crypto/tls: add a SignatureScheme type.

The SignatureAndHashAlgorithm from TLS 1.2[1] is being changed to
SignatureScheme in TLS 1.3[2]. (The actual values are compatible
however.)

Since we expect to support TLS 1.3 in the future, we're already using
the name and style of SignatureScheme in the recently augmented
ClientHelloInfo. As this is public API, it seems that SignatureScheme
should have its own type and exported values, which is implemented in
this change.

[1] https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
[2] https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.3

Change-Id: I0482755d02bb9a04eaf075c012696103eb806645
Reviewed-on: https://go-review.googlesource.com/32119Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/tls/common.go

@@ -215,6 +215,25 @@ type ClientSessionCache interface {
 	Put(sessionKey string, cs *ClientSessionState)
 }
 
+// SignatureScheme identifies a signature algorithm supported by TLS. See
+// https://tools.ietf.org/html/draft-ietf-tls-tls13-18#section-4.2.3.
+type SignatureScheme uint16
+
+const (
+	PKCS1WithSHA1   SignatureScheme = 0x0201
+	PKCS1WithSHA256 SignatureScheme = 0x0401
+	PKCS1WithSHA384 SignatureScheme = 0x0501
+	PKCS1WithSHA512 SignatureScheme = 0x0601
+
+	PSSWithSHA256 SignatureScheme = 0x0804
+	PSSWithSHA384 SignatureScheme = 0x0805
+	PSSWithSHA512 SignatureScheme = 0x0806
+
+	ECDSAWithP256AndSHA256 SignatureScheme = 0x0403
+	ECDSAWithP384AndSHA384 SignatureScheme = 0x0503
+	ECDSAWithP521AndSHA512 SignatureScheme = 0x0603
+)
+
 // ClientHelloInfo contains information from a ClientHello message in order to
 // guide certificate selection in the GetCertificate callback.
 type ClientHelloInfo struct {
@@ -244,7 +263,7 @@ type ClientHelloInfo struct {
 	// is willing to verify. SignatureSchemes is set only if the Signature
 	// Algorithms Extension is being used (see
 	// https://tools.ietf.org/html/rfc5246#section-7.4.1.4.1).
-	SignatureSchemes []uint16
+	SignatureSchemes []SignatureScheme
 
 	// SupportedProtos lists the application protocols supported by the client.
 	// SupportedProtos is set only if the Application-Layer Protocol

src/crypto/tls/handshake_server.go

@@ -822,9 +822,9 @@ func (hs *serverHandshakeState) clientHelloInfo() *ClientHelloInfo {
 		supportedVersions = suppVersArray[VersionTLS12-hs.clientHello.vers:]
 	}
 
-	signatureSchemes := make([]uint16, 0, len(hs.clientHello.signatureAndHashes))
+	signatureSchemes := make([]SignatureScheme, 0, len(hs.clientHello.signatureAndHashes))
 	for _, sah := range hs.clientHello.signatureAndHashes {
-		signatureSchemes = append(signatureSchemes, uint16(sah.hash)<<8+uint16(sah.signature))
+		signatureSchemes = append(signatureSchemes, SignatureScheme(sah.hash)<<8+SignatureScheme(sah.signature))
 	}
 
 	hs.cachedClientHelloInfo = &ClientHelloInfo{