crypto/ssh: add support for aes128-cbc cipher.

The aes128cbc cipher is commented out in cipher.go on purpose, anyone wants to
use the cipher needs to uncomment line 119 in cipher.go

Fixes #4274.

Change-Id: I4bbc88ab884bda821c5f155dcf495bb7235c8605
Reviewed-on: https://go-review.googlesource.com/8396
Reviewed-by: Adam Langley <agl@golang.org>

ssh/transport.go

@@ -12,6 +12,7 @@ import (
 
 const (
 	gcmCipherID = "aes128-gcm@openssh.com"
+	aes128cbcID = "aes128-cbc"
 )
 
 // packetConn represents a transport that implements packet based
@@ -218,6 +219,10 @@ func newPacketCipher(d direction, algs directionAlgorithms, kex *kexResult) (pac
 		return newGCMCipher(iv, key, macKey)
 	}
 
+	if algs.Cipher == aes128cbcID {
+		return newAESCBCCipher(iv, key, macKey, algs)
+	}
+
 	c := &streamPacketCipher{
 		mac: macModes[algs.MAC].new(macKey),
 	}