I spent some time today implementing support for loading U2F keys into the SSH agent from my AsyncSSH library. I got it working, but along the way I ran into a few issues I wanted to report: First, it looks like the value of SSH_AGENT_CONSTRAIN_EXTENSION has changed from the value 3 defined at https://tools.ietf.org/html/draft-miller-ssh-agent-02 <https://tools.ietf.org/html/draft-miller-ssh-agent-02> to the value 255 now, and somewhere along the way the constraint SSH_AGENT_CONSTRAIN_MAXSIGN was defined to use the value 3. Second, https://raw.githubusercontent.com/openssh/openssh-portable/master/PROTOCOL.u2f <https://raw.githubusercontent.com/openssh/openssh-portable/master/PROTOCOL.u2f> documents the new extension for loading SK keys as: byte SSH_AGENT_CONSTRAIN_EXTENSION string sk@xxxxxxxxxxx string middleware path However, the current OpenSSH agent code actually expects to receive: byte SSH_AGENT_CONSTRAIN_EXTENSION string sk-provider@xxxxxxxxxxx string middleware path Also, this documentation doesn’t define the format of the key data sent to the agent for SK keys with certificates. Similar to plain ECDSA keys with certificates, the key data sent for ECDSA SK keys omits the curve_id and Q value of the ECDSA key that would normally be written out when serializing a local private key. So, the data sent to the agent for an ECDSA SK key with certificate looks like: string "sk-ecdsa-sha2-nistp256-cert-v01@xxxxxxxxxxx" string nonce string curve name ec_point Q string application uint64 serial uint32 type string key id string valid principals uint64 valid after uint64 valid before string critical options string extensions string reserved string signature key string signature string application uint8 flags string key_handle string reserved If the instant was to avoid duplicating what was already in the certificate, though, I’m not sure why “application” is sent twice. It seems like that should have been left out along with the curve_id and Q value, appending only the flags, key_handle, and reserved values from the private key at the end. In the case of Ed25519 SK keys with certificates, nothing was removed. There, the format appears to currently be just the normal encoding of the certificate followed by the normal encoding of the private key, repeating the public key value and the application: string "sk-ssh-ed25519-cert-v01@xxxxxxxxxxx" string nonce string public key string application uint64 serial uint32 type string key id string valid principals uint64 valid after uint64 valid before string critical options string extensions string reserved string signature key string signature string public key string application uint8 flags string key_handle string reserved I also noticed that https://raw.githubusercontent.com/openssh/openssh-portable/master/PROTOCOL.u2f <https://raw.githubusercontent.com/openssh/openssh-portable/master/PROTOCOL.u2f> incorrectly documents the flags value as being a uint32 in the Ed25519 SK private key encoding: string "sk-ssh-ed25519@xxxxxxxxxxx" string public key string application (user-specified, but typically "ssh:") uint32 flags string key_handle string reserved This should be a uint8 for the flags, matching the ECDSA case. -- Ron Frederick ronf@xxxxxxxxxxxxx _______________________________________________ openssh-unix-dev mailing list openssh-unix-dev@xxxxxxxxxxx https://lists.mindrot.org/mailman/listinfo/openssh-unix-dev
