Function selectSignatureScheme

auth.go:234–257

selectSignatureScheme picks a SignatureScheme from the peer's preference list that works with the selected certificate. It's only called for protocol versions that support signature algorithms, so TLS 1.2 and 1.3.

(vers uint16, c *Certificate, peerAlgs []SignatureScheme)

// that works with the selected certificate. It's only called for protocol
// versions that support signature algorithms, so TLS 1.2 and 1.3.
func selectSignatureScheme(vers uint16, c *Certificate, peerAlgs []SignatureScheme) (SignatureScheme, error) {
	supportedAlgs := signatureSchemesForCertificate(vers, c)
	if len(supportedAlgs) == 0 {
		return 0, unsupportedCertificateError(c)
	}
	if len(peerAlgs) == 0 && vers == VersionTLS12 {
		// For TLS 1.2, if the client didn't send signature_algorithms then we
		// can assume that it supports SHA1. See RFC 5246, Section 7.4.1.4.1.
		// RFC 9155 made signature_algorithms mandatory in TLS 1.2, and we gated
		// it behind the tlssha1 GODEBUG setting.
		// if tlssha1.Value() != "1" {
			return 0, errors.New("tls: missing signature_algorithms from TLS 1.2 peer")
		// }
		// peerAlgs = []SignatureScheme{PKCS1WithSHA1, ECDSAWithSHA1}
	}
	// Pick signature scheme in the peer's preference order, as our
	// preference order is not configurable.
	for _, preferredAlg := range peerAlgs {
		if isSupportedSignatureAlgorithm(preferredAlg, supportedAlgs) {
			return preferredAlg, nil
		}
	}
	return 0, errors.New("tls: peer doesn't support any of the certificate's signature algorithms")
}

// unsupportedCertificateError returns a helpful error for certificates with
// an unsupported private key.