AES encryption/decryption in javascript using CryptoJS

Question

I am trying to send AES encrypted messages between javascript and php using a shared secret. In Javascript I am using the CryptoJS library. In php, I am using mycrypt. I am trying to construct an encrypted message in javascript and then decrypt it in php using the shared secret. I can encrypt and decrypt a message in Javascript. I can encrypt and decrypt the same message in php - but the encryption is not the same between the two.

The Javascript

var encrypted = CryptoJS.AES.encrypt("Message", "Secret Passphrase");
alert(encrypted);

Gives

U2FsdGVkX18+k3pba4l4MbGZfmDjMc1yQ6uj1fg+BGo=

In php

<?php
$Pass = "Secret Passphrase";
$Clear = "Message";

$crypted = fnEncrypt($Clear, $Pass);
echo "Encrypted: ".$crypted."</br>";

$newClear = fnDecrypt($crypted, $Pass);
echo "Decrypted: ".$newClear."</br>";

function fnEncrypt($sValue, $sSecretKey) {
return rtrim(
        base64_encode(
                mcrypt_encrypt(
                        MCRYPT_RIJNDAEL_256,
                        $sSecretKey, $sValue,
                        MCRYPT_MODE_ECB,
                        mcrypt_create_iv(
                                mcrypt_get_iv_size(
                                        MCRYPT_RIJNDAEL_256,
                                        MCRYPT_MODE_ECB
                                ),
                                MCRYPT_RAND
                        )
                )
        ),"\0"
);
}

function fnDecrypt($sValue, $sSecretKey) {
return rtrim(
        mcrypt_decrypt(
                MCRYPT_RIJNDAEL_256,
                $sSecretKey,
                base64_decode($sValue),
                MCRYPT_MODE_ECB,
                mcrypt_create_iv(
                        mcrypt_get_iv_size(
                                MCRYPT_RIJNDAEL_256,
                                MCRYPT_MODE_ECB
                        ),
                        MCRYPT_RAND
                )
        ),"\0"
);
}

The output is

Encrypted: iqJ0R5ahRP7GpWKtW7+OBSCGnudDr99VbJC36OQlmgE=

Decrypted: Message

My question is, why are these not the same?

the php is ascii and the js is unicode, so even though the strings look like they would have the same binary pattern, they don't, and when you jumble the bits, the difference becomes obvious. can php do unicode yet? — dandavis
– dandavis, Commented Feb 15, 2014 at 23:14
php is basically silent on unicode. a string is just a "string" (hah) of characters. whether those chars are 1byte-per-char ascii or multi-byte like utf16 or utf8 isn't PHP's job to figure out. — Marc B
– Marc B, Commented Feb 15, 2014 at 23:22

Chris · Accepted Answer · 2014-02-19 16:09:48Z

1

It would be a crypographic catastrophy if they would be the same...

However: Using the way you used your PHP Encryption (using EBC mode) you will always get the same result. You can see why this is a problem by looking at the tux pciture at http://en.wikipedia.org/wiki/Cipher_block_chaining#Electronic_codebook_.28ECB.29

CryptoJS seems to use CBC as a default block cipher mode (at least they say so in https://code.google.com/p/crypto-js/#Block_Modes_and_Padding), which has a random initial vector. This is better than CBC.

The result should be the same, if you use the same cipher, the same block cipher mode (e.g. CBC) and the same key and IV - and of course the same plaintext - the comments indicate that there also might be a unicode issue.

Additionally the MCRYPT_RIJNDAEL_256 is not AES. AES has a BLOCK size of 16 byte - AES-128 has a KEY size of 16 byte, AES-256 has a KEY size of 32 byte. MCRYPT_RIJNDAEL_256 has a BLOCK size of 32 byte - that's a big difference.

One last note: It is not enough to encrypt data! You must also authenticate it, by either using a HMAC or by using an authenticated block cipher mode like GCM - if you don't do this you may be vulnerable to at least a padding oracle attack: http://en.wikipedia.org/wiki/Padding_oracle_attack

answered Feb 19, 2014 at 16:09

Chris

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1 Comment

anthonyc Over a year ago

Excellent reference and very helpful in getting me up the learning curve. I settled on using GibberishAES which has both javascript and php libraries which co-ordinate well.

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