Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)
draft-ietf-curdle-ssh-kex-sha2-11
The information below is for an old version of the document.
| Document | Type | This is an older version of an Internet-Draft that was ultimately published as an RFC. | |
|---|---|---|---|
| Author | Mark D. Baushke | ||
| Last updated | 2020-07-13 | ||
| Replaces | draft-baushke-ssh-dh-group-sha2 | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text xml htmlized pdfized bibtex | ||
| Reviews |
GENART Last Call review
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||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Daniel Migault | ||
| Shepherd write-up | Show Last changed 2018-01-05 | ||
| IESG | IESG state | AD Evaluation::AD Followup | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Benjamin Kaduk | ||
| Send notices to | Daniel Migault <daniel.migaultf@ericsson.com> |
draft-ietf-curdle-ssh-kex-sha2-11
Internet Engineering Task Force M. Baushke
Internet-Draft Juniper Networks, Inc.
Updates: 4250 (if approved) July 13, 2020
Intended status: Standards Track
Expires: January 14, 2021
Key Exchange (KEX) Method Updates and Recommendations for Secure Shell
(SSH)
draft-ietf-curdle-ssh-kex-sha2-11
Abstract
This document is intended to update the recommended set of key
exchange methods for use in the Secure Shell (SSH) protocol to meet
evolving needs for stronger security. This document updates RFC
4250.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Overview and Rationale . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 4
3.1. curve25519-sha256 . . . . . . . . . . . . . . . . . . . . 4
3.2. curve448-sha512 . . . . . . . . . . . . . . . . . . . . . 4
3.3. diffie-hellman-group-exchange-sha1 . . . . . . . . . . . 4
3.4. diffie-hellman-group-exchange-sha256 . . . . . . . . . . 5
3.5. diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . 5
3.6. diffie-hellman-group14-sha1 . . . . . . . . . . . . . . . 5
3.7. diffie-hellman-group14-sha256 . . . . . . . . . . . . . . 5
3.8. diffie-hellman-group15-sha512 . . . . . . . . . . . . . . 6
3.9. diffie-hellman-group16-sha512 . . . . . . . . . . . . . . 6
3.10. diffie-hellman-group17-sha512 . . . . . . . . . . . . . . 6
3.11. diffie-hellman-group18-sha512 . . . . . . . . . . . . . . 6
3.12. ecdh-sha2-* . . . . . . . . . . . . . . . . . . . . . . . 6
3.12.1. ecdh-sha2-nistp256 . . . . . . . . . . . . . . . . . 6
3.12.2. ecdh-sha2-nistp384 . . . . . . . . . . . . . . . . . 7
3.12.3. ecdh-sha2-nistp521 . . . . . . . . . . . . . . . . . 7
3.13. ecmqv-sha2 . . . . . . . . . . . . . . . . . . . . . . . 7
3.14. ext-info-c . . . . . . . . . . . . . . . . . . . . . . . 7
3.15. ext-info-s . . . . . . . . . . . . . . . . . . . . . . . 8
3.16. gss-* . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.16.1. gss-gex-sha1-* . . . . . . . . . . . . . . . . . . . 8
3.16.2. gss-group1-sha1-* . . . . . . . . . . . . . . . . . 8
3.16.3. gss-group14-sha1-* . . . . . . . . . . . . . . . . . 8
3.16.4. gss-group14-sha256-* . . . . . . . . . . . . . . . . 9
3.16.5. gss-group15-sha512-* . . . . . . . . . . . . . . . . 9
3.16.6. gss-group16-sha512-* . . . . . . . . . . . . . . . . 9
3.16.7. gss-group17-sha512-* . . . . . . . . . . . . . . . . 9
3.16.8. gss-group18-sha512-* . . . . . . . . . . . . . . . . 9
3.16.9. gss-nistp256-sha256-* . . . . . . . . . . . . . . . 10
3.16.10. gss-nistp384-sha384-* . . . . . . . . . . . . . . . 10
3.16.11. gss-nistp521-sha512-* . . . . . . . . . . . . . . . 10
3.16.12. gss-curve25519-sha256-* . . . . . . . . . . . . . . 10
3.16.13. gss-curve448-sha512-* . . . . . . . . . . . . . . . 10
3.17. rsa1024-sha1 . . . . . . . . . . . . . . . . . . . . . . 10
3.18. rsa2048-sha256 . . . . . . . . . . . . . . . . . . . . . 10
4. Selecting an appropriate hashing algorithm . . . . . . . . . 11
5. Summary Guidance for Key Exchange Method Names . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
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9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. In [RFC4253], SSH originally defined two Key Exchange
Method Names that MUST be implemented. Over time, what was once
considered secure is no longer considered secure. The purpose of
this RFC is to recommend that some published key exchanges be
deprecated as well as recommending some that SHOULD and one that MUST
be adopted. This document updates [RFC4250].
