ssh: remove arcfour ciphers from the default preference list

OpenSSH removed these ciphers from sshd's default configuration with
release 6.7 in 2014.

Change-Id: Ia8b6d671dc8fa5d0493bf933d3b541f8ae5707a3
Reviewed-on: https://go-review.googlesource.com/86955
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/cipher_test.go

@@ -15,7 +15,12 @@ import (
 func TestDefaultCiphersExist(t *testing.T) {
 	for _, cipherAlgo := range supportedCiphers {
 		if _, ok := cipherModes[cipherAlgo]; !ok {
-			t.Errorf("default cipher %q is unknown", cipherAlgo)
+			t.Errorf("supported cipher %q is unknown", cipherAlgo)
+		}
+	}
+	for _, cipherAlgo := range preferredCiphers {
+		if _, ok := cipherModes[cipherAlgo]; !ok {
+			t.Errorf("preferred cipher %q is unknown", cipherAlgo)
 		}
 	}
 }

ssh/common.go

@@ -24,12 +24,21 @@ const (
 	serviceSSH      = "ssh-connection"
 )
 
-// supportedCiphers specifies the supported ciphers in preference order.
+// supportedCiphers lists ciphers we support but might not recommend.
 var supportedCiphers = []string{
 	"aes128-ctr", "aes192-ctr", "aes256-ctr",
 	"aes128-gcm@openssh.com",
 	chacha20Poly1305ID,
-	"arcfour256", "arcfour128",
+	"arcfour256", "arcfour128", "arcfour",
+	aes128cbcID,
+	tripledescbcID,
+}
+
+// preferredCiphers specifies the default preference for ciphers.
+var preferredCiphers = []string{
+	"aes128-ctr", "aes192-ctr", "aes256-ctr",
+	"aes128-gcm@openssh.com",
+	chacha20Poly1305ID,
 }
 
 // supportedKexAlgos specifies the supported key-exchange algorithms in
@@ -212,7 +221,7 @@ func (c *Config) SetDefaults() {
 		c.Rand = rand.Reader
 	}
 	if c.Ciphers == nil {
-		c.Ciphers = supportedCiphers
+		c.Ciphers = preferredCiphers
 	}
 	var ciphers []string
 	for _, c := range c.Ciphers {

ssh/test/session_test.go

@@ -324,13 +324,15 @@ func TestWindowChange(t *testing.T) {
 	}
 }
 
+var deprecatedCiphers = []string{
+	"aes128-cbc", "3des-cbc",
+	"arcfour128", "arcfour256",
+}
+
 func TestCiphers(t *testing.T) {
 	var config ssh.Config
 	config.SetDefaults()
-	cipherOrder := config.Ciphers
-	// These ciphers will not be tested when commented out in cipher.go it will
-	// fallback to the next available as per line 292.
-	cipherOrder = append(cipherOrder, "aes128-cbc", "3des-cbc")
+	cipherOrder := append(config.Ciphers, deprecatedCiphers...)
 
 	for _, ciph := range cipherOrder {
 		t.Run(ciph, func(t *testing.T) {