Internet Engineering Task Force (IETF) M. Baushke

Internet Engineering Task Force (IETF) M. Baushke
Request for Comments: 9142 January 2022
Updates: 4250, 4253, 4432, 4462
Category: Standards Track
ISSN: 2070-1721

Key Exchange (KEX) Method Updates and Recommendations for Secure Shell
(SSH)

Abstract

This document updates the recommended set of key exchange methods for
use in the Secure Shell (SSH) protocol to meet evolving needs for
stronger security. It updates RFCs 4250, 4253, 4432, and 4462.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9142.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Overview and Rationale
1.1. Selecting an Appropriate Hashing Algorithm
1.2. Selecting an Appropriate Public Key Algorithm
1.2.1. Elliptic Curve Cryptography (ECC)
1.2.2. Finite Field Cryptography (FFC)
1.2.3. Integer Factorization Cryptography (IFC)
2. Requirements Language
3. Key Exchange Methods
3.1. Elliptic Curve Cryptography (ECC)
3.1.1. curve25519-sha256 and gss-curve25519-sha256-*
3.1.2. curve448-sha512 and gss-curve448-sha512-*
3.1.3. ecdh-*, ecmqv-sha2, and gss-nistp*
3.2. Finite Field Cryptography (FFC)
3.2.1. FFC Diffie-Hellman Using Generated MODP Groups
3.2.2. FFC Diffie-Hellman Using Named MODP Groups
3.3. Integer Factorization Cryptography (IFC)
3.4. KDFs and Integrity Hashing
3.5. Secure Shell Extension Negotiation
4. Summary Guidance for Implementation of Key Exchange Method
Names
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Author's Address

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
(KEX) 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 or disallowed as well as to recommend some that SHOULD and
one that MUST be adopted.

This document updates [RFC4250], [RFC4253], [RFC4432], and [RFC4462]
by changing the requirement level ("MUST" moving to "SHOULD", "MAY",
or "SHOULD NOT", and "MAY" moving to "MUST", "SHOULD", "SHOULD NOT",
or "MUST NOT") of various key exchange mechanisms. Some
recommendations will be unchanged but are included for completeness.

Section 7.2 of [RFC4253] says the following:

| The key exchange produces two values: a shared secret K, and an
| exchange hash H. Encryption and authentication keys are derived
| from these. The exchange hash H from the first key exchange is
| additionally used as the session identifier, which is a unique
| identifier for this connection. It is used by authentication
| methods as a part of the data that is signed as a proof of
| possession of a private key. Once computed, the session
| identifier is not changed, even if keys are later re-exchanged.

The security strength of the public key exchange algorithm and the
hash used in the Key Derivation Function (KDF) both impact the
security of the shared secret K being used.

The hashing algorithms used by key exchange methods described in this
document are: sha1, sha256, sha384, and sha512. In many cases, the
hash name is explicitly appended to the public key exchange algorithm
name. However, some of them are implicit and defined in the RFC that
defines the key exchange algorithm name.

Various RFCs use different spellings and capitalizations for the
hashing function and encryption function names. For the purpose of
this document, the following are equivalent names: sha1, SHA1, and
SHA-1; sha256, SHA256, SHA-256, and SHA2-256; sha384, SHA384, SHA-
384, and SHA2-384; and sha512, SHA512, SHA-512, and SHA2-512.

For the purpose of this document, the following are equivalent:
aes128, AES128, AES-128; aes192, AES192, and AES-192; and aes256,
AES256, and AES-256.

It is good to try to match the security strength of the public key
exchange algorithm with the security strength of the symmetric
cipher.

There are many possible symmetric ciphers available with multiple
modes. The list in Table 1 is intended as a representative sample of
those that appear to be present in most SSH implementations. The
security strength estimates are generally available in [RFC4086] for
triple-DES and AES, as well as Section 5.6.1.1 of
[NIST.SP.800-57pt1r5].

+========================+=============================+
| Cipher Name (modes) | Estimated Security Strength |
+========================+=============================+
| 3des (cbc) | 112 bits |
+------------------------+-----------------------------+
| aes128 (cbc, ctr, gcm) | 128 bits |
+------------------------+-----------------------------+
| aes192 (cbc, ctr, gcm) | 192 bits |
+------------------------+-----------------------------+
| aes256 (cbc, ctr, gcm) | 256 bits |
+------------------------+-----------------------------+

Table 1: Symmetric Cipher Security Strengths

The following subsections describe how to select each component of
the key exchange.