New key exchange methods will use the SHA-2 family of hashes found in
[RFC6234] rather than the SHA-1 hash which is in the process of being
deprecated for many purposes as no longer providing enough security.
SSH uses multiple mathematically hard problems for doing Key
Exchange. Finite Field Cryptography (FFC) with "safe primes" for
diffie-hellman (DH) key exchange. Elliptic Curve Cryptography (ECC)
using NIST prime curves with Elliptic Curve Diffie-Hellman (ECDH) and
the similar Curve25519 and Curve448 key exchanges.
For FFC, many experts have suggested that a prime field of 2048-bits
is the minimum (2048-bits is said to have 112 bits of security and
3072-bits is said to have 128 bits of security) allowed and larger
sizes up to 8192 bits are considered to be much stronger. The
minimum MODP group that MAY be used is the 2048-bit MODP group14.
For ECC, many experts have suggested that a 256-bits curve is the
minimum allowed (256-bits is said to have 128 bits of security) and
larger sizes up to 521-bits are considered to be much stronger
(521-bits are considered to have around 256-bits of security).
When it comes to Secure Hashing functions, SHA2-256 is said to have
128-bits of security SHA2-384 to have 192-bits of security, and
SHA2-512 to have 256-bits of security. The older SHA-1 hash is
supposed to have about 80-bits of security. The minimum secure
hashing function that should be used is SHA2-256 in the year of this
RFC.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
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BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Key Exchange Methods
This memo adopts the style and conventions of [RFC4253] in specifying
how the use of data key exchange is indicated in SSH.
This RFC also collects Key Exchange Method Names in various existing
RFCs [RFC4253], [RFC4419], [RFC4432], [RFC4462], [RFC5656],
[RFC8268], [RFC8731] [RFC8732], and [RFC8308], and provides a
suggested suitability for implementation of MUST, SHOULD, SHOULD NOT,
and MUST NOT. Any method not explicitly listed MAY be implemented.
This document is intended to provide guidance as to what Key Exchange
Algorithms are to be considered for new or updated SSH
implementations. This document will be superseded when one or more
of the listed algorithms are considered too weak to continue to use
securely, in which case they will likely be downgraded to one of MAY,
SHOULD NOT, or MUST NOT. Or, when newer methods have been analyzed
and found to be secure with wide enough adoption, upgrade their
recommendation from MAY to SHOULD or MUST.
3.1. curve25519-sha256
Curve25519 is efficient on a wide range of architectures with
properties that allow higher performance implementations compared to
traditional elliptic curves. The use of SHA2-256 (also known as
SHA-256 and sha256) as defined in [RFC6234] for integrity is a
reasonable one for this method. This Key Exchange Method is
described in [RFC8731] and is similar to the IKEv2 Key Agreement
described in [RFC8031]. This Key Exchange Method has multiple
implementations and SHOULD be implemented in any SSH implementation
interested in using elliptic curve based key exchanges.
