ssh: Support ECDSA keys.
* agent/command-ssh.c (SPEC_FLAG_IS_ECDSA): New.
(struct ssh_key_type_spec): Add fields CURVE_NAME and HASH_ALGO.
(ssh_key_types): Add types ecdsa-sha2-nistp{256,384,521}.
(ssh_signature_encoder_t): Add arg spec and adjust all callers.
(ssh_signature_encoder_ecdsa): New.
(sexp_key_construct, sexp_key_extract, ssh_receive_key)
(ssh_convert_key_to_blob): Support ecdsa.
(ssh_identifier_from_curve_name): New.
(ssh_send_key_public): Retrieve and pass the curve_name.
(key_secret_to_public): Ditto.
(data_sign): Add arg SPEC and change callers to pass it.
(ssh_handler_sign_request): Get the hash algo from SPEC.
* common/ssh-utils.c (get_fingerprint): Support ecdsa.
* agent/protect.c (protect_info): Add flag ECC_HACK.
(agent_protect): Allow the use of the "curve" parameter.
* agent/t-protect.c (test_agent_protect): Add a test case for ecdsa.
* agent/command-ssh.c (ssh_key_grip): Print a better error code.The 3 standard curves are now supported in gpg-agent's ssh-agent
protocol implementation. I tested this with all 3 curves and keys
generated by OpenSSH 5.9p1.
Using existing non-ssh generated keys will likely fail for now. To fix
this, the code should first undergo some more cleanup; then the fixes
are pretty straightforward. And yes, the data structures are way too
complicated.