| Internet Engineering Task Force | M. Baushke |
| Internet-Draft | Juniper Networks, Inc. |
| Updates: 4250, 4253 (if approved) | April 14, 2017 |
| Intended status: Standards Track | |
| Expires: October 16, 2017 |
More Modular Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)
draft-ietf-curdle-ssh-modp-dh-sha2-04
This document defines added Modular Exponential (MODP) Groups for the Secure Shell (SSH) protocol using SHA-2 hashes. This document updates RFC 4250. This document updates RFC 4253.
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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.
It is intended that these new MODP groups with SHA-2 based hashes update the [RFC4253] section 6.4 and [RFC4250] 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 [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
Figure 1
The SHA-2 family of secure hash algorithms are defined in [RFC6234].
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 [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] and [RFC4250].
IANA is requested to add to the Key Exchange Method Names algorithm registry [IANA-KEX] with the following entries:
| Key Exchange Method Name | Reference |
|---|---|
| diffie-hellman-group14-sha256 | This Draft |
| diffie-hellman-group15-sha512 | This Draft |
| diffie-hellman-group16-sha512 | This Draft |
| diffie-hellman-group17-sha512 | This Draft |
| diffie-hellman-group18-sha512 | This 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 [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 2
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 ([SNFS]) by using "nothing up my sleve" parameters.
| [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. |
| [RFC4250] | Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, DOI 10.17487/RFC4250, January 2006. |
| [RFC4253] | Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006. |