upstream: changes to support FIDO attestation

Allow writing to disk the attestation certificate that is generated by
the FIDO token at key enrollment time. These certificates may be used
by an out-of-band workflow to prove that a particular key is held in
trustworthy hardware.

Allow passing in a challenge that will be sent to the card during
key enrollment. These are needed to build an attestation workflow
that resists replay attacks.

ok markus@

OpenBSD-Commit-ID: 457dc3c3d689ba39eed328f0817ed9b91a5f78f6

1 files changed, 12 insertions, 9 deletions

diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 58f75ba28..748111d56 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -141,17 +141,20 @@ least manufacturer and batch number granularity. For this reason, we
 choose not to include this information in the public key or save it by
 default.
 
-Attestation information is very useful however in an organisational
-context, where it may be used by a CA as part of certificate
-issuance. In this case, exposure to the CA of hardware identity is
-desirable. To support this case, OpenSSH optionally allows retaining the
-attestation information at the time of key generation. It will take the
-following format:
-
-        string          "sk-attest-v00"
-        uint32          version         (1 for U2F, 2 for FIDO2 in future)
+Attestation information is useful for out-of-band key and certificate
+registration worksflows, e.g. proving to a CA that a key is backed
+by trusted hardware before it will issue a certificate. To support this
+case, OpenSSH optionally allows retaining the attestation information
+at the time of key generation. It will take the following format:
+
+        string          "ssh-sk-attest-v00"
         string          attestation certificate
         string          enrollment signature
+        uint32          reserved flags
+        string          reserved string
+
+OpenSSH treats the attestation certificate and enrollment signatures as
+opaque objects and does no interpretation of them itself.
 
 SSH U2F signatures
 ------------------