3.2. curve448-sha512
The Curve448 requires more work than Curve25519. It uses SHA2-512
(also known as SHA-512) defined in [RFC6234] for integrity. This Key
Exchange Method is described in [RFC8731] and is similar to the IKEv2
Key Agreement described in [RFC8031]. This method MAY be
implemented.
3.3. diffie-hellman-group-exchange-sha1
This random selection from a set of pre-generated moduli for key
exchange uses SHA-1 as defined in [RFC4419]. However, SHA-1 has
security concerns provided in [RFC6194], so it would be better to use
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a key exchange method which uses a SHA-2 hash as in [RFC6234] for
integrity. This key exchange SHOULD NOT be used.
3.4. diffie-hellman-group-exchange-sha256
This random selection from a set of pre-generated moduli for key
exchange uses SHA2-256 as defined in [RFC4419]. [RFC8270] mandates
implementations avoid any MODP group with less than 2048 bits. Care
should be taken in the pre-generation of the moduli P and generator G
such that the generator provides a Q-ordered subgroup of P or the
parameter set may leak one bit of the shared private key leaving the
MODP group half as strong as desired as compared with the number of
bits. This key exchange MAY be used.
3.5. diffie-hellman-group1-sha1
This method is decribed in [RFC4253] and uses [RFC7296] Oakley Group
2 (a 1024-bit MODP group) and SHA-1 [RFC3174]. Due to recent
security concerns with SHA-1 [RFC6194] and with MODP groups with less
than 2048 bits (see [LOGJAM] and [NIST-SP-800-131Ar2]), this method
is considered insecure. This method is being moved from MUST to
SHOULD NOT instead of MUST NOT only to allow a transition time to get
off of it. There are many old implementations out there that may
still need to use this key exchange; it should be removed from server
implementations as quickly as possible.
3.6. diffie-hellman-group14-sha1
This method uses [RFC3526] group14 (a 2048-bit MODP group) which is
still a reasonable size. This key exchange group uses SHA-1 which
has security concerns [RFC6194]. However, this group is still strong
enough and is widely deployed. This method is being moved from MUST
to SHOULD to aid in transition to stronger SHA-2 based hashes. This
method will transition to SHOULD NOT when SHA-2 alternatives are more
generally available.
3.7. diffie-hellman-group14-sha256
This key exchange method is defined in [RFC8268] and uses the group14
(a 2048-bit MODP group) along with a SHA-2 (SHA2-256) hash as in
[RFC6234] for integrity. This represents the smallest FFC DH key
exchange method considered to be secure. It is a reasonably simple
transition to move from SHA-1 to SHA-2. This method SHOULD be
implemented.
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3.8. diffie-hellman-group15-sha512
This key exchange method is defined in [RFC8268] and uses group15 (a
3072-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. This modulus is the minimum required by
[CNSA-SUITE]. This method MAY be implemented.
3.9. diffie-hellman-group16-sha512
This key exchange method is defined in [RFC8268] and uses group16 (a
4096-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of FFC DH is well understood and
trusted. Adding larger modulus sizes and protecting with SHA2-512
should give enough head room to be ready for the next scare that
someone has pre-computed it. This method MAY be implemented.
3.10. diffie-hellman-group17-sha512
This key exchange method is defined in [RFC8268] and uses group17 (a
6144-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of this 6144-bit MODP group is
going to be slower than what may be desirable. It is provided to
help those who wish to avoid using ECC algorithms. This method MAY
be implemented.
3.11. diffie-hellman-group18-sha512
This key exchange method is defined in [RFC8268] and uses group18 (a
8192-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of this 8192-bit MODP group is
going to be slower than what may be desirable. It is provided to
help those who wish to avoid using ECC algorithms. This method MAY
be implemented.
3.12. ecdh-sha2-*
This namespace allows for other curves to be defined for the elliptic
curve Diffie Hellman key exchange. At present, there are three
members of this namespace They appear in [RFC5656] which are covered
below. This set of methods MAY be implemented.
