feat(webauthn): emit SubjectPublicKeyInfo from getPublicKey()#230
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I'm not a cryptography expert, but lgtm
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WebAuthn L3 §5.2.1.1 mandates DER-encoded SubjectPublicKeyInfo from AuthenticatorAttestationResponse.getPublicKey() for credentials using ES256, EdDSA Ed25519 and RS256, and null for any algorithm the user agent does not understand. Until now libwebauthn was emitting raw COSE_Key bytes there, which RPs could not feed into SubtleCrypto or the standard library X.509 parsers. A new cose::to_spki helper converts the stored COSE_Key bytes into SPKI for the mandatory floor. ESP256 (RFC 9864, -9) maps to the same SPKI as ES256 per WebAuthn L3 §5.8.5. Unsupported algorithms return Ok(None) so the JSON IDL response emits null per spec. Malformed keys for understood algorithms return Err. The conversion uses the spki and der crates, both already in the transitive dependency tree via p256.
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WebAuthn L3 §5.2.1.1 says
AuthenticatorAttestationResponse.getPublicKey()returns DER-encoded SubjectPublicKeyInfo for credentials using ES256, EdDSA Ed25519 and RS256, and null for any algorithm the user agent does not implement. libwebauthn was emitting raw COSE bytes there instead, which relying parties could not feed intoSubtleCryptoor standard X.509 parsers.A new converter produces SPKI for the WebAuthn L3 floor plus ESP256, which is equivalent to ES256 per RFC 9864. Algorithms outside the supported set return null per spec. Malformed keys for understood algorithms surface as an error.
The conversion uses the
spkianddercrates from RustCrypto, both already in the transitive dependency tree viap256, so no new external runtime crates land. Adding new algorithm support later is a small per-algorithm addition.Stacked on top of #229.