Hackathon 117: t_cose_sign_verify with T_COSE_OPT_DECODE_ONLY fails

[dthaler]

In the dev branch, when T_COSE_OPT_DECODE_ONLY is set but T_COSE_OPT_VERIFY_ALL_SIGNATURES is clear, the verification fails when multiple signatures exist and one succeeds and another fails.

The root cause seems to be in
https://github.com/laurencelundblade/t_cose/blob/dev/src/t_cose_sign_verify.c#L284

        if(me->option_flags & (T_COSE_OPT_VERIFY_ALL_SIGNATURES | T_COSE_OPT_DECODE_ONLY)) {
            if(return_value == T_COSE_ERR_DECLINE) {
                /* When verifying all, there can be no declines.
                 * Also only decoding (not verifying) there can be
                 * no declines because every signature must be
                 * decoded so its parameters can be returned.
                 * TODO: is this really true? It might be OK to
                 * only decode some as long as the caller knows
                 * that some weren't decoded. How to indicate this
                 * if it happens? An error code? A special
                 * indicator parameter in the returned list?
                 */
                break;

Specifically, setting T_COSE_OPT_DECODE_ONLY causes it to take the T_COSE_OPT_VERIFY_ALL_SIGNATURES code path and fail unless all succeed, which breaks the ability to compute auxiliary buffer size (by using T_COSE_OPT_DECODE_ONLY) in the just-one-success-needed case.

[laurencelundblade]

Hi Dave, are you setting a kid for the EdDSA verifier?

Can you point me to your repo (if it is public)?

I'm having trouble replicating so far.