| Internet-Draft | KEX Method Updates/Recommendations for S | March 2021 |
| Baushke | Expires 18 September 2021 | [Page] |
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, RFC 4253, RFC 4432, and RFC 4462.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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 recommending some that SHOULD and one that MUST be adopted.¶
This document updates [RFC4250] [RFC4253] [RFC4432] [RFC4462] by changing the requirement level ("MUST" moving to "SHOULD" or "MAY" or "SHOULD NOT", and "MAY" moving to "MUST" or "SHOULD" or "SHOULD NOT" or "MUST NOT") of various key exchange mechanisms.¶
[RFC4253] section 7.2 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.¶
It is good to try to match the security strength of the public key exchange algorithm with 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 which appear to be present in most SSH implementations.¶
| 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 |
The following subsections describe how to select each component of the key exchange.¶
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.¶
At present, the attacks against SHA-1 are collision attacks that usually rely on human help, rather than a pre-image attack. SHA-1 resistance against 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 [TRANS-COLL]. This paper shows that the man in the middle 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 is is desirable that any key exchanging 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 it be listed 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 that should be used for key exchange methods is SHA2-256.¶
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 for the hash algorithm.¶
Table 2 provides a summary of security strength for hashing functions.¶
| Hash Name | Estimated Security Strength |
|---|---|
| sha1 | 80 bits (before attacks) |
| sha256 | 128 bits |
| sha384 | 192 bits |
| sha512 | 256 bits |
SSH uses mathematically hard problems for doing key exchanges:¶
It is desirable for 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 to select a minimum of 112 bits of security strength 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 MODP group, the ECC curve size, or the RSA key length, the more computation power will be required to perform the key exchange.¶
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 which 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 and not including the hashing algorithm used. The hashing algorithm defined 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 |
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. [RFC3526] section 1 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 |
The minimum MODP group is the 2048-bit MODP group14. When used with sha1, this group provides approximately 80 bits of security. When used with sha256, 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 at 112 bits of security.¶
A 3072-bit MODP group with sha256 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.¶
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.¶
| Key Exchange Method | Estimated Security Strength |
|---|---|
| rsa1024-sha1 | 80 bits |
| rsa2048-sha256 | 112 bits |
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.¶
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, MAY, 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.¶
The 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; curve25519, curve448, the NIST prime curves (nistp256, nistp384, nistp521) as well as other curves allowed for by [RFC5656] section 6. There are GSSAPI forms for these curves as well which have a 'gss-' prefix.¶
Curve25519 is efficient on a wide range of architectures with properties that allow higher performance implementations compared to traditional elliptic curves. The use of hash SHA2-256 (also known as SHA-256 and sha256) as defined in [RFC6234] for integrity is a reasonable one for both the KDF and integrity for use with 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 |
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] for integrity is a reasonable one for both the KDF and integrity for use with 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 |
The ecdh-sha2-* name-space allows for both the named NIST prime curves (nistp256, nistp384, nistp521) as well as other curves to be defined for the Elliptic-curve Diffie-Hellman key exchange. At the time of this writing, there are three named curves in this name-space which SHOULD be supported. They appear in [RFC5656] in section 10.1 ("Required Curves"). If implemented, the named curves SHOULD always be enabled unless specifically disabled by local security policy. In [RFC5656], section 6.1, the method to name other ECDH curves using OIDs is specified. These other curves MAY be implemented.¶
The GSS-API name-space 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 |
It is advisable to match the ECDSA and ECDH algorithms to use the same curve for both to maintain the same security strength in the connection.¶