1.1. Selecting an Appropriate Hashing Algorithm

The SHA-1 hash is in the process of being deprecated for many
reasons.

There have been attacks against SHA-1, and it is no longer strong
enough for SSH security requirements. Therefore, it is desirable to
move away from using it before attacks become more serious.

The SHA-1 hash provides for approximately 80 bits of security
strength. This means that the shared key being used has at most 80
bits of security strength, which may not be sufficient for most
users.

For purposes of key exchange methods, attacks against SHA-1 are
collision attacks that usually rely on human help rather than a pre-
image attack. The SHA-1 hash resistance against a second pre-image
is still at 160 bits, but SSH does not depend on second pre-image
resistance but rather on chosen-prefix collision resistance.

Transcript Collision attacks are documented in [TRANSCRIPTION]. This
paper shows that an on-path attacker does not tamper with the Diffie-
Hellman values and does not know the connection keys. The attack
could be used to tamper with both I_C and I_S (as defined in
Section 7.3 of [RFC4253]) and might potentially be able to downgrade
the negotiated ciphersuite to a weak cryptographic algorithm that the
attacker knows how to break.

These attacks are still computationally very difficult to perform,
but it is desirable that any key exchange using SHA-1 be phased out
as soon as possible.

If there is a need for using SHA-1 in a key exchange for
compatibility, it would be desirable to list it last in the
preference list of key exchanges.

Use of the SHA-2 family of hashes found in [RFC6234] rather than the
SHA-1 hash is strongly advised.

When it comes to the SHA-2 family of secure hashing functions,
SHA2-256 has 128 bits of security strength; SHA2-384 has 192 bits of
security strength; and SHA2-512 has 256 bits of security strength.
It is suggested that the minimum secure hashing function used for key
exchange methods should be SHA2-256 with 128 bits of security
strength. Other hashing functions may also have the same number of
bits of security strength, but none are as yet defined in any RFC for
use in a KEX for SSH.

To avoid combinatorial explosion of key exchange names, newer key
exchanges are generally restricted to *-sha256 and *-sha512. The
exceptions are ecdh-sha2-nistp384 and gss-nistp384-sha384-*, which
are defined to use SHA2-384 (also known as SHA-384) defined in
[RFC6234] for the hash algorithm.

Table 2 provides a summary of security strength for hashing functions
for collision resistance. You may consult [NIST.SP.800-107r1] for
more information on hash algorithm security strength.

+===========+=============================+
| Hash Name | Estimated Security Strength |
+===========+=============================+
| sha1 | 80 bits (before attacks) |
+-----------+-----------------------------+
| sha256 | 128 bits |
+-----------+-----------------------------+
| sha384 | 192 bits |
+-----------+-----------------------------+
| sha512 | 256 bits |
+-----------+-----------------------------+

Table 2: Hashing Function Security
Strengths

1.2. Selecting an Appropriate Public Key Algorithm

SSH uses mathematically hard problems for doing key exchanges:

* Elliptic Curve Cryptography (ECC) has families of curves for key
exchange methods for SSH. NIST prime curves with names and other
curves are available using an object identifier (OID) with
Elliptic Curve Diffie-Hellman (ECDH) via [RFC5656]. Curve25519
and curve448 key exchanges are used with ECDH via [RFC8731].

* Finite Field Cryptography (FFC) is used for Diffie-Hellman (DH)
key exchange with "safe primes" either from a specified list found
in [RFC3526] or generated dynamically via [RFC4419] as updated by
[RFC8270].

* Integer Factorization Cryptography (IFC) using the RSA algorithm
is provided for in [RFC4432].

It is desirable that the security strength of the key exchange be
chosen to be comparable with the security strength of the other
elements of the SSH handshake. Attackers can target the weakest
element of the SSH handshake.

It is desirable that a minimum of 112 bits of security strength be
selected to match the weakest of the symmetric cipher (3des-cbc)
available. Based on implementer security needs, a stronger minimum
may be desired.

The larger the Modular Exponentiation (MODP) group, the ECC curve
size, or the RSA key length, the more computation power will be
required to perform the key exchange.

