| Internet Engineering Task Force | M. Baushke |
| Internet-Draft | Juniper Networks, Inc. |
| Updates: 4253, 4432, 4462 (if approved) | September 11, 2016 |
| Intended status: Standards Track | |
| Expires: March 15, 2017 |
More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)
draft-ietf-curdle-ssh-modp-dh-sha2-00
This document defines added Modular Exponential (MODP) Groups for the Secure Shell (SSH) protocol using SHA-2 hashes.
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Secure Shell (SSH) is a common protocol for secure communication on the Internet. Due to recent security concerns with SHA-1 [RFC6194] and with MODP groups with less than 2048 bits [NIST-SP-800-131Ar1] 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 [MFQ-U-OO-815099-15] 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.
In [RFC4462], there is another method for providing DH key exchange with MODP Groups using "Generic Security Service Application Program Interface (GSS-API)". This RFC extends the "gss-*" MODP DH groups and provides for using SHA-2 based hashes for them as well.
Please send comments on this draft to ietf-ssh@NetBSD.org and ietf-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 [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
diffie-hellman-group14-sha256
diffie-hellman-group15-sha512
diffie-hellman-group16-sha512
diffie-hellman-group17-sha512
diffie-hellman-group18-sha512
gss-group14-sha256-*
gss-group15-sha512-*
gss-group16-sha512-*
gss-group17-sha512-*
gss-group18-sha512-*
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" except that the SHA2-256 hash algorithm is used.
The method of key exchange used for the name "gss-group14-sha256-*" is the same as that for "gss-group14-sha1-*" except that the SHA2-256 hash algorithm is used.
The group15 through group18 names are the same as those specified in [RFC3526] 3071-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 [RFC4253].
IANA is requested to update the SSH algorithm registry with the following entries:
Key Exchange Method Name Reference Note
diffie-hellman-group14-sha256 This Draft MAY
diffie-hellman-group15-sha512 This Draft MAY
diffie-hellman-group16-sha512 This Draft MAY
diffie-hellman-group17-sha512 This Draft MAY
diffie-hellman-group18-sha512 This Draft MAY
gss-group14-sha256-* This Draft MAY
gss-group15-sha512-* This Draft MAY
gss-group16-sha512-* This Draft MAY
gss-group17-sha512-* This Draft MAY
gss-group18-sha512-* This Draft MAY
Figure 2
The Note in the above table is not 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. This RFC is intended to provide IANA defined names for these groups for interoperability.
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. They are based on [RFC3766] 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- |
+--------+----------+---------------------+---------------------+
Figure 3
Many users seem to be interested in the perceived safety of using larger MODP groups and hashing with SHA2-based algorithms.
| [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. |
| [MFQ-U-OO-815099-15] | "National Security Agency/Central Security Service", "CNSA Suite and Quantum Computing FAQ", January 2016. |
| [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. |
| [RFC3766] | Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, DOI 10.17487/RFC3766, April 2004. |
| [RFC4462] | Hutzelman, J., Salowey, J., Galbraith, J. and V. Welch, "Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May 2006. |
| [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. |