Key Exchange Method Updates for Secure Shell (SSH)

Secure Shell (SSH) is a common protocol for secure communication on the Internet. In , SSH originally defined the Key Exchange Method Name diffie-hellman-group1-sha1 which used Oakley Group 1 (a MODP group with 768 bits) and SHA-1 . Due to recent security concerns with SHA-1 and with MODP groups with less than 2048 bits implementer and users request support for larger 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 that the use of ECDH using the nistp256 curve and SHA-2 based hashes less than SHA2-384 are no longer sufficient for transport of Top Secret information. It is for this reason that this draft moves ecdh-sha2-nistp256 from a REQUIRED to OPTIONAL as a key exchange method. This is the same reason that the stronger MODP groups being introduced are using SHA2-512 as the hash algorithm. Group14 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 has been observed in that the NIST recommended Elliptic Curve Prime Curves (P-256, P-384, and P-521) are perhaps not the best available for Elliptic Curve Cryptography Security. For this reason, none of the curves are marked as a MUST implement. However, the requirement that "every compliant SSH ECC implementation MUST implement ECDH key exchange" is now taken to mean that if ecdsa-sha2-[identifier] is implemented, then ecdh-sha2-[identifier] MUST be implemented. Please send comments on this draft to ietf-ssh@NetBSD.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 .

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 Note diffie-hellman-group14-sha256 MAY/OPTIONAL diffie-hellman-group16-sha512 SHOULD/RECOMMENDED diffie-hellman-group18-sha512 MAY/OPTIONAL 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. The group16 and group18 names are the same as those specified in 4096-bit MODP Group 16 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 . It downgrades the use of SHA-1 hashing for key exchange methods in , , and . It also moves from MUST to MAY the ecdh-sha2-nistp256 given in . It is desirable to also include the ssh-curves from the in this list. The "curve25519-sha256" is currently available in some Secure Shell implementations under the name "curve25519-sha256@libssh.org" and is the best candidate for a fast, safe, and secure key exchange method.

IANA is requested to update the SSH algorithm registry with the following entries: Key Exchange Method Name Reference Note diffie-hellman-group-exchange-sha1 RFC4419 SHOULD NOT diffie-hellman-group-exchange-sha256 RFC4419 MAY diffie-hellman-group1-sha1 RFC4253 SHOULD NOT diffie-hellman-group14-sha1 RFC4253 SHOULD ecdh-sha2-nistp256 RFC5656 MAY ecdh-sha2-nistp384 RFC5656 SHOULD ecdh-sha2-nistp521 RFC5656 SHOULD ecdh-sha2-* RFC5656 MAY ecmqv-sha2 RFC5656 MAY gss-gex-sha1-* RFC4462 SHOULD NOT gss-group1-sha1-* RFC4462 SHOULD NOT gss-group14-sha1-* RFC4462 MAY gss-* RFC4462 MAY rsa1024-sha1 RFC4432 SHOULD NOT rsa2048-sha256 RFC4432 MAY diffie-hellman-group14-sha256 This Draft MAY diffie-hellman-group16-sha512 This Draft SHOULD diffie-hellman-group18-sha512 This Draft MAY curve25519-sha256 ssh-curves MUST curve448-sha512 ssh-curves MAY The Note in the above table is an implementation suggestion/recommendation for the listed key exchange method. It is up to the end-user as to what algorithms they choose to be able to negotiate. The guidance of his document is that the SHA-1 algorithm hashing SHOULD NOT be used. If it is used, it should only be provided for backwards compatibility, should not be used in new designs, and should be phased out of existing key exchanges as quickly as possible because of its known weaknesses. Any key exchange using SHA-1 SHOULD NOT be in a default key exchange list if at all possible. If they are needed for backward compatibility, they SHOULD be listed after all of the SHA-2 based key exchanges. The RFC4253 REQUIRED diffie-hellman-group14-sha1 method SHOULD be retained for compatibility with older Secure Shell implementations. It is intended that this key exchange be phased out as soon as possible.

Thanks to the following people for review and comments: Denis Bider, Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto Kouichi, Simon Josefsson, Dave Dugal. Thanks to the following people for code to implement interoperable exchanges using some of these groups as found in an the -01 draft: Darren Tucker for OpenSSH and Matt Johnston for Dropbear. And thanks to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh) and Poderosa implementations also adopting new Diffie-Hellman groups based on the -01 draft.

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- | +--------+----------+---------------------+---------------------+ Many users seem to be interested in the perceived safety of using larger MODP groups and hashing with SHA2-based algorithms.