Internet Engineering Task Force (IETF) D. Bider
Request for Comments: 8332 Bitvise Limited
Updates: 4252, 4253 March 2018
Category: Standards Track
Use of RSA Keys with SHA-256 and SHA-512
in the Secure Shell (SSH) Protocol
This memo updates RFCs 4252 and 4253 to define new public key
algorithms for use of RSA keys with SHA-256 and SHA-512 for server
and client authentication in SSH connections.
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.
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Table of Contents
1. Overview and Rationale . . . . . . . . . . . . . . . . . . . 31.1. Requirements Terminology . . . . . . . . . . . . . . . . 31.2. Wire Encoding Terminology . . . . . . . . . . . . . . . . 32. Public Key Format vs. Public Key Algorithm . . . . . . . . . 33. New RSA Public Key Algorithms . . . . . . . . . . . . . . . . 43.1. Use for Server Authentication . . . . . . . . . . . . . . 53.2. Use for Client Authentication . . . . . . . . . . . . . . 53.3. Discovery of Public Key Algorithms Supported by Servers . 64. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 65. Security Considerations . . . . . . . . . . . . . . . . . . . 75.1. Key Size and Signature Hash . . . . . . . . . . . . . . . 75.2. Transition . . . . . . . . . . . . . . . . . . . . . . . 75.3. PKCS #1 v1.5 Padding and Signature Verification . . . . . 76. References . . . . . . . . . . . . . . . . . . . . . . . . . 86.1. Normative References . . . . . . . . . . . . . . . . . . 86.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Overview and Rationale
Secure Shell (SSH) is a common protocol for secure communication on
the Internet. In [RFC4253], SSH originally defined the public key
algorithms "ssh-rsa" for server and client authentication using RSA
with SHA-1, and "ssh-dss" using 1024-bit DSA and SHA-1. These
algorithms are now considered deficient. For US government use, NIST
has disallowed 1024-bit RSA and DSA, and use of SHA-1 for signing
This memo updates RFCs 4252 and 4253 to define new public key
algorithms allowing for interoperable use of existing and new RSA
keys with SHA-256 and SHA-512.
1.1. Requirements Terminology
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.
1.2. Wire Encoding Terminology
The wire encoding types in this document -- "boolean", "byte",
"string", "mpint" -- have meanings as described in [RFC4251].
2. Public Key Format vs. Public Key Algorithm
In [RFC4252], the concept "public key algorithm" is used to establish
a relationship between one algorithm name, and:
A. procedures used to generate and validate a private/public
B. a format used to encode a public key; and
C. procedures used to calculate, encode, and verify a signature.
This document uses the term "public key format" to identify only A
and B in isolation. The term "public key algorithm" continues to
identify all three aspects -- A, B, and C.
3. New RSA Public Key Algorithms
This memo adopts the style and conventions of [RFC4253] in specifying
how use of a public key algorithm is indicated in SSH.
The following new public key algorithms are defined:
rsa-sha2-256 RECOMMENDED sign Raw RSA key
rsa-sha2-512 OPTIONAL sign Raw RSA key
These algorithms are suitable for use both in the SSH transport layer
[RFC4253] for server authentication and in the authentication layer
[RFC4252] for client authentication.
Since RSA keys are not dependent on the choice of hash function, the
new public key algorithms reuse the "ssh-rsa" public key format as
defined in [RFC4253]:
All aspects of the "ssh-rsa" format are kept, including the encoded
string "ssh-rsa". This allows existing RSA keys to be used with the
new public key algorithms, without requiring re-encoding or affecting
already trusted key fingerprints.
Signing and verifying using these algorithms is performed according
to the RSASSA-PKCS1-v1_5 scheme in [RFC8017] using SHA-2 [SHS] as
For the algorithm "rsa-sha2-256", the hash used is SHA-256.
For the algorithm "rsa-sha2-512", the hash used is SHA-512.
The resulting signature is encoded as follows:
string "rsa-sha2-256" / "rsa-sha2-512"
The value for 'rsa_signature_blob' is encoded as a string that
contains an octet string S (which is the output of RSASSA-PKCS1-v1_5)
and that has the same length (in octets) as the RSA modulus. When S
contains leading zeros, there exist signers that will send a shorter
encoding of S that omits them. A verifier MAY accept shorter
encodings of S with one or more leading zeros omitted.
3.1. Use for Server Authentication
To express support and preference for one or both of these algorithms
for server authentication, the SSH client or server includes one or
both algorithm names, "rsa-sha2-256" and/or "rsa-sha2-512", in the
name-list field "server_host_key_algorithms" in the SSH_MSG_KEXINIT
packet [RFC4253]. If one of the two host key algorithms is
negotiated, the server sends an "ssh-rsa" public key as part of the
negotiated key exchange method (e.g., in SSH_MSG_KEXDH_REPLY) and
encodes a signature with the appropriate signature algorithm name --
either "rsa-sha2-256" or "rsa-sha2-512".