1.2.1. Elliptic Curve Cryptography (ECC)

For ECC, across all of the named curves, the minimum security
strength is approximately 128 bits. The [RFC5656] key exchanges for
the named curves use a hashing function with a matching security
strength. Likewise, the [RFC8731] key exchanges use a hashing
function that has more security strength than the curves. The
minimum strength will be the security strength of the curve. Table 3
contains a breakdown of just the ECC security strength by curve name;
it does include the hashing algorithm used. The curve25519 and
curve488 security-level numbers are in [RFC7748]. The nistp256,
nistp384, and nistp521 (NIST prime curves) are provided in [RFC5656].
The hashing algorithm designated for use with the individual curves
have approximately the same number of bits of security as the named
curve.

+============+=============================+
| Curve Name | Estimated Security Strength |
+============+=============================+
| nistp256 | 128 bits |
+------------+-----------------------------+
| nistp384 | 192 bits |
+------------+-----------------------------+
| nistp521 | 512 bits |
+------------+-----------------------------+
| curve25519 | 128 bits |
+------------+-----------------------------+
| curve448 | 224 bits |
+------------+-----------------------------+

Table 3: ECC Security Strengths

1.2.2. Finite Field Cryptography (FFC)

For FFC, it is recommended to use a modulus with a minimum of 2048
bits (approximately 112 bits of security strength) with a hash that
has at least as many bits of security as the FFC. The security
strength of the FFC and the hash together will be the minimum of
those two values. This is sufficient to provide a consistent
security strength for the 3des-cbc cipher. Section 1 of [RFC3526]
notes that the Advanced Encryption Standard (AES) cipher, which has
more strength, needs stronger groups. For the 128-bit AES, we need
about a 3200-bit group. The 192- and 256-bit keys would need groups
that are about 8000 and 15400 bits, respectively. Table 4 provides
the security strength of the MODP group. When paired with a hashing
algorithm, the security strength will be the minimum of the two
algorithms.

+==================+=============================+============+
| Prime Field Size | Estimated Security Strength | Example |
| | | MODP Group |
+==================+=============================+============+
| 2048-bit | 112 bits | group14 |
+------------------+-----------------------------+------------+
| 3072-bit | 128 bits | group15 |
+------------------+-----------------------------+------------+
| 4096-bit | 152 bits | group16 |
+------------------+-----------------------------+------------+
| 6144-bit | 176 bits | group17 |
+------------------+-----------------------------+------------+
| 8192-bit | 200 bits | group18 |
+------------------+-----------------------------+------------+

Table 4: FFC MODP Security Strengths

The minimum MODP group is the 2048-bit MODP group14. When used with
a SHA-1 hash, this group provides approximately 80 bits of security.
When used with a SHA2-256 hash, this group provides approximately 112
bits of security. The 3des-cbc cipher itself provides at most 112
bits of security, so the group14-sha256 key exchanges is sufficient
to keep all of the 3des-cbc key, for 112 bits of security.

A 3072-bit MODP group when used with a SHA2-256 hash will provide
approximately 128 bits of security. This is desirable when using a
cipher such as aes128 or chacha20-poly1305 that provides
approximately 128 bits of security.

The 8192-bit group18 MODP group when used with sha512 provides
approximately 200 bits of security, which is sufficient to protect
aes192 with 192 bits of security.

1.2.3. Integer Factorization Cryptography (IFC)

The only IFC algorithm for key exchange is the RSA algorithm
specified in [RFC4432]. RSA 1024-bit keys have approximately 80 bits
of security strength. RSA 2048-bit keys have approximately 112 bits
of security strength. It is worth noting that the IFC types of key
exchange do not provide Forward Secrecy, which both FFC and ECC do
provide.

In order to match the 112 bits of security strength needed for 3des-
cbc, an RSA 2048-bit key matches the security strength. The use of a
SHA-2 family hash with RSA 2048-bit keys has sufficient security to
match the 3des-cbc symmetric cipher. The rsa1024-sha1 key exchange
has approximately 80 bits of security strength and is not desirable.

Table 5 summarizes the security strengths of these key exchanges
without including the hashing algorithm strength. Guidance for these
strengths can be found in Section 5.6.1.1 of [NIST.SP.800-57pt1r5].

+=====================+=============================+
| Key Exchange Method | Estimated Security Strength |
+=====================+=============================+
| rsa1024-sha1 | 80 bits |
+---------------------+-----------------------------+
| rsa2048-sha256 | 112 bits |
+---------------------+-----------------------------+

Table 5: IFC Security Strengths

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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Key Exchange Methods

This document 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], [RFC8308], [RFC8731], and [RFC8732]) and provides a
suggested suitability for implementation of MUST, SHOULD, MAY, SHOULD
NOT, and MUST NOT. Any method not explicitly listed MAY be
implemented.

