go.crypto/ssh: improve support for MAC algorithms

Also, add support for hmac-sha1.

At the suggestion of AGL hmac-md5, and hmac-md5-96
support was not included.

Fixes golang/go#3095.

R=golang-dev, agl, huin
CC=golang-dev
https://golang.org/cl/5696065

ssh/common.go

@@ -17,7 +17,6 @@ const (
 	kexAlgoDH14SHA1 = "diffie-hellman-group14-sha1"
 	hostAlgoRSA     = "ssh-rsa"
 	hostAlgoDSA     = "ssh-dss"
-	macSHA196       = "hmac-sha1-96"
 	compressionNone = "none"
 	serviceUserAuth = "ssh-userauth"
 	serviceSSH      = "ssh-connection"
@@ -25,7 +24,6 @@ const (
 
 var supportedKexAlgos = []string{kexAlgoDH14SHA1}
 var supportedHostKeyAlgos = []string{hostAlgoRSA}
-var supportedMACs = []string{macSHA196}
 var supportedCompressions = []string{compressionNone}
 
 // dhGroup is a multiplicative group suitable for implementing Diffie-Hellman key agreement.
@@ -134,6 +132,9 @@ type CryptoConfig struct {
 	// The allowed cipher algorithms. If unspecified then DefaultCipherOrder is
 	// used.
 	Ciphers []string
+
+	// The allowed MAC algorithms. If unspecified then DefaultMACOrder is used.
+	MACs []string
 }
 
 func (c *CryptoConfig) ciphers() []string {
@@ -143,6 +144,13 @@ func (c *CryptoConfig) ciphers() []string {
 	return c.Ciphers
 }
 
+func (c *CryptoConfig) macs() []string {
+	if c.MACs == nil {
+		return DefaultMACOrder
+	}
+	return c.MACs
+}
+
 // serialize a signed slice according to RFC 4254 6.6.
 func serializeSignature(algoname string, sig []byte) []byte {
 	switch algoname {