3.12.1. ecdh-sha2-nistp256
This key exchange method is defined in [RFC5656]. ECDH methods are
often implemented because they are smaller and faster than using
large FFC DH parameters. However, given [CNSA-SUITE] and
[safe-curves], this curve may not be as useful and strong as desired
for handling TOP SECRET information for some applications. The SSH
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development community is divided on this and many implementations do
exist. If traditional ECDH key exchange methods are implemented,
then this method SHOULD be implemented.
It is advisable to match the ECDSA and ECDH algorithms to use the
same curve for both.
3.12.2. ecdh-sha2-nistp384
This key exchange method is defined in [RFC5656]. This ECDH method
should be implemented because it is smaller and faster than using
large FFC primes with traditional DH. Given [CNSA-SUITE], it is
considered good enough for TOP SECRET. However, given
[ECDSA-Nonce-Leak], care must be used when using this algorithm. If
traditional ECDH key exchange methods are implemented, then this
method SHOULD be implemented.
Concerns raised in [safe-curves] may mean that this algorithm will
need to be downgraded in the future along the other ECDSA NIST Prime
curves.
3.12.3. ecdh-sha2-nistp521
This key exchange method is defined in [RFC5656]. This ECDH method
may be implemented because it is smaller and faster than using large
FFC DH parameters. It is not listed in [CNSA-SUITE], so it is not
currently appropriate for TOP SECRET. It is possible that the
mismatch between the 521-bit key and the 512-bit hash could mean that
as many as nine bits of this key could be at risk of leaking if
appropriate padding measures are not taken. This method MAY be
implemented.
3.13. ecmqv-sha2
This key exchange method is defined in [RFC5656]. This method MAY be
implemented.
3.14. ext-info-c
This key exchange method is defined in [RFC8308]. This method is not
actually a key exchange, but proivides a method to provide support
for extensions to other Secure Shell negotations. Being able to
extend functionality is desirable, This method SHOULD be implemented.
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3.15. ext-info-s
This key exchange method is defined in [RFC8308]. This method is not
actually a key exchange, but proivides a method to provide support
for extensions to other Secure Shell negotations. Being able to
extend functionality is desirable, This method SHOULD be implemented.
3.16. gss-*
This family of key exchange methods is defined in [RFC4462] and
[RFC8732] for the GSS-API key exchange methods. The family of
methods MAY be implemented for those who need GSS-API methods.
3.16.1. gss-gex-sha1-*
This key exchange method is defined in [RFC4462]. This set of
ephemerally generated key exchange groups uses SHA-1 which has
security concerns [RFC6194]. This key exchange SHOULD NOT be used.
It is intended that it move to MUST NOT as soon as the majority of
server implementations no longer offer it. It should be removed from
server implementations as quickly as possible.
3.16.2. gss-group1-sha1-*
This key exchange method is defined in [RFC4462]. This method
suffers from the same problems of diffie-hellman-group1-sha1. It
uses [RFC7296] Oakley Group 2 (a 1024-bit MODP group) and SHA-1
[RFC3174]. Due to recent security concerns with SHA-1 [RFC6194] and
with MODP groups with less than 2048 bits (see [LOGJAM] and
[NIST-SP-800-131Ar2]), this method is considered insecure. This
method SHOULD NOT be implemented. It is intended that it move to
MUST NOT as soon as the majority of server implementations no longer
offer it. It should be removed from server implementations as
quickly as possible.
3.16.3. gss-group14-sha1-*
This key exchange method is defined in [RFC4462]. This generated key
exchange groups uses SHA-1 which has security concerns [RFC6194]. If
GSS-API key exchange methods are being used, then this one SHOULD be
implemented until such time as SHA-2 variants may be implemented and
deployed. This method will transition to SHOULD NOT when SHA-2
alternatives are more generally available. No other standard
indicated that this method was anything other than optional even
though it was implemented in all GSS-API systems. This method MAY be
implemented.
