crypto/tls: add VersionName

Fixes #46308

Change-Id: I5162b26cbce61ae5df5d2e093cf8a28406d15863
Reviewed-on: https://go-review.googlesource.com/c/go/+/497377
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Marten Seemann <martenseemann@gmail.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Samuli Silvius <samuli.silvius@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Filippo Valsorda <filippo@golang.org>

common.go

@@ -36,6 +36,26 @@ const (
 	VersionSSL30 = 0x0300
 )
 
+// VersionName returns the name for the provided TLS version number
+// (e.g. "TLS 1.3"), or a fallback representation of the value if the
+// version is not implemented by this package.
+func VersionName(version uint16) string {
+	switch version {
+	case VersionSSL30:
+		return "SSLv3"
+	case VersionTLS10:
+		return "TLS 1.0"
+	case VersionTLS11:
+		return "TLS 1.1"
+	case VersionTLS12:
+		return "TLS 1.2"
+	case VersionTLS13:
+		return "TLS 1.3"
+	default:
+		return fmt.Sprintf("0x%04X", version)
+	}
+}
+
 const (
 	maxPlaintext       = 16384        // maximum plaintext payload length
 	maxCiphertext      = 16384 + 2048 // maximum ciphertext payload length

tls_test.go

@@ -1590,6 +1590,15 @@ func TestCipherSuites(t *testing.T) {
 	}
 }
 
+func TestVersionName(t *testing.T) {
+	if got, exp := VersionName(VersionTLS13), "TLS 1.3"; got != exp {
+		t.Errorf("unexpected VersionName: got %q, expected %q", got, exp)
+	}
+	if got, exp := VersionName(0x12a), "0x012A"; got != exp {
+		t.Errorf("unexpected fallback VersionName: got %q, expected %q", got, exp)
+	}
+}
+
 // http2isBadCipher is copied from net/http.
 // TODO: if it ends up exposed somewhere, use that instead.
 func http2isBadCipher(cipher uint16) bool {