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More Modular
Exponential (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups
for Secure Shell (SSH)
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Internet Engineering Task Force
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
including an errata fix for checking the Peer's DH Public Key.
Secure Shell (SSH) is a common protocol for secure
communication on the Internet. Security protocols and
primitives are an active area for research and help to suggest
updates to SSH.
Section 3 of the contains a small
errata for checking the Peer's DH Public key. Section 4 of this
document provides the correction.
Due to security concerns with SHA-1
and with MODP groups with less than 2048 bits 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 use of larger MODP groups and the move to the SHA-2 family
of hashes are important features to strengthen the key exchange
algorithms available to the SSH client and server.
DH primes being adopted by this document are all "safe primes"
such that p = 2q + 1 where q is also a prime. New MODP groups
are being introduced starting with the MODP 3072-bit group 15.
All use SHA512 as the hash algorithm.
The DH 2048-bit MODP group 14 is already present in most SSH
implementations and most implementations already have a SHA256
implementation, so diffie-hellman-group14-sha256 is provided as
easy to implement.
It is intended that these new MODP groups with SHA-2 based
hashes update the section 6.4 and
section 4.10 standards.
The United States Information Assurance Directorate (IAD) at
the National Security Agency (NSA) has published "Commercial
National Security Algorithm (CNSA) Suite and Quantum Computing
Frequently Asked Questions (FAQ)" addressed to organizations that
run classified or unclassified national security systems (NSS)
and vendors that build products used in NSS.
This FAQ document indicates that NSS should no longer use:
ECDH and ECDSA with NIST P-256
SHA-256
AES-128
RSA with 2048-bit keys
Diffie-Hellman with 2048-bit keys

The FAQ also states that NSS users should select DH groups
based upon well established and validated parameter sets that
comply with the minimum required sizes. Some specific examples
include:
Elliptic Curves are currently restricted to the NIST
P-384 group only for both ECDH and ECDSA, in accordance
with existing NIST and NIAP standards.
RSA moduli should have a minimum size of 3072 bits
(other than the noted PKI exception), and keys should be
generated in accordance with all relevant NIST standards.
For Diffie-Hellman use a Diffie-Hellman prime modulus of at
least 3072 bits as specified in IETF
RFC 3526 (Groups 15-18).

Although SSH may not always be used to protect Top Secret
communications, this document adopts the use of the DH groups
provided as an example in the FAQ as well as the use of SHA512
rather than SHA256 for the new DH groups.
[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.
This document adds some new Key Exchange Algorithm Method Names
in and .
This document 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 method algorithms are defined:
diffie-hellman-group14-sha256
diffie-hellman-group15-sha512
diffie-hellman-group16-sha512
diffie-hellman-group17-sha512
diffie-hellman-group18-sha512

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 SHA256 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 3072-bit MODP Group 15,
4096-bit MODP Group 16, 6144-bit MODP Group 17, and 8192-bit
MODP Group 18.
The SHA512 algorithm is to be used when "sha512" is
specified as a part of the key exchange method name.
Section 3 of contains a small errata.
When checking e (client public key) and f (server public key)
values, an incorrect range is provided. The erroneous text is:
Values of 'e' or 'f' that are not in the range [1, p-1]
MUST NOT be sent or accepted by either side. If this
condition is violated, the key exchange fails.

The errata is that the range should have been an open interval
excluding the end point values. (i.e "(1, p-1)").
This document amends that document text as follows:
DH Public key values MUST be checked and both conditions:
1 < e < p-1
1 < f < p-1

MUST be true. Values not within these bounds MUST NOT be
sent or accepted by either side. If either one of these
condition is violated, then the key exchange fails.

This simple check ensures:
The remote peer behaves properly.
The local system is not forced into the two-element
subgroup.

IANA is requested to add to the Key Exchange Method Names
algorithm registry with the following
entries:
Key Exchange Method NameReference
diffie-hellman-group14-sha256This Draft
diffie-hellman-group15-sha512This Draft
diffie-hellman-group16-sha512This Draft
diffie-hellman-group17-sha512This Draft
diffie-hellman-group18-sha512This 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>]
Thanks to the following people for review and comments: Denis
Bider, Peter Gutmann, Damien Miller, Niels Möller, Matt
Johnston, Iwamoto Kouichi, Dave Dugal, Daniel Migault, Anna
Johnston, Ron Frederick, Rich Salz, Travis Finkenauer, Eric
Rescorla.
The security considerations of apply
to this document.
The security considerations of suggest
that MODP group14 through group18 have security strengths that
range between 110 bits of security through 310 bits of
security. They are based on
Determining Strengths For Public Keys Used For Exchanging
Symmetric Keys. Care should be taken to use sufficient entropy
and/or DRBG algorithms to maximize the true security strength
of the key exchange and ciphers selected.
Using a fixed set of Diffie-Hellman parameters makes them a
high value target for pre-computation.
Generating additional sets of primes to be used, or moving to
larger values is a mitigation against this issue.
&rfc2119;
&rfc3526;
&rfc4250;
&rfc4253;
&rfc6234;
Secure Shell (SSH) Protocol Parameters:
Key Exchange Method Names
Internet Assigned Numbers Authority (IANA)
CNSA Suite and Quantum Computing FAQ
"National Security Agency/Central Security Service"
Transitions: Recommendation for the Transitioning of
the Use of Cryptographic Algorithms and Key Lengths
&rfc3766;
&rfc6194;