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3.16.4. gss-group14-sha256-*
This key exchange method is defined in [RFC8732]. This key exchange
uses the group14 (a 2048-bit MODP group) along with a SHA-2
(SHA2-256) hash. This represents the smallest Finite Field
Cryptography (FFC) Diffie-Hellman (DH) key exchange method considered
to be secure. It is a reasonably simple transition to move from
SHA-1 to SHA-2. If the GSS-API is to be used, then this method
SHOULD be implemented.
3.16.5. gss-group15-sha512-*
This key exchange method is defined in [RFC8732] and uses group15 (a
3072-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. This modulus is the minimum required by
[CNSA-SUITE]. If the GSS-API is to be used, then this method MAY be
implemented.
3.16.6. gss-group16-sha512-*
This key exchange method is defined in [RFC8732] and uses group16 (a
4096-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of FFC DH is well understood and
trusted. Adding larger modulus sizes and protecting with SHA2-512
should give enough head room to be ready for the next scare that
someone has pre-computed it. If the GSS-API is to be used, then this
method MAY be implemented.
3.16.7. gss-group17-sha512-*
This key exchange method is defined in [RFC8732]and uses group17 (a
6144-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of this 6144-bit MODP group is
going to be slower than what may be desirable. It is provided to
help those who wish to avoid using ECC algorithms. If the GSS-API is
to be used, then this method MAY be implemented.
3.16.8. gss-group18-sha512-*
This key exchange method is defined in [RFC8732] and uses group18 (a
8192-bit MODP group) along with a SHA-2 (SHA2-512) hash as in
[RFC6234] for integrity. The use of this 8192-bit MODP group is
going to be slower than what may be desirable. It is provided to
help those who wish to avoid using ECC algorithms. If the GSS-API is
to be used, then this method MAY be implemented.
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3.16.9. gss-nistp256-sha256-*
This key exchange method is defined in [RFC8732]. If the GSS-API is
to be used with ECC algorithms, then this method SHOULD be
implemented.
3.16.10. gss-nistp384-sha384-*
This key exchange method is defined in [RFC8732]. If the GSS-API is
to be used with ECC algorithms, then this method SHOULD be
implemented to permit TOP SECRET information to be communicated.
3.16.11. gss-nistp521-sha512-*
This key exchange method is defined in [RFC8732]. If the GSS-API is
to be used with ECC algorithms, then this method MAY be implemented.
3.16.12. gss-curve25519-sha256-*
This key exchange method is defined in [RFC8732]. If the GSS-API is
to be used with ECC algorithms, then this method SHOULD be
implemented.
3.16.13. gss-curve448-sha512-*
This key exchange method is defined in [RFC8732]. If the GSS-API is
to be used with ECC algorithms, then this method MAY be implemented.
3.17. rsa1024-sha1
This key exchange method is defined in [RFC4432]. The security of
RSA 1024-bit modulus keys is not good enough any longer per
[NIST-SP-800-131Ar2], an RSA key size should be a minimum of
2048-bits. This key exchange group uses SHA-1 which has security
concerns [RFC6194]. This method MUST NOT be implemented.
3.18. rsa2048-sha256
This key exchange method is defined in [RFC4432]. An RSA 2048-bit
modulus key with a SHA2-256 hash. At the present time, a 2048-bit
RSA key is considered to be suffiently strong in [NIST-SP-800-131Ar2]
to be permitted. In addition, the use of a SHA-2 hash as defined in
[RFC6234] is a good integrity measure. This method MAY be
implemented.
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4. Selecting an appropriate hashing algorithm
As may be seen from the above, the Key Exchange Methods area all
using either SHA256 or SHA512 with the exception of the ecdh-
sha2-nistp384 which uses SHA384.
The cited CNSA Suite specifies the use of SHA384 and says that SHA256
is no longer good enough for TOP SECRET. Nothing is said about the
use of SHA512. It may be that the internal state of 1024 bits in
both SHA384 and SHA512 makes the SHA384 more secure because it does
not leak an additional 128 bits of state. Of course, the use of
SHA384 also reduces the security strength to 384 bits instead of
being 512 bits. This seems to contradict the desire to double the
symmetric key strength in order to try to be safe from Post Quantum
Computing (PQC) attacks given a session key derived from the key
exchange will be limited to the security strength of the hash being
used.
