[gnutls-devel] GnuTLS | server auth: disable TLS 1.3 if no signature algorithm is usable (!987)

[Development of GNU's TLS library]

Daiki Ueno commented on a discussion on lib/auth.c: https://gitlab.com/gnutls/gnutls/merge_requests/987#note_166099829

>  		gnutls_certificate_credentials_t c = cred;
> -		unsigned i;
> +		unsigned i, j;
>  		bool allow_tls13 = 0;
>  		unsigned key_usage;
> +		const gnutls_sign_algorithm_t *sign_algos = gnutls_sign_list();
> +		const gnutls_sign_entry_st *se;
>  
>  		if (c != NULL && c->ncerts != 0) {
>  			for (i = 0; i < c->ncerts; i++) {
>  				key_usage = get_key_usage(session, c->certs[i].cert_list[0].pubkey);
>  				if (key_usage == 0 || (key_usage & GNUTLS_KEY_DIGITAL_SIGNATURE)) {
> -					allow_tls13 = 1;
> -					break;
> +					/* check if the private key is usable for signing in TLS 1.3 */
> +					if (session->security_parameters.entity == GNUTLS_SERVER) {

I think we nevertheless would need to check all the signature algorithms, for the case where ECDSA/EdDSA keys are available but RSA-PSS is not usable.  I'm moving the iteration logic into `lib/algorithms/sign.c` so it doesn't have thread-safety issue.

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