More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)

Secure Shell (SSH) is a common protocol for secure communication on the Internet. Due to recent security concerns with SHA-1 and with MODP groups with less than 2048 bits implementer and users request support for larger Diffie Hellman (DH) MODP group sizes with data integrity verification using the SHA-2 family of secure hash algorithms as well as MODP groups providing more security. The United States Information Assurance Directorate at the National Security Agency has published a FAQ suggesting both: a) DH groups using less than 3072-bits, and b) the use of SHA-2 based hashes less than SHA2-384, are no longer sufficient for transport of Top Secret information. For this reason, the new MODP groups are being introduced starting with the MODP 3072-bit group 15 are all using SHA2-512 as the hash algorithm. The DH 2048-bit MODP group 14 is already present in most SSH implementations and most implementations already have a SHA2-256 implementation, so diffie-hellman-group14-sha256 is provided as an easy to implement and faster to use key exchange for small embedded applications. It is intended that these new MODP groups with SHA-2 based hashes update the section 6.4 and section 4.10 standards. [TO BE REMOVED: Please send comments on this draft to curdle@ietf.org.]

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 RFC 2119.

This memo adopts the style and conventions of in specifying how the use of new data key exchange is indicated in SSH.

The following new key exchange algorithms are defined: Key Exchange Method Name diffie-hellman-group14-sha256 diffie-hellman-group15-sha512 diffie-hellman-group16-sha512 diffie-hellman-group17-sha512 diffie-hellman-group18-sha512 The SHA-2 family of secure hash algorithms are defined in . The method of key exchange used for the name "diffie-hellman-group14-sha256" is the same as that for "diffie-hellman-group14-sha1" except that the SHA2-256 hash algorithm is used. It is recommended that diffie-hellman-group14-sha256 SHOULD be supported to smooth the transition to newer group sizes. The group15 through group18 names are the same as those specified in 3072-bit MODP Group 15, 4096-bit MODP Group 16, 6144-bit MODP Group 17, and 8192-bit MODP Group 18. The SHA2-512 algorithm is to be used when "sha512" is specified as a part of the key exchange method name.

This document augments the Key Exchange Method Names in and . IANA is requested to add to the Key Exchange Method Names algorithm registry with the following entries: Key Exchange Method NameReference diffie-hellman-group14-sha256This Draft diffie-hellman-group15-sha512This Draft diffie-hellman-group16-sha512This Draft diffie-hellman-group17-sha512This Draft diffie-hellman-group18-sha512This Draft [TO BE REMOVED: This registration should take place at the following location: <http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-16>]

The security considerations of apply to this document. The security considerations of suggest that these MODP groups have security strengths given in this table. They are based on Determining Strengths For Public Keys Used For Exchanging Symmetric Keys.

Group modulus security strength estimates (RFC3526) +--------+----------+---------------------+---------------------+ | Group | Modulus | Strength Estimate 1 | Strength Estimate 2 | | | +----------+----------+----------+----------+ | | | | exponent | | exponent | | | | in bits | size | in bits | size | +--------+----------+----------+----------+----------+----------+ | 14 | 2048-bit | 110 | 220- | 160 | 320- | | 15 | 3072-bit | 130 | 260- | 210 | 420- | | 16 | 4096-bit | 150 | 300- | 240 | 480- | | 17 | 6144-bit | 170 | 340- | 270 | 540- | | 18 | 8192-bit | 190 | 380- | 310 | 620- | +--------+----------+---------------------+---------------------+ Using a fixed set of Diffie-Hellman parameters makes them a high value target for precomputation. Generating additional sets of primes to be used, or moving to larger values is a mitigation against this issue. Care should be taken to avoid backdoored primes () by using "nothing up my sleve" parameters.