• Cipher: AES in CBC mode with PKCS#5 padding (equivalent to PKCS#7 for AES)
• Key size: 256 bits
• IV: all zeros, 16 bytes (AES block size)
• Ciphertext encoding: hex

Common pitfalls include using ECB instead of CBC, omitting PKCS#5 padding, mismatching key sizes, or using a nonzero/encorrect IV length. AES-CBC requires a 16-byte IV.

PKCS#5 padding helpers

<?php
function pkcs5_pad(string $data, int $blockSize = 16): string {
    $padLen = $blockSize - (strlen($data) % $blockSize);
    return $data . str_repeat(chr($padLen), $padLen);
}

function pkcs5_unpad(string $data, int $blockSize = 16): string {
    $len = strlen($data);
    if ($len === 0 || $len % $blockSize !== 0) {
        return '';
    }
    $padLen = ord($data[$len - 1]);
    if ($padLen < 1 || $padLen > $blockSize) {
        return '';
    }
    // Verify padding bytes
    for ($i = 1; $i <= $padLen; $i++) {
        if (ord($data[$len - $i]) !== $padLen) {
            return '';
        }
    }
    return substr($data, 0, $len - $padLen);
}

All-zero IV

<?php
function zero_iv(int $len = 16): string {
    return str_repeat("\x00", $len);
}

// Alternative equivalent construction via pack (16 zero bytes):
function zero_iv_pack(): string {
    $blk = pack('N', 0); // 4 zero bytes
    return str_repeat($blk, 4); // 16 bytes total
}

AES-256-CBC encryption/decryption using OpenSSL (manual PKCS#5)

<?php
function aes256cbc_encrypt(string $plaintext, string $secret): string {
    $key = hash('sha256', $secret, true);      // 32 bytes
    $iv  = zero_iv(16);                        // 16 zero bytes
    $padded = pkcs5_pad($plaintext, 16);

    $raw = openssl_encrypt(
        $padded,
        'AES-256-CBC',
        $key,
        OPENSSL_RAW_DATA | OPENSSL_ZERO_PADDING,
        $iv
    );

    return bin2hex($raw);
}

function aes256cbc_decrypt(string $cipherHex, string $secret): string {
    $key = hash('sha256', $secret, true);
    $iv  = zero_iv(16);

    $raw = hex2bin($cipherHex);
    $padded = openssl_decrypt(
        $raw,
        'AES-256-CBC',
        $key,
        OPENSSL_RAW_DATA | OPENSSL_ZERO_PADDING,
        $iv
    );

    return pkcs5_unpad($padded, 16);
}

Quick self-check

<?php
$pass = 'test-key';
$msg  = 'test str';

$cipher = aes256cbc_encrypt($msg, $pass);
$plain  = aes256cbc_decrypt($cipher, $pass);

var_dump($msg === $plain); // bool(true)

Legacy mcrypt-compatible variant (for outdated environments)

<?php
function aes256cbc_encrypt_mcrypt(string $plaintext, string $secret): string {
    $key = hash('sha256', $secret, true); // 32 bytes
    $iv  = zero_iv(16);

    $enc = mcrypt_encrypt(
        MCRYPT_RIJNDAEL_128,
        $key,
        pkcs5_pad($plaintext, 16),
        MCRYPT_MODE_CBC,
        $iv
    );

    return bin2hex($enc);
}

function aes256cbc_decrypt_mcrypt(string $cipherHex, string $secret): string {
    $key = hash('sha256', $secret, true);
    $iv  = zero_iv(16);

    $dec = mcrypt_decrypt(
        MCRYPT_RIJNDAEL_128,
        $key,
        hex2bin($cipherHex),
        MCRYPT_MODE_CBC,
        $iv
    );

    return pkcs5_unpad($dec, 16);
}