What is an openssl iv, and why do I need a key and an iv?

Question

I am about to use the following script to encrypt and decrypt some data. I am using it because my current encryption does not work on our new server. We are currently using mcrypt so I want to change to openssl.

In our database we use aes encryption which uses a 128bit key so I know what a key is, but I do not know what an openssl iv is? And why would I need a key and an iv.

The code I am about to use is this, which I found on a website because I don't understand encryption very well.

Obviously I will modify it so that the key is kept somewhere else.

function encrypt_decrypt($action, $string) {
    $output = false;

    $encrypt_method = "AES-256-CBC";
    $secret_key = 'This is my secret key';
    $secret_iv = 'This is my secret iv';

    // hash
    $key = hash('sha256', $secret_key);

    // iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
    $iv = substr(hash('sha256', $secret_iv), 0, 16);

    if( $action == 'encrypt' ) {
        $output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
        $output = base64_encode($output);
    }
    else if( $action == 'decrypt' ){
        $output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
    }

    return $output;
}

$plain_txt = "This is my plain text";
echo "Plain Text = $plain_txt\n";

$encrypted_txt = encrypt_decrypt('encrypt', $plain_txt);
echo "Encrypted Text = $encrypted_txt\n";

$decrypted_txt = encrypt_decrypt('decrypt', $encrypted_txt);
echo "Decrypted Text = $decrypted_txt\n";

if( $plain_txt === $decrypted_txt ) echo "SUCCESS";
else echo "FAILED";

echo "\n";

Answer 1

The Initialization Vector is part of what makes AES in CBC (Cipher Block Chaining) mode work - IVs are not unique to OpenSSL. CBC works by XORing the previous block with the current block. The very first block has no previous block, so the IV serves that purpose.

Why this is necessary requires a bit of understanding of how block ciphers work. Without this chaining and IV, we're left with a mode of AES called ECB, or Electronic Code Book. ECB has weaknesses that allow a chosen plaintext attack, among many other problems.

I would recommend spending a bit of time with best practices for CBC initialization vectors. Using them incorrectly can weaken the overall security of AES. The short explanation is:

• IVs should be random and generated by a CSPRNG.
• IVs should not be reused. That is, don't encrypt plaintext "A" and plaintext "B" with the same IV. Every record should have its own IV.
• The IV is not a secret like the key. It can be stored in plaintext along with the cipher text.

Also keep in mind that this advice only applies to AES-CBC. If you ever investigate other modes of AES, such as GCM, this does not apply.