webauthn signatures: SecurityKeyProvider, json parsing

[Scott C Wang]

Greetings,

I was recently exploring OpenSSH's support for security keys and webauthn signatures, and had the following questions.

(1) Could you confirm that the client doesn't support creating signatures of type "webauthn-sk-ecdsa-sha2-nistp256 at openssh.com"? To me, it looked like SecurityKeyProvider implementers of sk-api.h aren't provided fields in the sk_sign_response struct in which to return the origin, clientData, and extensions signature fields, and sshsk_sign in ssh-sk.c cannot create signatures of type "webauthn-sk-ecdsa-sha2-nistp256 at openssh.com". What considerations would there be against augmenting sk-api.h with a webauthn signature type with fields for origin, clientData, and extensions, so that SecurityKeyProvider implementers can provide webauthn signatures?

(2) Am I correct to understand, from my reading of webauthn_check_prepare_hash in ssh-ecdsa-sk.c, that the server requires the webauthn signature's clientData field to begin with the type, challenge, and origin json fields, and ignores all fields after origin; and the hash is computed over the entire clientData blob? I'm asking to clarify since I notice Google Chromium [0] and the webauthn spec [1] warn that the json could be extended in the future, so they discourage verifiers from relying upon the order of the keys.

(3) I happened upon regress/unittests/sshsig/webauthn.html, which is very useful; however, the script hardcodes the relying party as "mindrot.org". I edited out that domain and hosted the site on my own domain to get this page to work. Is this page already hosted on mindrot.org, and if so, what's the path?

Thanks for your patience with my questions,


Scott C Wang

[0] https://goo.gl/yabPex
[1] https://w3c.github.io/webauthn/#dictdef-collectedclientdata