The move away from SHA256 to SHA512 for the newer key exchange
methods is more to try to slow Grover's algorithm (a PQC attack)
slightly. It is also the case that SHA2-512 may, in many modern
CPUs, be implemented more efficiently using 64-bit arithmetic than
SHA256 which is faster on 32-bit CPUs. The selection of SHA384 vs
SHA512 is more about reducing the number of code point alternatives
to negotiate. There seemed to be consensus in favor of SHA2-512 over
SHA2-384 for key exchanges.
5. Summary Guidance for Key Exchange Method Names
The Implement column is the current recommendations of this RFC. Key
Exchange Method Names are listed alphabetically.
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Key Exchange Method Name Reference Implement
------------------------------------ --------- ----------
curve25519-sha256 RFC8731 SHOULD
curve448-sha512 RFC8731 MAY
diffie-hellman-group-exchange-sha1 RFC4419 SHOULD NOT
diffie-hellman-group-exchange-sha256 RFC4419 MAY
diffie-hellman-group1-sha1 RFC4253 SHOULD NOT
diffie-hellman-group14-sha1 RFC4253 SHOULD
diffie-hellman-group14-sha256 RFC8268 SHOULD
diffie-hellman-group15-sha512 RFC8268 MAY
diffie-hellman-group16-sha512 RFC8268 MAY
diffie-hellman-group17-sha512 RFC8268 MAY
diffie-hellman-group18-sha512 RFC8268 MAY
ecdh-sha2-* RFC5656 MAY
ecdh-sha2-nistp256 RFC5656 SHOULD
ecdh-sha2-nistp384 RFC5656 SHOULD
ecdh-sha2-nistp521 RFC5656 MAY
ecmqv-sha2 RFC5656 MAY
ext-info-c RFC8308 MAY
ext-info-s RFC8308 MAY
gss-* RFC4462 MAY
gss-curve25519-sha256-* RFC8732 SHOULD
gss-curve448-sha512-* RFC8732 MAY
gss-gex-sha1-* RFC4462 SHOULD NOT
gss-group1-sha1-* RFC4462 SHOULD NOT
gss-group14-sha256-* RFC8732 SHOULD
gss-group15-sha512-* RFC8732 MAY
gss-group16-sha512-* RFC8732 MAY
gss-group17-sha512-* RFC8732 MAY
gss-group18-sha512-* RFC8732 MAY
gss-nistp256-sha256-* RFC8732 SHOULD
gss-nistp384-sha384-* RFC8732 SHOULD
gss-nistp521-sha512-* RFC8732 MAY
rsa1024-sha1 RFC4432 MUST NOT
rsa2048-sha256 RFC4432 MAY
The full set of official [IANA-KEX] key algorithm method names not
otherwise mentioned in this document MAY be implemented.
The guidance of this document is that the SHA-1 algorithm hashing
SHOULD NOT be used. If it is used in implementations, it should only
be provided for backwards compatibility, should not be used in new
designs, and should be phased out of existing key exchanges as
quickly as possible because of its known weaknesses. Any key
exchange using SHA-1 should not be in a default key exchange list if
at all possible. If they are needed for backward compatibility, they
SHOULD be listed after all of the SHA-2 based key exchanges.
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The [RFC4253] MUST diffie-hellman-group14-sha1 method SHOULD be
retained for compatibility with older Secure Shell implementations.
It is intended that this key exchange method be phased out as soon as
possible. It SHOULD be listed after all possible SHA-2 based key
exchanges.
It is believed that all current SSH implementations should be able to
achieve an implementation of the "diffie-hellman-group14-sha256"
method. To that end, this is one method that MUST be implemented.
