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6.6. Public Key Algorithms

6.6. Public Key Algorithms​

This protocol has been designed to operate with almost any public key format, encoding, and algorithm (signature and/or encryption).

There are several aspects that define a public key type:

  • Key format: how is the key encoded and how are certificates represented. The key blobs in this protocol MAY contain certificates in addition to keys.

  • Signature and/or encryption algorithms: Some key types may not support both signing and encryption. Key usage may also be restricted by policy statements (e.g., in certificates). In this case, different key types SHOULD be defined for the different policy alternatives.

  • Encoding of signatures and/or encrypted data: This includes but is not limited to padding, byte order, and data formats.

The following public key and/or certificate formats are currently defined:

ssh-dss           REQUIRED     sign   Raw DSS Key
ssh-rsa           RECOMMENDED  sign   Raw RSA Key
pgp-sign-rsa      OPTIONAL     sign   OpenPGP certificates (RSA key)
pgp-sign-dss      OPTIONAL     sign   OpenPGP certificates (DSS key)

Additional key types may be defined, as specified in [SSH-ARCH] and in [SSH-NUMBERS].

The key type MUST always be explicitly known (from algorithm negotiation or some other source). It is not normally included in the key blob.

Certificates and public keys are encoded as follows:

string    certificate or public key format identifier
byte[n]   key/certificate data

The certificate part may be a zero length string, but a public key is required. This is the public key that will be used for authentication. The certificate sequence contained in the certificate blob can be used to provide authorization.

Public key/certificate formats that do not explicitly specify a signature format identifier MUST use the public key/certificate format identifier as the signature identifier.

Signatures are encoded as follows:

string    signature format identifier (as specified by the public key/certificate format)
byte[n]   signature blob in format specific encoding

Detailed encoding specifications for "ssh-dss" and "ssh-rsa" key formats can be found in the complete specification documents.

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