Section 7.2 of [RFC4253] defines the generation of a shared secret K
(really the output of the KDF) and an exchange key hash H. Each key
exchange method uses a specified HASH function, which must be the
same for both key exchange and Key Derivation. H is used for key
exchange integrity across the SSH session as it is computed only
once. It is noted at the end of Section 7.2 of [RFC4253] that:

| This process will lose entropy if the amount of entropy in K is
| larger than the internal state size of HASH.

So, care must be taken that the hashing algorithm used is well chosen
("reasonable") for the key exchange algorithms being used.

This document provides guidance as to what key exchange algorithms
are to be considered for new or updated SSH implementations.

In general, key exchange methods that are considered "weak" are being
moved to either deprecated ("SHOULD NOT") or disallowed ("MUST NOT").
Methods that are newer or considered to be stronger usually require
more device resources than many administrators and/or developers need
are to be allowed ("MAY"). (Eventually, some of these methods could
be moved by consensus to "SHOULD" to increase interoperability and
security.) Methods that are not "weak" and have implementation
consensus are encouraged ("SHOULD"). There needs to be at least one
consensus method promoted to a status of mandatory to implement
(MTI). This should help to provide continued interoperability even
with the loss of one of the now disallowed MTI methods.

For this document, 112 bits of security strength is the minimum. Use
of either or both of SHA-1 and RSA 1024 bits at an approximate 80
bits of security fall below this minimum and should be deprecated and
moved to disallowed as quickly as possible in configured deployments
of SSH. It seems plausible that this minimum may be increased over
time, so authors and administrators may wish to prepare for a switch
to algorithms that provide more security strength.

3.1. Elliptic Curve Cryptography (ECC)

The Elliptic Curve (EC) key exchange algorithms used with SSH include
the ECDH and EC Menezes-Qu-Vanstone (ECMQV).

The ECC curves defined for the key exchange algorithms above include
the following: curve25519, curve448, the NIST prime curves (nistp256,
nistp384, and nistp521), as well as other curves allowed for by
Section 6 of [RFC5656]. There are key exchange mechanisms based on
the Generic Security Service Application Program Interface (GSS-API)
that use these curves as well that have a "gss-" prefix.

3.1.1. curve25519-sha256 and gss-curve25519-sha256-*

Curve25519 is efficient on a wide range of architectures with
properties that allow higher-performance implementations compared to
the patented elliptic curve parameters purchased by NIST for the
general public to use as described in [RFC5656]. The corresponding
key exchange methods use SHA2-256 (also known as SHA-256) defined in
[RFC6234]. SHA2-256 is a reasonable hash for use in both the KDF and
session integrity. It is reasonable for both gss and non-gss uses of
curve25519 key exchange methods. These key exchange methods are
described in [RFC8731] and [RFC8732] and are similar to the IKEv2 key
agreement described in [RFC8031]. The curve25519-sha256 key exchange
method has multiple implementations and SHOULD be implemented. The
gss-curve25519-sha256-* key exchange method SHOULD also be
implemented because it shares the same performance and security
characteristics as curve25519-sha256.

Table 6 contains a summary of the recommendations for
curve25519-based key exchanges.

+==========================+==========+
| Key Exchange Method Name | Guidance |
+==========================+==========+
| curve25519-sha256 | SHOULD |
+--------------------------+----------+
| gss-curve25519-sha256-* | SHOULD |
+--------------------------+----------+

Table 6: Curve25519 Implementation
Guidance

3.1.2. curve448-sha512 and gss-curve448-sha512-*

Curve448 provides more security strength than curve25519 at a higher
computational and bandwidth cost. The corresponding key exchange
methods use SHA2-512 (also known as SHA-512) defined in [RFC6234].
SHA2-512 is a reasonable hash for use in both the KDF and session
integrity. It is reasonable for both gss and non-gss uses of
curve448 key exchange methods. These key exchange methods are
described in [RFC8731] and [RFC8732] and are similar to the IKEv2 key
agreement described in [RFC8031]. The curve448-sha512 key exchange
method MAY be implemented. The gss-curve448-sha512-* key exchange
method MAY also be implemented because it shares the same performance
and security characteristics as curve448-sha512.

Table 7 contains a summary of the recommendations for curve448-based
key exchanges.

