Function signatureSchemesForCertificate

auth.go:171–229

signatureSchemesForCertificate returns the list of supported SignatureSchemes for a given certificate, based on the public key and the protocol version, and optionally filtered by its explicit SupportedSignatureAlgorithms.

(version uint16, cert *Certificate)

// for a given certificate, based on the public key and the protocol version,
// and optionally filtered by its explicit SupportedSignatureAlgorithms.
func signatureSchemesForCertificate(version uint16, cert *Certificate) []SignatureScheme {
	priv, ok := cert.PrivateKey.(crypto.Signer)
	if !ok {
		return nil
	}

	var sigAlgs []SignatureScheme
	switch pub := priv.Public().(type) {
	case *ecdsa.PublicKey:
		if version != VersionTLS13 {
			// In TLS 1.2 and earlier, ECDSA algorithms are not
			// constrained to a single curve.
			sigAlgs = []SignatureScheme{
				ECDSAWithP256AndSHA256,
				ECDSAWithP384AndSHA384,
				ECDSAWithP521AndSHA512,
				ECDSAWithSHA1,
			}
			break
		}
		switch pub.Curve {
		case elliptic.P256():
			sigAlgs = []SignatureScheme{ECDSAWithP256AndSHA256}
		case elliptic.P384():
			sigAlgs = []SignatureScheme{ECDSAWithP384AndSHA384}
		case elliptic.P521():
			sigAlgs = []SignatureScheme{ECDSAWithP521AndSHA512}
		default:
			return nil
		}
	case *rsa.PublicKey:
		size := pub.Size()
		sigAlgs = make([]SignatureScheme, 0, len(rsaSignatureSchemes))
		for _, candidate := range rsaSignatureSchemes {
			if size >= candidate.minModulusBytes && version <= candidate.maxVersion {
				sigAlgs = append(sigAlgs, candidate.scheme)
			}
		}
	case ed25519.PublicKey:
		sigAlgs = []SignatureScheme{Ed25519}
	default:
		return nil
	}

	if cert.SupportedSignatureAlgorithms != nil {
		sigAlgs = slices.DeleteFunc(sigAlgs, func(sigAlg SignatureScheme) bool {
			return !isSupportedSignatureAlgorithm(sigAlg, cert.SupportedSignatureAlgorithms)
		})
	}

	// Filter out any unsupported signature algorithms, for example due to
	// FIPS 140-3 policy, tlssha1=0, or any downstream changes to defaults.go.
	supportedAlgs := supportedSignatureAlgorithms(version)
	sigAlgs = slices.DeleteFunc(sigAlgs, func(sigAlg SignatureScheme) bool {
		return !isSupportedSignatureAlgorithm(sigAlg, supportedAlgs)
	})
	
	return sigAlgs