[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>]
6. Acknowledgements
Thanks to the following people for review and comments: Denis Bider,
Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto
Kouichi, Simon Josefsson, Dave Dugal, Daniel Migault, Anna Johnston,
and Tero Kivinen.
Thanks to the following people for code to implement interoperable
exchanges using some of these groups as found in an this draft:
Darren Tucker for OpenSSH and Matt Johnston for Dropbear. And thanks
to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh)
and Poderosa implementations also adopting new Diffie-Hellman groups
based on this draft.
7. Security Considerations
This SSH protocol provides a secure encrypted channel over an
insecure network. It performs server host authentication, key
exchange, encryption, and integrity protection. It also derives a
unique session ID that may be used by higher-level protocols.
Full security considerations for this protocol are provided in
[RFC4251].
It is desirable to deprecate or remove key exchange method name that
are considered weak. A key exchange method may be weak because too
few bits are used, or the hashing algorithm is considered too weak.
The diffie-hellman-group1-sha1 is being moved from MUST to MUST NOT.
This method used [RFC7296] Oakley Group 2 (a 1024-bit MODP group) and
SHA-1 [RFC3174]. Due to recent security concerns with SHA-1
[RFC6194] and with MODP groups with less than 2048 bits
[NIST-SP-800-131Ar2], this method is no longer considered secure.
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The United States Information Assurance Directorate (IAD) at the
National Security Agency (NSA) has published a FAQ
[MFQ-U-OO-815099-15] suggesting that the use of Elliptic Curve
Diffie-Hellman (ECDH) using the nistp256 curve and SHA-2 based hashes
less than SHA2-384 are no longer sufficient for transport of TOP
SECRET information. If your systems need to be concerned with TOP
SECRET information, then the guidance for supporting lesser security
strength key exchanges may be omitted for your implementations.
The MODP group14 is already required for 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. Small embedded applications may find this KEX
desirable to use.
The NSA Information Assurance Directorate (IAD) has also published
the Commercial National Security Algorithm Suite (CNSA Suite)
[CNSA-SUITE] in which the 3072-bit MODP Group 15 in [RFC3526] is
explicitly mentioned as the minimum modulus to protect TOP SECRET
communications.
It has been observed in [safe-curves] that the NIST Elliptic Curve
Prime Curves (P-256, P-384, and P-521) are perhaps not the best
available for Elliptic Curve Cryptography (ECC) Security. For this
reason, none of the [RFC5656] curves are mandatory to implement.
However, the requirement that "every compliant SSH ECC implementation
MUST implement ECDH key exchange" is now taken to mean that if ecdsa-
sha2-[identifier] is implemented, then ecdh-sha2-[identifier] MUST be
implemented.
In a Post-Quantum Computing (PQC) world, it will be desirable to use
larger cyclic subgroups. To do this using Elliptic Curve
Cryptography will require much larger prime base fields, greatly
reducing their efficiency. Finite Field based Cryptography already
requires large enough base fields to accommodate larger cyclic
subgroups. Until such time as a PQC method of key exchange is
developed and adopted, it may be desirable to generate new and larger
DH groups to avoid pre-calculation attacks that are provably not
backdoored.
8. IANA Considerations
IANA is requested to annotate entries in [IANA-KEX] which MUST NOT be
implemented as being deprecated by this document.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3526>.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250,
DOI 10.17487/RFC4250, January 2006,
<https://www.rfc-editor.org/info/rfc4250>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC8031] Nir, Y. and S. Josefsson, "Curve25519 and Curve448 for the
Internet Key Exchange Protocol Version 2 (IKEv2) Key
Agreement", RFC 8031, DOI 10.17487/RFC8031, December 2016,
<https://www.rfc-editor.org/info/rfc8031>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8268] Baushke, M., "More Modular Exponentiation (MODP) Diffie-
Hellman (DH) Key Exchange (KEX) Groups for Secure Shell
(SSH)", RFC 8268, DOI 10.17487/RFC8268, December 2017,
<https://www.rfc-editor.org/info/rfc8268>.
