| Internet Engineering Task Force | M. Baushke |
| Internet-Draft | Juniper Networks, Inc. |
| Updates: 4253 (if approved) | December 10, 2015 |
| Intended status: Standards Track | |
| Expires: June 12, 2016 |
More Modular Exponential (MODP) Diffie-Hellman Groups for SSH
draft-baushke-ssh-dh-group-sha2-01
This document defines two added Modular Exponential (MODP) Groups for the Secure Shell (SSH) protocol. It also updates [RFC4253] by specifying new RECOMMENDED and new OPTIONAL Diffie-Hellman key exchange algorithms using SHA-2 hashes.
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Secure Shell (SSH) is a common protocol for secure communication on the Internet. In [RFC4253], SSH originally defined the Key Exchange Method Name diffie-hellman-group1-sha1 which used [RFC2409] Oakley Group 1 (a MODP group with 768 bits) and SHA-1 [RFC3174]. Due to recent security concerns with SHA-1 [RFC6194] and with MODP groups with less than 2048 bits [NIST-SP-800-131Ar1] implementors 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.
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 [RFC2119].
This memo adopts the style and conventions of [RFC4253] 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-group1-sha1 NOT RECOMMENDED
diffie-hellman-group14-sha256 RECOMMENDED
diffie-hellman-group15-sha256 RECOMMENDED
diffie-hellman-group16-sha256 OPTIONAL
Figure 1
The SHA-2 family of secure hash algorithms are defined in [FIPS-180-4].
The method of key exchange used for the name "diffie-hellman-group14-sha256" is the same as that for "diffie-hellman-group14-sha1" escept that the SHA2-256 hash algorith is used.
The group15 and group16 names are the same as those specified in [RFC3526] as 3072-bit MODP Group 14 and 4096-bit MODP Group 15.
This document augments the Key Exchange Method Names in [RFC4253].
IANA is requested to update the SSH algorithm registry with the following entries:
Key Exchange Method Name Reference Note
diffie-hellman-group1-sha1 RFC4253 NOT RECOMMENDED
diffie-hellman-group14-sha256 This draft RECOMMENDED
diffie-hellman-group15-sha256 This draft RECOMMENDED
diffie-hellman-group16-sha256 This draft OPTIONAL
Figure 2
It is RECOMMENDED that the new diffie-hellman-group14-sha256 method be proposed before the diffie-hellman-group14-sha1 method.
The security considerations of [RFC4253] apply to this document.
The security considerations of [RFC3526] suggest that these MODP groups have security strengths given in this table.
Group modulus security strength estimates
+--------+----------+---------------------+---------------------+
| 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- |
+--------+----------+---------------------+---------------------+
Figure 3
Many users seem to be interested in the perceived safety of using the SHA2-based algorithms for hashing.
| [FIPS-180-4] | National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3526] | Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, DOI 10.17487/RFC3526, May 2003. |
| [RFC4253] | Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006. |
| [NIST-SP-800-131Ar1] | Barker, and Roginsky, "Transitions: Recommendation for the Transitioning of the Use of Cryptographic Algorithms and Key Lengths", NIST Special Publication 800-131A Revision 1, November 2015. |
| [RFC2409] | Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998. |
| [RFC3174] | Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001. |
| [RFC6194] | Polk, T., Chen, L., Turner, S. and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011. |