Detect wrong decryption key while working with mcrypt functions in PHP

Question

I've wrote the codes below to make an encryption and decryption mechanism. Everything works fine, but I would like to prevent showing unreadable characters if somebody enter a wrong key while decryption. Instead, I would like to show him a certain error message. How can I do it?
My PHP codes:

<?php

$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

function encrypt($plaintext,$key) {
    global $iv, $iv_size;
    $ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $plaintext, MCRYPT_MODE_CBC, $iv);
    $ciphertext = $iv . $ciphertext;
    return base64_encode($ciphertext);
}

function decrypt($ciphertext_base64,$key) {
    global $iv, $iv_size;
    $ciphertext_dec = base64_decode($ciphertext_base64);
    $iv_dec = substr($ciphertext_dec, 0, $iv_size);
    $ciphertext_dec = substr($ciphertext_dec, $iv_size);
    return mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
}

echo decrypt(encrypt('Hello World','123'),'123');
// Correct key used for decryption and result will be `Hello world`

echo decrypt(encrypt('Hello World','123'),'321');
// Wrong key used for decryption and result will be something like :ŘI¨ĄěđŘcSNŔ¶¸˘ÚE‘Z‰ŃZŃ9

?>

Answer 1

You can only detect whether a key was correct if you already know something about the plain text.

(Bad) Method 1: Add a static string to the plain text

You could modify your encrypt() function so that it always adds something to the plain text, e.g. $plaintext = 'correct key' . $plaintext;.

In your decrypt() function you could now check whether this string exists and remove it before returning.

You would have to replace the last line of decrypt() with this:

$decrypted_text = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
if (strpos($decrypted_text, 'correct key') === 0) {
    return substr($decrypted_text, strlen('correct key'));
} else {
    // key was wrong
    return false;
}

The problem here is that knowing part of the encrypted message potentially weakens the security of the encryption. So don't do that.

(Better) Method 2: Validate the plain text with a hash

Instead of modifying the plain text, you could just save a finger print (e.g. a SHA1 hash) of it.

Modify the encrypt() function like this:

return sha1($plaintext) . base64_encode($ciphertext);

And the decrypt() function like this:

function decrypt($ciphertext, $key) {
    global $iv, $iv_size;

    $fingerprint = substr($ciphertext, 0, 32);
    $ciphertext_base64 = substr($ciphertext, 32);

    $ciphertext_dec = base64_decode($ciphertext_base64);
    $iv_dec = substr($ciphertext_dec, 0, $iv_size);
    $ciphertext_dec = substr($ciphertext_dec, $iv_size);

    $decrypted_text = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
    if (sha1($decrypted_text) == $fingerprint) {
        return $decrypted_text;
    } else {
        // key was wrong
        return false;
    }
}