This random selection from a set of pre-generated moduli for key exchange uses SHA2-256 as defined in [RFC4419]. [RFC8270] mandates that implementations avoid any MODP group whose modulus size is 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. Otherwise, the parameter set may leak one bit of the shared secret. The diffie-hellman-group-exchange-sha1 uses SHA-1 which is being deprecated. This key exchange SHOULD NOT be used. The diffie-hellman-group-exchange-sha256 uses SHA2-256 which is reasonable for MODP groups less than 4K bits. The diffie-hellman-group-exchange-sha256 key exchange MAY be used.¶
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 |
The diffie-hellman-group14-sha256 key exchange method is defined in [RFC8268] and represents a key exchange which 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. Given that diffie-hellman-group14-sha1 is being removed from 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 below 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 |
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 provide a summary of the guidance for IFC key exchanges.¶
| Key Exchange Method Name | Guidance |
|---|---|
| rsa1024-sha1 | MUST NOT |
| rsa2048-sha256 | MAY |
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 exchanges 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 laying 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 which uses the SHA-3 family of Secure Hashing functions or the Extendable Output Functions.¶
Prior to the changes made by this document, diffie-hellman-group1-sha1 and diffie-hellman-group14-sha1 were mandatory to implement (MTI). diffie-hellman-group14-sha1 is the stronger of the two. Group14 (a 2048-bit MODP group) is defined in [RFC3526]. The group1 MODP group with approximately 80 bits of security is too weak to be retained. However, rather than jumping from the MTI 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 sha1 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. Transitioning from MTI to one that provides for continued use with the expectation of deprecating or disallowing it in the future was able to find consensus. 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, diffie-hellman-group-exchange-sha1, gss-gex-sha1-*, and gss-group1-sha1-* key exchanges SHOULD NOT be implemented.¶
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.¶
The Implement column is the current recommendations of this RFC. Table 12 provides the existing key exchange method names listed alphabetically.¶
| Key Exchange Method Name | Reference | Previous Recommendation | RFCxxxxx Implement |
|---|---|---|---|
| curve25519-sha256 | RFC8731 | none | SHOULD |
| curve448-sha512 | RFC8731 | none | MAY |
| diffie-hellman-group-exchange-sha1 | RFC4419 RFC8270 | none | SHOULD NOT |
| diffie-hellman-group-exchange-sha256 | RFC4419 RFC8720 | none | MAY |
| diffie-hellman-group1-sha1 | RFC4253 | MUST | SHOULD NOT |
| diffie-hellman-group14-sha1 | RFC4253 | MUST | MAY |
| diffie-hellman-group14-sha256 | RFC8268 | none | MUST |
| diffie-hellman-group15-sha512 | RFC8268 | none | MAY |
| diffie-hellman-group16-sha512 | RFC8268 | none | SHOULD |
| diffie-hellman-group17-sha512 | RFC8268 | none | MAY |
| diffie-hellman-group18-sha512 | RFC8268 | none | MAY |
| 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 | none | MAY |
| gss-curve25519-sha256-* | RFC8732 | SHOULD | SHOULD |
| gss-curve448-sha512-* | RFC8732 | MAY | MAY |
| gss-gex-sha1-* | RFC4462 | none | SHOULD NOT |
| gss-group1-sha1-* | RFC4462 | none | SHOULD NOT |
| gss-group14-sha256-* | RFC8732 | none | SHOULD |
| gss-group15-sha512-* | RFC8732 | none | MAY |
| gss-group16-sha512-* | RFC8732 | none | MAY |
| gss-group17-sha512-* | RFC8732 | none | MAY |
| gss-group18-sha512-* | RFC8732 | none | MAY |
| gss-nistp256-sha256-* | RFC8732 | none | SHOULD |
| gss-nistp384-sha384-* | RFC8732 | none | SHOULD |
| gss-nistp521-sha512-* | RFC8732 | none | SHOULD |
| rsa1024-sha1 | RFC4432 | none | MUST NOT |
| rsa2048-sha256 | RFC4432 | none | MAY |
The full set of official [IANA-KEX] key algorithm method names not otherwise mentioned in this document MAY be implemented.¶
[TO BE REMOVED: This registration should take place at the following location URL: http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ssh-parameters-16 It is hoped that the Table 12 in section 4 of this draft provide guidance information to be merged into the IANA ssh-parameters-16 table. Future RFCs may update the these Implementation Guidance notations. ]¶
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 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.¶
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]. In addition, the security considerations provided in [RFC4432]. 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 in 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.¶
At this time, the 1024-bit MODP group used by diffie-hellman-group1-sha1 is too small for the symmetric ciphers used in SSH.¶
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 as a last resort in key exchange negotiations. All key exchanges methods using the SHA-1 hash are to be considered as deprecated.¶
IANA is requested to annotate entries in [IANA-KEX] with the suggested implementation guidance provided in section 4 "Summary Guidance for Key Exchange Method Names Implementation" in this document. A summary may be found in Table 12 in section 4. The entry with "MUST NOT" should be considered disallowed. An entry with "SHOULD NOT" is deprecated and may be disallowed in the future.¶