[RFC8270] Velvindron, L. and M. Baushke, "Increase the Secure Shell
Minimum Recommended Diffie-Hellman Modulus Size to 2048
Bits", RFC 8270, DOI 10.17487/RFC8270, December 2017,
<https://www.rfc-editor.org/info/rfc8270>.
[RFC8308] Bider, D., "Extension Negotiation in the Secure Shell
(SSH) Protocol", RFC 8308, DOI 10.17487/RFC8308, March
2018, <https://www.rfc-editor.org/info/rfc8308>.
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9.2. Informative References
[CNSA-SUITE]
NSA/IAD, "Commercial National Security Algorithm Suite",
September 2016, <https://apps.nsa.gov/iaarchive/programs/
iad-initiatives/cnsa-suite.cfm>.
[ECDSA-Nonce-Leak]
Mulder, E. D., Hutter, M., Marson, M. E., and P. Pearson,
"Using Bleichenbacher's Solution to the Hidden Number
Problem to Attack Nonce Leaks in 384-Bit ECDSA", IACR
Cryptology ePrint Archive 2013, August 2013,
<https://eprint.iacr.org/2013/346.pdf>.
[IANA-KEX]
Internet Assigned Numbers Authority (IANA), "Secure Shell
(SSH) Protocol Parameters: Key Exchange Method Names",
July 2020, <http://www.iana.org/assignments/ssh-
parameters/ssh-parameters.xhtml#ssh-parameters-16>.
[LOGJAM] Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
Green, M., Halderman, J., Heninger, N., Springall, D.,
Thome, E., Valenta, L., VanderSloot, B., Wustrow, E.,
Zanella-Beguelin, S., and P. Zimmermann, "Imperfect
Forward Secrecy: How Diffie-Hellman Fails in Practice",
ACM Conference on Computer and Communications Security
(CCS) 2015, 2015,
<https://weakdh.org/imperfect-forward-secrecy-ccs15.pdf>.
[MFQ-U-OO-815099-15]
NSA/CSS, "CNSA Suite and Quantum Computing FAQ", January
2016, <https://www.iad.gov/iad/library/ia-guidance/ia-
solutions-for-classified/algorithm-guidance/cnsa-suite-
and-quantum-computing-faq.cfm>.
[NIST-SP-800-131Ar2]
Barker, E. and A. Roginsky, "Transitions: Recommendation
for the Transitioning of the Use of Cryptographic
Algorithms and Key Lengths", NIST Special
Publication 800-131A Revision 2, March 2019,
<http://doi.org/10.6028/NIST.SP.800-131Ar2.pdf>.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001,
<https://www.rfc-editor.org/info/rfc3174>.
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[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC4419] Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
Group Exchange for the Secure Shell (SSH) Transport Layer
Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006,
<https://www.rfc-editor.org/info/rfc4419>.
[RFC4432] Harris, B., "RSA Key Exchange for the Secure Shell (SSH)
Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432,
March 2006, <https://www.rfc-editor.org/info/rfc4432>.
[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, <https://www.rfc-editor.org/info/rfc4462>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>.
[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,
<https://www.rfc-editor.org/info/rfc6194>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8731] Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
Shell (SSH) Key Exchange Method Using Curve25519 and
Curve448", RFC 8731, DOI 10.17487/RFC8731, February 2020,
<https://www.rfc-editor.org/info/rfc8731>.
[RFC8732] Sorce, S. and H. Kario, "Generic Security Service
Application Program Interface (GSS-API) Key Exchange with
SHA-2", RFC 8732, DOI 10.17487/RFC8732, February 2020,
<https://www.rfc-editor.org/info/rfc8732>.
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[safe-curves]
Bernstein, D. J. and T. Lange, "SafeCurves: choosing safe
curves for elliptic-curve cryptography.", February 2016,
<https://safecurves.cr.yp.to/>.
Author's Address
Mark D. Baushke
Juniper Networks, Inc.
Email: mdb@juniper.net
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