go.crypto/ssh: never negotiate unsupported ciphers

Fixes golang/go#4285.

Adding a new cipher that is supported by the remote end, but not supported by our client causes that cipher to be considered a valid candidate. This fails later in setupKeys when there is no cipherModes configuration.

In summary, unsupported ciphers cannot be willed into existence by adding them to the client config. This change enforces this.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/6780047

ssh/common.go

@@ -8,8 +8,8 @@ import (
 	"crypto/dsa"
 	"crypto/rsa"
 	"errors"
+	"fmt"
 	"math/big"
-	"strconv"
 	"sync"
 )
 
@@ -77,7 +77,7 @@ type UnexpectedMessageError struct {
 }
 
 func (u UnexpectedMessageError) Error() string {
-	return "ssh: unexpected message type " + strconv.Itoa(int(u.got)) + " (expected " + strconv.Itoa(int(u.expected)) + ")"
+	return fmt.Sprintf("ssh: unexpected message type %d (expected %d)", u.got, u.expected)
 }
 
 // ParseError results from a malformed SSH message.
@@ -86,7 +86,7 @@ type ParseError struct {
 }
 
 func (p ParseError) Error() string {
-	return "ssh: parse error in message type " + strconv.Itoa(int(p.msgType))
+	return fmt.Sprintf("ssh: parse error in message type %d", p.msgType)
 }
 
 type handshakeMagics struct {
@@ -102,7 +102,18 @@ func findCommonAlgorithm(clientAlgos []string, serverAlgos []string) (commonAlgo
 			}
 		}
 	}
+	return
+}
 
+func findCommonCipher(clientCiphers []string, serverCiphers []string) (commonCipher string, ok bool) {
+	for _, clientCipher := range clientCiphers {
+		for _, serverCipher := range serverCiphers {
+			// reject the cipher if we have no cipherModes definition
+			if clientCipher == serverCipher && cipherModes[clientCipher] != nil {
+				return clientCipher, true
+			}
+		}
+	}
 	return
 }
 
@@ -117,12 +128,12 @@ func findAgreedAlgorithms(transport *transport, clientKexInit, serverKexInit *ke
 		return
 	}
 
-	transport.writer.cipherAlgo, ok = findCommonAlgorithm(clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer)
+	transport.writer.cipherAlgo, ok = findCommonCipher(clientKexInit.CiphersClientServer, serverKexInit.CiphersClientServer)
 	if !ok {
 		return
 	}
 
-	transport.reader.cipherAlgo, ok = findCommonAlgorithm(clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient)
+	transport.reader.cipherAlgo, ok = findCommonCipher(clientKexInit.CiphersServerClient, serverKexInit.CiphersServerClient)
 	if !ok {
 		return
 	}