# Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol

RFC 4419

Document | |||
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Type | RFC - Proposed Standard (March 2006; No errata) | ||

Last updated | 2013-03-02 | ||

Stream | IETF | ||

Formats | plain text pdf html | ||

Stream | |||

WG state | WG Document | ||

Consensus | Unknown | ||

Document shepherd | No shepherd assigned | ||

IESG | |||

IESG state | RFC 4419 (Proposed Standard) | ||

Telechat date | |||

Responsible AD | Russ Housley | ||

Send notices to | sommerfeld@east.sun.com |

Network Working Group M. Friedl Request for Comments: 4419 N. Provos Category: Standards Track W. Simpson March 2006 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This memo describes a new key exchange method for the Secure Shell (SSH) protocol. It allows the SSH server to propose new groups on which to perform the Diffie-Hellman key exchange to the client. The proposed groups need not be fixed and can change with time. 1. Overview and Rationale SSH [RFC4251] is a very common protocol for secure remote login on the Internet. Currently, SSH performs the initial key exchange using the "diffie-hellman-group1-sha1" method [RFC4253]. This method prescribes a fixed group on which all operations are performed. The Diffie-Hellman key exchange provides a shared secret that cannot be determined by either party alone. Furthermore, the shared secret is known only to the participant parties. In SSH, the key exchange is signed with the host key to provide host authentication. The security of the Diffie-Hellman key exchange is based on the difficulty of solving the Discrete Logarithm Problem (DLP). Since we expect that the SSH protocol will be in use for many years in the future, we fear that extensive precomputation and more efficient algorithms to compute the discrete logarithm over a fixed group might pose a security threat to the SSH protocol. Friedl, et al. Standards Track [Page 1] RFC 4419 SSH DH Group Exchange March 2006 The ability to propose new groups will reduce the incentive to use precomputation for more efficient calculation of the discrete logarithm. The server can constantly compute new groups in the background. 2. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 3. Diffie-Hellman Group and Key Exchange The server keeps a list of safe primes and corresponding generators that it can select from. A prime p is safe if p = 2q + 1 and q is prime. New primes can be generated in the background. The generator g should be chosen such that the order of the generated subgroup does not factor into small primes; that is, with p = 2q + 1, the order has to be either q or p - 1. If the order is p - 1, then the exponents generate all possible public values, evenly distributed throughout the range of the modulus p, without cycling through a smaller subset. Such a generator is called a "primitive root" (which is trivial to find when p is "safe"). The client requests a modulus from the server indicating the preferred size. In the following description (C is the client, S is the server, the modulus p is a large safe prime, and g is a generator for a subgroup of GF(p), min is the minimal size of p in bits that is acceptable to the client, n is the size of the modulus p in bits that the client would like to receive from the server, max is the maximal size of p in bits that the client can accept, V_S is S's version string, V_C is C's version string, K_S is S's public host key, I_C is C's KEXINIT message, and I_S is S's KEXINIT message that has been exchanged before this part begins): 1. C sends "min || n || max" to S, indicating the minimal acceptable group size, the preferred size of the group, and the maximal group size in bits the client will accept. 2. S finds a group that best matches the client's request, and sends "p || g" to C. 3. C generates a random number x, where 1 < x < (p-1)/2. It computes e = g^x mod p, and sends "e" to S. Friedl, et al. Standards Track [Page 2]Show full document text