Generation of symetric key AES-256-CBC with PHP & OpenSSL

Question

Anyone knows how to generate symetric key AES-256-CBC with OpenSSL. I generate IV with openssl_random_pseudo_bytes function. I tried to use openssl_encrypt - but it doesn't work. I found implementation of it on the web, but in Java language.

KeyGenerator keyGenerator = KeyGenerator.getInstance("AES") ;
keyGenerator.init(256);
SecretKey secretKey = keyGenerator.generateKey() ;
return secretKey.getEncoded();

Answer 1

The random generation of an AES encryption key works exact the same as for the initialization vector by using "openssl_random_pseudo_bytes" function.

Below you find a full running example for an AES 256 CBC encryption of a string with randomly generated key and IV. As you need the same IV for decryption the output is (Base64 encoded) IV : (Base64 encoded) ciphertext - in real programs you would concatenate both on binary basis before encoding.

This is the output:

AES CBC 256 String encryption with random key full
plaintext: The quick brown fox jumps over the lazy dog
encryptionKey (Base64): wJknn3c1MHWhDQaqCVYU648EbkdxIVjWghiJoXrpva8=

* * * Encryption * * *
ciphertext: s7L1BK/ds97P+abbuHtNUA==:5hWlNtTdEUgjQTkOVWV/QG2RauFjXNWwr+2Dczgx55pTg4azuNhwZyTFhyYFQSVh
output is (Base64) iv : (Base64) ciphertext

Cross platform cryptography: AES CBC 256 String encryption with random key (PHP)

* * * Decryption * * *
decryptionKey (Base64): JJu2xyi4sP7lTfxVi8iPvKIIOWiwkgr7spyUEhsnjek=
ciphertextDecryption (Base64): +KOM4ASOY0cMNrM9zV/qvw==:HuoyDiusSplYfd04XSNLOqtcWyOFwmeHNzXS5ywmqRkgAXi8do/6dKppo2U3ZoKl
input is (Base64) iv : (Base64) ciphertext
plaintext: The quick brown fox jumps over the lazy dog

Kindly note that the code has not exception handling and is for educational purpose only.

<?php
function generateRandomAesKey()
{
    return openssl_random_pseudo_bytes(32, $crypto_strong);
}

function generateRandomInitvector()
{
    return openssl_random_pseudo_bytes(16, $crypto_strong);
}

function base64Encoding($input)
{
    return base64_encode($input);
}

function base64Decoding($input)
{
    return base64_decode($input);
}

function aesCbcEncryptToBase64($key, $data)
{
    $iv = generateRandomInitvector();
    $ciphertext = openssl_encrypt($data, 'aes-256-cbc', $key, OPENSSL_RAW_DATA, $iv);
    return base64_encode($iv) . ':' . base64_encode($ciphertext);
}

function aesCbcDecryptFromBase64($key, $data)
{
    list($iv, $encryptedData) = explode(':', $data, 2);
    return openssl_decrypt(base64_decode($encryptedData), 'aes-256-cbc', $key, OPENSSL_RAW_DATA, base64_decode($iv));
}

echo 'AES CBC 256 String encryption with random key full' . PHP_EOL;

$plaintext = 'The quick brown fox jumps over the lazy dog';
echo 'plaintext: ' . $plaintext . PHP_EOL;

// generate random key
$encryptionKey = generateRandomAesKey();
$encryptionKeyBase64 = base64Encoding($encryptionKey);
echo 'encryptionKey (Base64): ' . $encryptionKeyBase64 . PHP_EOL;

// encryption
echo PHP_EOL . '* * * Encryption * * *' . PHP_EOL;
$ciphertextBase64 = aesCbcEncryptToBase64($encryptionKey, $plaintext);
echo 'ciphertext: ' . $ciphertextBase64 . PHP_EOL;
echo 'output is (Base64) iv : (Base64) ciphertext' .PHP_EOL;

echo PHP_EOL;
echo 'Cross platform cryptography: AES CBC 256 String encryption with random key (PHP)' . PHP_EOL;
// decryption
echo PHP_EOL . '* * * Decryption * * *' . PHP_EOL;
$decryptionKeyBase64 = $encryptionKeyBase64;
$ciphertextDecryptionBase64 = $ciphertextBase64;
echo 'decryptionKey (Base64): ' . $decryptionKeyBase64 . PHP_EOL;

echo 'ciphertextDecryption (Base64): ' . $ciphertextDecryptionBase64 . PHP_EOL;
echo 'input is (Base64) iv : (Base64) ciphertext' .PHP_EOL;
$decryptionKey = base64Decoding($decryptionKeyBase64);
$decryptedtext = aesCbcDecryptFromBase64($decryptionKey, $ciphertextDecryptionBase64);
echo 'plaintext: ' . $decryptedtext . PHP_EOL;
?>