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


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.

Status of This Memo

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This Internet-Draft will expire on June 12, 2016.

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1. Overview and Rationale

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

2. Requirements Language

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. Key Exchange Algorithms

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.

4. IANA Considerations

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.

5. Security Considerations

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.

6. References

6.1. Normative References

[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.

6.2. Informative References

[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.

Author's Address

Mark D. Baushke Juniper Networks, Inc. 1133 Innovation Way Sunnyvale, CA 94089-1228 US Phone: +1 408 745 2952 EMail: URI:

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