net/smtp: add CRAM-MD5 authentication

R=golang-dev, edsrzf, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5451087

src/pkg/net/smtp/auth.go

@@ -4,7 +4,11 @@
 
 package smtp
 
-import "errors"
+import (
+	"crypto/hmac"
+	"errors"
+	"fmt"
+)
 
 // Auth is implemented by an SMTP authentication mechanism.
 type Auth interface {
@@ -65,3 +69,29 @@ func (a *plainAuth) Next(fromServer []byte, more bool) ([]byte, error) {
 	}
 	return nil, nil
 }
+
+type cramMD5Auth struct {
+	username, secret string
+}
+
+// CRAMMD5Auth returns an Auth that implements the CRAM-MD5 authentication
+// mechanism as defined in RFC 2195.
+// The returned Auth uses the given username and secret to authenticate
+// to the server using the challenge-response mechanism.
+func CRAMMD5Auth(username, secret string) Auth {
+	return &cramMD5Auth{username, secret}
+}
+
+func (a *cramMD5Auth) Start(server *ServerInfo) (string, []byte, error) {
+	return "CRAM-MD5", nil, nil
+}
+
+func (a *cramMD5Auth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		d := hmac.NewMD5([]byte(a.secret))
+		d.Write(fromServer)
+		s := make([]byte, 0, d.Size())
+		return []byte(fmt.Sprintf("%s %x", a.username, d.Sum(s))), nil
+	}
+	return nil, nil
+}

src/pkg/net/smtp/smtp_test.go

@@ -23,6 +23,7 @@ type authTest struct {
 var authTests = []authTest{
 	{PlainAuth("", "user", "pass", "testserver"), []string{}, "PLAIN", []string{"\x00user\x00pass"}},
 	{PlainAuth("foo", "bar", "baz", "testserver"), []string{}, "PLAIN", []string{"foo\x00bar\x00baz"}},
+	{CRAMMD5Auth("user", "pass"), []string{"<123456.1322876914@testserver>"}, "CRAM-MD5", []string{"", "user 287eb355114cf5c471c26a875f1ca4ae"}},
 }
 
 func TestAuth(t *testing.T) {