code review 6061052: ssh: cosmetic cleanups

ssh/common.go

 // Copyright 2011 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
 
 import (
         "crypto/dsa"
         "crypto/rsa"
+        "errors"
         "math/big"
         "strconv"
         "sync"
 )
 
 // These are string constants in the SSH protocol.
 const (
         keyAlgoDH1SHA1  = "diffie-hellman-group1-sha1"
         kexAlgoDH14SHA1 = "diffie-hellman-group14-sha1"
         hostAlgoRSA     = "ssh-rsa"
         hostAlgoDSA     = "ssh-dss"
         compressionNone = "none"
         serviceUserAuth = "ssh-userauth"
         serviceSSH      = "ssh-connection"
 )
 
 var supportedKexAlgos = []string{kexAlgoDH14SHA1, keyAlgoDH1SHA1}
 var supportedHostKeyAlgos = []string{hostAlgoRSA}
 var supportedCompressions = []string{compressionNone}
 
 // dhGroup is a multiplicative group suitable for implementing Diffie-Hellman key agreement.
 type dhGroup struct {
         g, p *big.Int
+}
+
+func (group *dhGroup) diffieHellman(theirPublic, myPrivate *big.Int) (*big.Int, error) {
+        if theirPublic.Sign() <= 0 || theirPublic.Cmp(group.p) >= 0 {
+                return nil, errors.New("ssh: DH parameter out of bounds")
+        }
+        return new(big.Int).Exp(theirPublic, myPrivate, group.p), nil
 }
 
 // dhGroup1 is the group called diffie-hellman-group1-sha1 in RFC 4253 and
 // Oakley Group 2 in RFC 2409.
 var dhGroup1 *dhGroup
 
 var dhGroup1Once sync.Once
 
 func initDHGroup1() {
         p, _ := new(big.Int).SetString("FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF", 16)
@@ skipping 227 matching lines @@
         return append(buf, byte(n>>8), byte(n))
 }
 
 func appendU32(buf []byte, n uint32) []byte {
         return append(buf, byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
 }
 
 func appendInt(buf []byte, n int) []byte {
         return appendU32(buf, uint32(n))
 }