3.2. Use for Client Authentication
To use this algorithm for client authentication, the SSH client sends
an SSH_MSG_USERAUTH_REQUEST message [RFC4252] encoding the
"publickey" method and encoding the string field "public key
algorithm name" with the value "rsa-sha2-256" or "rsa-sha2-512". The
"public key blob" field encodes the RSA public key using the
"ssh-rsa" public key format.
For example, as defined in [RFC4252] and [RFC4253], an SSH
"publickey" authentication request using an "rsa-sha2-512" signature
would be properly encoded as follows:
string user name
string service name
string public key blob:
If the client includes the signature field, the client MUST encode
the same algorithm name in the signature as in
SSH_MSG_USERAUTH_REQUEST -- either "rsa-sha2-256" or "rsa-sha2-512".
If a server receives a mismatching request, it MAY apply arbitrary
authentication penalties, including but not limited to authentication
failure or disconnect.
OpenSSH 7.2 (but not 7.2p2) incorrectly encodes the algorithm in the
signature as "ssh-rsa" when the algorithm in SSH_MSG_USERAUTH_REQUEST
is "rsa-sha2-256" or "rsa-sha2-512". In this case, the signature
does actually use either SHA-256 or SHA-512. A server MAY, but is
not required to, accept this variant or another variant that
corresponds to a good-faith implementation and is considered safe to
3.3. Discovery of Public Key Algorithms Supported by Servers
Implementation experience has shown that there are servers that apply
authentication penalties to clients attempting public key algorithms
that the SSH server does not support.
Servers that accept rsa-sha2-* signatures for client authentication
SHOULD implement the extension negotiation mechanism defined in
[RFC8308], including especially the "server-sig-algs" extension.
When authenticating with an RSA key against a server that does not
implement the "server-sig-algs" extension, clients MAY default to an
"ssh-rsa" signature to avoid authentication penalties. When the new
rsa-sha2-* algorithms have been sufficiently widely adopted to
warrant disabling "ssh-rsa", clients MAY default to one of the new
4. IANA Considerations
IANA has updated the "Secure Shell (SSH) Protocol Parameters"
registry, established with [RFC4250], to extend the table "Public Key
Algorithm Names" [IANA-PKA] as follows.
- To the immediate right of the column "Public Key Algorithm Name",
a new column has been added, titled "Public Key Format". For
existing entries, the column "Public Key Format" has been assigned
the same value as under "Public Key Algorithm Name".
- Immediately following the existing entry for "ssh-rsa", two
sibling entries have been added:
P. K. Alg. Name P. K. Format Reference Note
rsa-sha2-256 ssh-rsa RFC 8332 Section 3
rsa-sha2-512 ssh-rsa RFC 8332 Section 3
5. Security Considerations
The security considerations of [RFC4251] apply to this document.
5.1. Key Size and Signature Hash
The National Institute of Standards and Technology (NIST) Special
Publication 800-131A, Revision 1 [NIST.800-131A] disallows RSA and
DSA keys shorter than 2048 bits for US government use. The same
document disallows the SHA-1 hash function for digital signature
generation, except under NIST's protocol-specific guidance.
It is prudent to follow this advice also outside of US government
This document is based on the premise that RSA is used in
environments where a gradual, compatible transition to improved
algorithms will be better received than one that is abrupt and
incompatible. It advises that SSH implementations add support for
new RSA public key algorithms along with SSH_MSG_EXT_INFO and the
"server-sig-algs" extension to allow coexistence of new deployments
with older versions that support only "ssh-rsa". Nevertheless,
implementations SHOULD start to disable "ssh-rsa" in their default
configurations as soon as the implementers believe that new RSA
signature algorithms have been widely adopted.
5.3. PKCS #1 v1.5 Padding and Signature Verification
This document prescribes RSASSA-PKCS1-v1_5 signature padding because:
(1) RSASSA-PSS is not universally available to all implementations;
(2) PKCS #1 v1.5 is widely supported in existing SSH
(3) PKCS #1 v1.5 is not known to be insecure for use in this scheme.
Implementers are advised that a signature with RSASSA-PKCS1-v1_5
padding MUST NOT be verified by applying the RSA key to the
signature, and then parsing the output to extract the hash. This may
give an attacker opportunities to exploit flaws in the parsing and
vary the encoding. Verifiers MUST instead apply RSASSA-PKCS1-v1_5
padding to the expected hash, then compare the encoded bytes with the
output of the RSA operation.