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/mac.go

@@ -0,0 +1,58 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssh
+
+// Message authentication support
+
+import (
+	"crypto/hmac"
+	"crypto/sha1"
+	"hash"
+)
+
+type macMode struct {
+	keySize int
+	new     func(key []byte) hash.Hash
+}
+
+// truncatingMAC wraps around a hash.Hash and truncates the output digest to
+// a given size.
+type truncatingMAC struct {
+	length int
+	hmac   hash.Hash
+}
+
+func (t truncatingMAC) Write(data []byte) (int, error) {
+	return t.hmac.Write(data)
+}
+
+func (t truncatingMAC) Sum(in []byte) []byte {
+	out := t.hmac.Sum(in)
+	return out[:len(in)+t.length]
+}
+
+func (t truncatingMAC) Reset() {
+	t.hmac.Reset()
+}
+
+func (t truncatingMAC) Size() int {
+	return t.length
+}
+
+func (t truncatingMAC) BlockSize() int { return t.hmac.BlockSize() }
+
+// Specifies a default set of MAC algorithms and a preference order. 
+// This is based on RFC 4253, section 6.4, with the removal of the 
+// hmac-md5 variants as they have reached the end of their useful life.
+var DefaultMACOrder = []string{"hmac-sha1", "hmac-sha1-96"}
+
+var macModes = map[string]*macMode{
+	"hmac-sha1": {20, func(key []byte) hash.Hash {
+		return hmac.New(sha1.New, key)
+	}},
+	"hmac-sha1-96": {20, func(key []byte) hash.Hash {
+		return truncatingMAC{12, hmac.New(sha1.New, key)}
+	}},
+}