared key computed in Section 2.1.1 OtherInfo is the DER encoding of the following structure: OtherInfo ::= SEQUENCE { keyInfo KeySpecificInfo, pubInfo [2] OCTET STRING OPTIONAL, } KeySpecificInfo ::= SEQUENCE { algorithm OBJECT IDENTIFIER, counter OCTET STRING SIZE (4..4) } algorithm is the ASN.1 algorithm OID of the symmetric algorithm with which this KEK will be used. counter is a 32 bit number, represented in network byte order. Its initial value is 1, i.e. the byte sequence 00 00 00 01 (hex) pubInfo is a random string provided by the sender. In CMS, it is provided as a parameter in the UserKeyingMaterial field (a 512 bit byte string). 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 ZZ, even if more data than ZZ is generated. However, since pubInfo is dif- ferent for each message, a different KEK will be generated for each message. 2.1.3. KEK Computation 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. 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 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 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 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. Rescorla [Page 3]Internet-Draft Diffie-Hellman Key Agreement Method 2.1.4. Keylengths for common algorithms Some common key encryption algorithms have KEKs of the following lengths. DES-ECB 64 bits 3DES-EDE-ECB 192 bits RC2 (all) 128 bits 2.1.5. Public Key Validation 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, the key is invalid. 2. Compute y^q (mod p). If the result == 1, the key is valid. Otherwise the key is invalid. The primary purpose of public key validation is to prevent a small subgroup attack [SUBGROUP] on the sender's key pair. If Ephemeral- Static mode is used, this check is unnecessary. Note that this pro- cedure may be subject to pending patents. 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 The key encryption algorithm is 3DES-EDE. No pubInfo is used 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 ; ZZ 30 13 30 11 06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 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 0e 8a c1 96 8f fb 94 b3 The input to the second invocation of SHA-1 is: Rescorla [Page 4]Internet-Draft Diffie-Hellman Key Agreement Method 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f ; ZZ 30 13 30 11 06 09 2a 86 48 86 f7 0d 03 06 00 ; 3DES-EDE OID 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 bd 0c 12 5e e0 f9 1a cc Consequently, K1=a8 c6 4e 46 1a aa c2 36 K2=45 c9 2e c6 0e 8a c1 96 K3=8f fb 94 b3 49 eb c8 09 Note: These keys are not parity adjusted 2.2. Key and Parameter Requirements 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- erating primes of this form can be found in FIPS PUB 186-1[DSS] and ANSI, X9.30-1 1996 [X930], as well as in [X942]. X9.42 requires that the private key x be in the interval [2^(m-1) + 1, (q - 2)]. x should be randomly generated in this interval. y is then computed by calculating g^x (mod p). To comply with this draft, m MUST be >=128, (consequently, q and x MUST each be at least 128 bits long). When symmetric ciphers stronger than DES are to be used, a larger m may be advisable. 2.2.1. Group Parameter Generation Agents SHOULD generate domain parameters (g,p,q) using the algorithms specified in Appendixes 2 and 3 of [FIPS-186]. 2.2.2. Group Parameter 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 group parameters were correctly generated. Two checks are possible: Rescorla [Page 5]Internet-Draft Diffie-Hellman Key Agreement Method 1. Verify that p=qj + 1. This demonstrates that the parameters meet the X9.42 parameter criteria. 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. This demonstrates that the parameters were randomly chosen and do not have a special form. Whether agents provide validation information in their certificates is a local matter between the agents and their CA. 2.3. Ephemeral-Static Mode In Ephemeral-Static mode, the recipient has a static (and certified) 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 is freshly generated for each message, the shared secret ZZ will be similarly different for each message and pubInfo MAY be omitted, since it serves merely to decouple multiple KEKs generated by the 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 optimization) then a separate pubInfo MUST be used for each message. All implementations of this standard MUST implement Ephemeral-Static mode. Acknowledgements 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 thanks for their assistance. The author also wishes to thank Paul Hoffman, Russ Housley, Brian Korver, Mark Schertler, and Peter Yee for their expert advice and review. References [CMS] Housley, R., "Cryptographic Message Syntax", draft-ietf-smime-cms-05.txt. [FIPS-46-1] Federal Information Processing Standards Publication (FIPS PUB) 46-1, Data Encryption Standard, Reaffirmed 1988 January 22 (supersedes FIPS PUB 46, 1977 January 15). [FIPS-81] Federal Information Processing Standards Publication (FIPS PUB) 81, DES Modes of Operation, 1980 December 2. [FIPS-180] Federal Information Processing Standards Publication (FIPS PUB) 180-1, "Secure Hash Standard", 1995 April 17. Rescorla [Page 6]Internet-Draft Diffie-Hellman Key Agreement Method [FIPS-186] Federal Information Processing Standards Publication (FIPS PUB) 186, "Digital Signature Standard", 1994 May 19. [PKIX] Housley, R., Ford, W., Polk, W., Solo, D., "Internet X.509 Public 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", ANSI draft, 1998. Security Considerations All the security in this system is provided by the secrecy of the private keying material. If either sender or recipient private keys are disclosed, all messages sent or received using that key are compromised. Similarly, loss of the private key results in an inabil- ity to read messages sent using that key. Author's Address Eric Rescorla <ekr@rtfm.com> RTFM Inc. 30 Newell Road, #16 East Palo Alto, CA 94303 Rescorla [Page 7]Internet-Draft Diffie-Hellman Key Agreement Method Table of Contents 1. Introduction ................................................... 1 1.1. Discussion of this Draft ..................................... 1 1.2. Requirements Terminology ..................................... 2 2. Overview Of Method ............................................. 2 2.1. Key Agreement ................................................ 2 2.1.1. Generation of ZZ ........................................... 2 2.1.2. Generation of Keying Material .............................. 3 2.1.3. KEK Computation ............................................ 3 2.1.4. Keylengths for common algorithms ........................... 4 2.1.5. Public Key Validation ...................................... 4 2.1.6. Example .................................................... 4 2.2. Key and Parameter Requirements ............................... 5 2.2.1. Group Parameter Generation ................................. 5 2.2.2. Group Parameter Validation ................................. 5 2.3. Ephemeral-Static Mode ........................................ 6 2.3. Acknowledgements ............................................. 6 2.3. References ................................................... 6 Security Considerations ........................................... 7 Author's Address .................................................. 7