+==========================+==========+
| Key Exchange Method Name | Guidance |
+==========================+==========+
| curve448-sha512 | MAY |
+--------------------------+----------+
| gss-curve448-sha512-* | MAY |
+--------------------------+----------+

Table 7: Curve448 Implementation
Guidance

3.1.3. ecdh-*, ecmqv-sha2, and gss-nistp*

The ecdh-sha2-* namespace allows for both the named NIST prime curves
(nistp256, nistp384, and nistp521) as well as other curves to be
defined for the ECDH key exchange. At the time of this writing,
there are three named curves in this namespace that SHOULD be
supported. They appear in Section 10.1 of [RFC5656]. If
implemented, the named curves SHOULD always be enabled unless
specifically disabled by local security policy. In Section 6.1 of
[RFC5656], the method to name other ECDH curves using OIDs is
specified. These other curves MAY be implemented.

The GSS-API namespace with gss-nistp*-sha* mirrors the algorithms
used by ecdh-sha2-* names. They are described in [RFC8732].

ECDH reduces bandwidth of key exchanges compared to FFC DH at a
similar security strength.

Table 8 lists algorithms as "SHOULD" where implementations may be
more efficient or widely deployed. The items listed as "MAY" in
Table 8 are potentially less efficient.

+==========================+==========+
| Key Exchange Method Name | Guidance |
+==========================+==========+
| ecdh-sha2-* | MAY |
+--------------------------+----------+
| ecdh-sha2-nistp256 | SHOULD |
+--------------------------+----------+
| gss-nistp256-sha256-* | SHOULD |
+--------------------------+----------+
| ecdh-sha2-nistp384 | SHOULD |
+--------------------------+----------+
| gss-nistp384-sha384-* | SHOULD |
+--------------------------+----------+
| ecdh-sha2-nistp521 | SHOULD |
+--------------------------+----------+
| gss-nistp521-sha512-* | SHOULD |
+--------------------------+----------+
| ecmqv-sha2 | MAY |
+--------------------------+----------+

Table 8: ECDH Implementation Guidance

It is advisable to match the Elliptic Curve Digital Signature
Algorithm (ECDSA) and ECDH algorithm to use the same curve for both
to maintain the same security strength in the connection.

3.2. Finite Field Cryptography (FFC)

3.2.1. FFC Diffie-Hellman Using Generated MODP Groups

[RFC4419] defines two key exchange methods that use a random
selection from a set of pre-generated moduli for key exchange: the
diffie-hellman-group-exchange-sha1 method and the diffie-hellman-
group-exchange-sha256 method. Per [RFC8270], implementations SHOULD
use a MODP group whose modulus size is equal to or greater than 2048
bits. MODP groups with a modulus size less than 2048 bits are weak
and MUST NOT be used.

The diffie-hellman-group-exchange-sha1 key exchange method SHOULD NOT
be used. This method uses SHA-1, which is being deprecated.

The diffie-hellman-group-exchange-sha256 key exchange method MAY be
used. This method uses SHA2-256, which is reasonable for MODP groups
less than 4096 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. Otherwise, the parameter set may leak one bit of the shared
secret.

Table 9 provides a summary of the guidance for these exchanges.

+======================================+============+
| Key Exchange Method Name | Guidance |
+======================================+============+
| diffie-hellman-group-exchange-sha1 | SHOULD NOT |
+--------------------------------------+------------+
| diffie-hellman-group-exchange-sha256 | MAY |
+--------------------------------------+------------+

Table 9: FFC Generated MODP Group Implementation
Guidance

3.2.2. FFC Diffie-Hellman Using Named MODP Groups

The diffie-hellman-group14-sha256 key exchange method is defined in
[RFC8268] and represents a key exchange that has approximately 112
bits of security strength that matches 3des-cbc symmetric cipher
security strength. It is a reasonably simple transition from SHA-1
to SHA-2, and given that diffie-hellman-group14-sha1 and diffie-
hellman-group14-sha256 share a MODP group and only differ in the hash
function used for the KDF and integrity, it is a correspondingly
simple transition from implementing diffie-hellman-group14-sha1 to
implementing diffie-hellman-group14-sha256. Given that diffie-
hellman-group14-sha1 is being removed from mandatory to implement
(MTI) status, the diffie-hellman-group14-sha256 method MUST be
implemented. The rest of the FFC MODP group from [RFC8268] have a
larger number of security bits and are suitable for symmetric ciphers
that also have a similar number of security bits.

Table 10 provides explicit guidance by name.

