ssh: add aes192-cbc and aes256-cbc cipher modes

Some legacy devices only offer AES-256 in CBC mode, which the package
did not implement. Register both the 192-bit and 256-bit variants of
the existing AES-CBC packet cipher so they can be enabled through a
custom Config. aes.NewCipher already handles 24- and 32-byte keys, so
both registrations reuse the existing implementation.

ssh/cipher.go

@@ -129,6 +129,10 @@ func init() {
 	// You should expect that an active attacker can recover plaintext if
 	// you do.
 	cipherModes[InsecureCipherAES128CBC] = &cipherMode{16, aes.BlockSize, newAESCBCCipher}
+	// AES-192 and AES-256 reuse the same CBC packet cipher; aes.NewCipher
+	// selects the key schedule from the key length.
+	cipherModes[InsecureCipherAES192CBC] = &cipherMode{24, aes.BlockSize, newAESCBCCipher}
+	cipherModes[InsecureCipherAES256CBC] = &cipherMode{32, aes.BlockSize, newAESCBCCipher}
 	cipherModes[InsecureCipherTripleDESCBC] = &cipherMode{24, des.BlockSize, newTripleDESCBCCipher}
 }