On Sun, 7 Feb 2021, Brian Candler wrote: > Does the ssh-agent protocol allow adding a certificate for a private key > which it already has? The idea is to issue a certificate for a key the > agent already has, to avoid the entropy drain of generating a new key. > > https://tools.ietf.org/html/draft-miller-ssh-agent-04 shows private > keys, and doesn't mention certificates at all. However it does say: > > "Typically only the public components of any keys supported on a > hardware token will be loaded into an agent" - which suggests that the > SSH_AGENTC_ADD_IDENTITY message might be able to carry only the public > parts of a key. > > https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.certkeys > defines new *public* key formats for certificates - they don't contain > the private key components as far as I can see. > > However, looking at the Go ssh-agent client, it inserts a private key > and certificate in a single SSH_AGENTC_ADD_IDENTITY or > SSH_AGENTC_ADD_ID_CONSTRAINED message: > > https://github.com/golang/crypto/blob/master/ssh/agent/client.go#L664 > > (and I haven't been able to find documentation which defines that > private key + certificate message format). > > So basically: can I send just a certificate to ssh-agent? And if so, > how is that done? Yes, it is possible but poorly documented (patches welcome as always). The format for encoding a certificate with private key is is roughly {cert, private fields}. See sshkey.c:sshkey_private_serialize_opt() for the actual code, but it's basically the following, where "certificate blob" is the entire public certificate key. RSA string "ssh-rsa-cert-v01@xxxxxxxxxxx" string certificate blob mpint rsa_n mpint rsa_e mpint rsa_d mpint rsa_iqmp mpint rsa_p mpint rsa_q DSA string "ssh-dsa-cert-v01@xxxxxxxxxxx" string certificate blob mpint dsa_priv ECDSA string "ecdsa-sha2-nistp256-cert-v01@xxxxxxxxxxx" / "ecdsa-sha2-nistp384-cert-v01@xxxxxxxxxxx" / "ecdsa-sha2-nistp521-cert-v01@xxxxxxxxxxx" string certificate blob mpint ecdsa_priv ED25519 string "ssh-ed25519-cert-v01@xxxxxxxxxxx" string certificate blob string ed25519_pubkey string ed25519_privkey ECDSA/FIDO string "sk-ecdsa-sha2-nistp256-cert-v01@xxxxxxxxxxx" string certificate blob string sk_application uint8 sk_flags string sk_key_handle string sk_reserved ED25519/FIDO string "sk-ssh-ed25519-cert-v01@xxxxxxxxxxx" string certificate blob string ed25519_pubkey string sk_application uint8 sk_flags string sk_key_handle string sk_reserved Note in ED25519 and FIDO key types there are some redundant fields between the cert blob and the subsequent fields. _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