+===============================+==========+
| Key Exchange Method Name | Guidance |
+===============================+==========+
| diffie-hellman-group14-sha256 | MUST |
+-------------------------------+----------+
| gss-group14-sha256-* | SHOULD |
+-------------------------------+----------+
| diffie-hellman-group15-sha512 | MAY |
+-------------------------------+----------+
| gss-group15-sha512-* | MAY |
+-------------------------------+----------+
| diffie-hellman-group16-sha512 | SHOULD |
+-------------------------------+----------+
| gss-group16-sha512-* | MAY |
+-------------------------------+----------+
| diffie-hellman-group17-sha512 | MAY |
+-------------------------------+----------+
| gss-group17-sha512-* | MAY |
+-------------------------------+----------+
| diffie-hellman-group18-sha512 | MAY |
+-------------------------------+----------+
| gss-group18-sha512-* | MAY |
+-------------------------------+----------+

Table 10: FFC Named Group Implementation
Guidance

3.3. Integer Factorization Cryptography (IFC)

The rsa1024-sha1 key exchange method is defined in [RFC4432] and uses
an RSA 1024-bit modulus with a SHA-1 hash. This key exchange does
NOT meet security requirements. This method MUST NOT be implemented.

The rsa2048-sha256 key exchange method is defined in [RFC4432] and
uses an RSA 2048-bit modulus with a SHA2-256 hash. This key exchange
meets 112-bit minimum security strength. This method MAY be
implemented.

Table 11 provides a summary of the guidance for IFC key exchanges.

+==========================+==========+
| Key Exchange Method Name | Guidance |
+==========================+==========+
| rsa1024-sha1 | MUST NOT |
+--------------------------+----------+
| rsa2048-sha256 | MAY |
+--------------------------+----------+

Table 11: IFC Implementation Guidance

3.4. KDFs and Integrity Hashing

The SHA-1 and SHA-2 family of hashing algorithms are combined with
the FFC, ECC, and IFC algorithms to comprise a key exchange method
name.

The selected hash algorithm is used both in the KDF as well as for
the integrity of the response.

All of the key exchange methods using the SHA-1 hashing algorithm
should be deprecated and phased out due to security concerns for SHA-
1, as documented in [RFC6194].

Unconditionally deprecating and/or disallowing SHA-1 everywhere will
hasten the day when it may be simply removed from implementations
completely. Leaving partially broken algorithms lying around is not
a good thing to do.

The SHA-2 family of hashes [RFC6234] is more secure than SHA-1. They
have been standardized for use in SSH with many of the currently
defined key exchanges.

Please note that at the present time, there is no key exchange method
for Secure Shell that uses the SHA-3 family of secure hashing
functions or the Extendable-Output Functions [NIST.FIPS.202].

Prior to the changes made by this document, diffie-hellman-
group1-sha1 and diffie-hellman-group14-sha1 were MTI. diffie-
hellman-group14-sha1 is the stronger of the two. Group14 (a 2048-bit
MODP group) is defined in Section 3 of [RFC3526]. The SSH group1 is
defined in Section 8.1 of [RFC4253] as using the Oakley Group 2
provided in Section 6.2 of [RFC2409] (a 1024-bit MODP group). This
group1 MODP group with approximately 80 bits of security is too weak
to be retained. However, rather than jumping from the MTI status to
making it disallowed, many implementers suggested that it should
transition to deprecated first and be disallowed at a later time.
The group14 MODP group using a SHA-1 hash for the KDF is not as weak
as the group1 MODP group. There are some legacy situations where it
will still provide administrators with value, such as small hardware
Internet of Things (IOT) devices that have insufficient compute and
memory resources to use larger MODP groups before a timeout of the
session occurs. There was consensus to transition from MTI to a
requirement status that provides for continued use with the
expectation that it would be deprecated or disallowed in the future.
Therefore, it is considered reasonable to retain the diffie-hellman-
group14-sha1 exchange for interoperability with legacy
implementations. The diffie-hellman-group14-sha1 key exchange MAY be
implemented, but should be put at the end of the list of negotiated
key exchanges.

The diffie-hellman-group1-sha1 and diffie-hellman-group-exchange-sha1
SHOULD NOT be implemented. The gss-group1-sha1-*, gss-
group14-sha1-*, and gss-gex-sha1-* key exchanges are already
specified as SHOULD NOT be implemented by [RFC8732].

3.5. Secure Shell Extension Negotiation

There are two methods, ext-info-c and ext-info-s, defined in
[RFC8308]. They provide a mechanism to support other Secure Shell
negotiations. Being able to extend functionality is desirable. Both
ext-info-c and ext-info-s SHOULD be implemented.

