Issue with PHP mcrypt function

Question

I use the following function to decrypt data on my server:

function decrypt($key, $text) {
    return trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, base64_decode($text), MCRYPT_MODE_ECB, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB), MCRYPT_RAND)));
}

I have read a lot about NOT using ECB however (and know it is deprecated so wanted to switch to CBC. Simply switching the mode to:

function decrypt($key, $text) {
    return trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, base64_decode($text), MCRYPT_MODE_CBC, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC), MCRYPT_RAND)));
}

does not work however. No errors are generated but the data returned is still encrypted.

What am I missing?

Updated code - still with errors:

$key = "hello"; 

$iv = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC), MCRYPT_DEV_RANDOM);

function encrypt($key, $text) {
    return trim(base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $text, MCRYPT_MODE_CBC, $iv)));
}


function decrypt($key, $text) { 
    return trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, base64_decode($text), MCRYPT_MODE_CBC, $iv));
}


$text = 12345;

echo "Plain Number : " . $text . "<br><br>";

$encrypted = encrypt($key, $text);
echo "AES Number : " . $encrypted . "<br><br>";

echo "Plain Number : ". decrypt($key, $encrypted) . "<br><br>";

this should work - but it returns the error:

blocksize in

blocksize in> Warning: mcrypt_encrypt()

[function.mcrypt-encrypt]: The IV parameter must be as long as the blocksize inblocksize in

blocksize in

Answer 1

When you decrypt you need to use the same IV as when you encrypted. It looks like you're generating a new, random IV during decryption.

It's OK to append or prepend the IV to the ciphertext. IVs are not secret but they should be unique for each encrypted message and only used once.