CODEWITHSUNDEEP
phpencryptionbase64aesdecoding
hackartist
hackartist

Reputation: 5264

AES decrypt in php

I am new to AES but from what I have found there are several modes (ECB,CBC, etc.) and different modes need different initialization vector requirements, blocks, and encodings. I am trying to decode the following

Xrb9YtT7cHUdpHYIvEWeJIAbkxWUtCNcjdzOMgyxJzU/vW9xHivdEDFKeszC93B6MMkhctR35e+YkmYI5ejMf5ofNxaiQcZbf3OBBsngfWUZxfvnrE2u1lD5+R6cn88vk4+mwEs3WoAht1CAkjr7P+fRIaCTckWLaF9ZAgo1/rvYA8EGDc+uXgWv9KvYpDDsCd1JStrD96IACN3DNuO28lVOsKrhcEWhDjAx+yh72wM=

using php and the (text) key "043j9fmd38jrr4dnej3FD11111111111" with mode CBC and an IV of all zeros. I am able to get it to work with this tool but can't get it in php. Here is the code I am using:

function decrypt_data($data, $iv, $key) {
    $data   = base64_decode($data);
    $cypher = mcrypt_module_open(MCRYPT_RIJNDAEL_256, '', MCRYPT_MODE_CBC, '');

    // initialize encryption handle
    if (mcrypt_generic_init($cypher, $key, $iv) != -1) {
            // decrypt
            $decrypted = mdecrypt_generic($cypher, $data);

            // clean up
            mcrypt_generic_deinit($cypher);
            mcrypt_module_close($cypher);

            return $decrypted;
    }

    return false;
}

I think I may be missing something relating to base 64 encoding or turning the key into binary first. I have tried decoding many things and all I can produce is gibberish. Any help would be very appreciated.

Upvotes: 0

Views: 10021

Answers (1)

vstm
vstm

Reputation: 12537

Well the tool itself does not say how exactly it's encrypted. And you can't set the IV either so it's hard to get the parameters right (because they have to be equal).

After some guesswork I found out the following:

  • The IV is prepended to the ciphertext
  • The ciphertext is encrypted with aes-128-cbc

So you have to modify the code:

function decrypt_data($data, $iv, $key) {
    $cypher = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');

    if(is_null($iv)) {
        $ivlen = mcrypt_enc_get_iv_size($cypher);
        $iv = substr($data, 0, $ivlen);
        $data = substr($data, $ivlen);
    }

    // initialize encryption handle
    if (mcrypt_generic_init($cypher, $key, $iv) != -1) {
            // decrypt
            $decrypted = mdecrypt_generic($cypher, $data);

            // clean up
            mcrypt_generic_deinit($cypher);
            mcrypt_module_close($cypher);

            return $decrypted;
    }

    return false;
}

$ctext = "Xrb9YtT7cHUdpHYIvEWeJIAbkxWUtCNcjdzOMgyxJzU/vW9x" .
         "HivdEDFKeszC93B6MMkhctR35e+YkmYI5ejMf5ofNxaiQcZb" .
         "f3OBBsngfWUZxfvnrE2u1lD5+R6cn88vk4+mwEs3WoAht1CA" .
         "kjr7P+fRIaCTckWLaF9ZAgo1/rvYA8EGDc+uXgWv9KvYpDDs" .
         "Cd1JStrD96IACN3DNuO28lVOsKrhcEWhDjAx+yh72wM=";

$key = "043j9fmd38jrr4dnej3FD11111111111";

$res = decrypt_data(base64_decode($ctext), null, $key);

I'm not sure why the key length is not used to encrypt it with aes-256-cbc - I've checked out the source of that as3crypto-library and it kind of supported it, but I would have to debug it to really verify it.

Upvotes: 3

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