4. Summary Guidance for Implementation of Key Exchange Method Names

Table 12 provides the existing key exchange method names listed
alphabetically. The Implement column contains the current
recommendations of this RFC.

+=======================+============+================+===========+
| Key Exchange Method | Reference | Previous | RFC 9142 |
| Name | | Recommendation | Implement |
+=======================+============+================+===========+
| curve25519-sha256 | [RFC8731] | none | SHOULD |
+-----------------------+------------+----------------+-----------+
| curve448-sha512 | [RFC8731] | none | MAY |
+-----------------------+------------+----------------+-----------+
| diffie-hellman-group- | [RFC4419], | none | SHOULD |
| exchange-sha1 | [RFC8270] | | NOT |
+-----------------------+------------+----------------+-----------+
| diffie-hellman-group- | [RFC4419], | none | MAY |
| exchange-sha256 | [RFC8270] | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC4253] | MUST | SHOULD |
| group1-sha1 | | | NOT |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC4253] | MUST | MAY |
| group14-sha1 | | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC8268] | none | MUST |
| group14-sha256 | | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC8268] | none | MAY |
| group15-sha512 | | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC8268] | none | SHOULD |
| group16-sha512 | | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC8268] | none | MAY |
| group17-sha512 | | | |
+-----------------------+------------+----------------+-----------+
| diffie-hellman- | [RFC8268] | none | MAY |
| group18-sha512 | | | |
+-----------------------+------------+----------------+-----------+
| ecdh-sha2-* | [RFC5656] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| ecdh-sha2-nistp256 | [RFC5656] | MUST | SHOULD |
+-----------------------+------------+----------------+-----------+
| ecdh-sha2-nistp384 | [RFC5656] | MUST | SHOULD |
+-----------------------+------------+----------------+-----------+
| ecdh-sha2-nistp521 | [RFC5656] | MUST | SHOULD |
+-----------------------+------------+----------------+-----------+
| ecmqv-sha2 | [RFC5656] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| ext-info-c | [RFC8308] | SHOULD | SHOULD |
+-----------------------+------------+----------------+-----------+
| ext-info-s | [RFC8308] | SHOULD | SHOULD |
+-----------------------+------------+----------------+-----------+
| gss- | [RFC4462] | reserved | reserved |
+-----------------------+------------+----------------+-----------+
| gss- | [RFC8732] | SHOULD | SHOULD |
| curve25519-sha256-* | | | |
+-----------------------+------------+----------------+-----------+
| gss-curve448-sha512-* | [RFC8732] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| gss-gex-sha1-* | [RFC4462], | SHOULD NOT | SHOULD |
| | [RFC8732] | | NOT |
+-----------------------+------------+----------------+-----------+
| gss-group1-sha1-* | [RFC4462], | SHOULD NOT | SHOULD |
| | [RFC8732] | | NOT |
+-----------------------+------------+----------------+-----------+
| gss-group14-sha1-* | [RFC4462], | SHOULD NOT | SHOULD |
| | [RFC8732] | | NOT |
+-----------------------+------------+----------------+-----------+
| gss-group14-sha256-* | [RFC8732] | SHOULD | SHOULD |
+-----------------------+------------+----------------+-----------+
| gss-group15-sha512-* | [RFC8732] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| gss-group16-sha512-* | [RFC8732] | SHOULD | MAY |
+-----------------------+------------+----------------+-----------+
| gss-group17-sha512-* | [RFC8732] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| gss-group18-sha512-* | [RFC8732] | MAY | MAY |
+-----------------------+------------+----------------+-----------+
| gss-nistp256-sha256-* | [RFC8732] | SHOULD | SHOULD |
+-----------------------+------------+----------------+-----------+
| gss-nistp384-sha384-* | [RFC8732] | MAY | SHOULD |
+-----------------------+------------+----------------+-----------+
| gss-nistp521-sha512-* | [RFC8732] | MAY | SHOULD |
+-----------------------+------------+----------------+-----------+
| rsa1024-sha1 | [RFC4432] | MAY | MUST NOT |
+-----------------------+------------+----------------+-----------+
| rsa2048-sha256 | [RFC4432] | MAY | MAY |
+-----------------------+------------+----------------+-----------+

Table 12: IANA Guidance for Implementation of Key Exchange
Method Names

The full set of official [IANA-SSH] "Key Exchange Method Names" not
otherwise mentioned in this document MAY be implemented.

5. Security Considerations

This SSH protocol provides a secure encrypted channel over an
insecure network. It performs server host authentication, key
exchange, encryption, and integrity checks. It also derives a unique
session ID that may be used by higher-level protocols. The key
exchange itself generates a shared secret and uses the hash function
for both the KDF and integrity.

Full security considerations for this protocol are provided in
[RFC4251] and continue to apply. In addition, the security
considerations provided in [RFC4432] apply. Note that Forward
Secrecy is NOT available with the rsa1024-sha1 or rsa2048-sha256 key
exchanges.

It is desirable to deprecate or disallow key exchange methods that
are considered weak so they are not still actively in operation when
they are broken.

A key exchange method is considered weak when the security strength
is insufficient to match the symmetric cipher or the algorithm has
been broken.

The 1024-bit MODP group used by diffie-hellman-group1-sha1 is too
small for the symmetric ciphers used in SSH.

MODP groups with a modulus size less than 2048 bits are too small for
the symmetric ciphers used in SSH. If the diffie-hellman-group-
exchange-sha256 or diffie-hellman-group-exchange-sha1 key exchange
method is used, the modulus size of the MODP group used needs to be
at least 2048 bits.

At this time, the rsa1024-sha1 key exchange is too small for the
symmetric ciphers used in SSH.

The use of SHA-1 for use with any key exchange may not yet be
completely broken, but it is time to retire all uses of this
algorithm as soon as possible.

The diffie-hellman-group14-sha1 algorithm is not yet completely
deprecated. This is to provide a practical transition from the MTI
algorithms to a new one. However, it would be best to only be used
as a last resort in key exchange negotiations. All key exchange
methods using the SHA-1 hash are to be considered as deprecated.

6. IANA Considerations

IANA has added a new column to the "Key Exchange Method Names"
registry [IANA-SSH] with the heading "OK to Implement" and annotated
entries therein with the implementation guidance provided in
Section 4, "Summary Guidance for Implementation of Key Exchange
Method Names", in this document. IANA also added entries for ecdh-
sha2-nistp256, ecdh-sha2-nistp384, and ecdh-sha2-nistp521, and added
references to [RFC4462] and [RFC8732] for gss-gex-sha1-*, gss-
group1-sha1-*, gss-group14-sha1-*, diffie-hellman-group-exchange-
sha1, and diffie-hellman-group-exchange-sha256. A summary may be
found in Table 12 in Section 4. IANA has also included this document
as an additional registry reference for the suggested implementation
guidance provided in Section 4 of this document and added a note
indicating the following:

| OK to Implement guidance entries for registrations that pre-date
| [RFC9142] are found in Table 12 in Section 4 of [RFC9142].

Registry entries annotated with "MUST NOT" are considered disallowed.
Registry entries annotated with "SHOULD NOT" are deprecated and may
be disallowed in the future.

7. References

7.1. 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>.

[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>.

[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>.

[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>.

7.2. Informative References

[IANA-SSH] IANA, "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/>.

[NIST.FIPS.202]
National Institute of Standards and Technology, "SHA-3
Standard: Permutation-Based Hash and Extendable-Output
Functions", FIPS PUB 202, DOI 10.6028/NIST.FIPS.202,
August 2015, <https://doi.org/10.6028/NIST.FIPS.202>.

[NIST.SP.800-107r1]
Dang, Q., "Recommendation for applications using approved
hash algorithms", DOI 10.6028/NIST.SP.800-107r1, August
2012, <https://doi.org/10.6028/NIST.SP.800-107r1>.

[NIST.SP.800-57pt1r5]
Barker, E., "Recommendation for Key Management: Part 1 -
General", DOI 10.6028/NIST.SP.800-57pt1r5, May 2020,
<https://doi.org/10.6028/NIST.SP.800-57pt1r5>.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
<https://www.rfc-editor.org/info/rfc2409>.

[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>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[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>.

[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.

[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>.

[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>.

[TRANSCRIPTION]
Bhargavan, K. and G. Leurent, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
Network and Distributed System Security Symposium (NDSS),
DOI 10.14722/ndss.2016.23418, February 2016,
<https://doi.org/10.14722/ndss.2016.23418>.

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,
Tero Kivinen, and Travis Finkenauer.

Thanks to the following people for code to implement interoperable
exchanges using some of these groups as found in this document:
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 document.

Author's Address

Mark D. Baushke

Email: [email protected]