AES-256 encryption and decryption in PHP and C#

Daniel Opitz

10 Aug 2017

Attention: This article is from 2017, some information may be out of date.

PHP

<?php

$plaintext = 'My secret message 1234';
$password = '3sc3RLrpd17';

// CBC has an IV and thus needs randomness every time a message is encrypted
$method = 'aes-256-cbc';

// Must be exact 32 chars (256 bit)
// You must store this secret random key in a safe place of your system.
$key = substr(hash('sha256', $password, true), 0, 32);
echo "Password:" . $password . "\n";

// Most secure key
//$key = openssl_random_pseudo_bytes(openssl_cipher_iv_length($method));

// IV must be exact 16 chars (128 bit)
$iv = chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0) . chr(0x0);

// Most secure iv
// Never ever use iv=0 in real life. Better use this iv:
// $ivlen = openssl_cipher_iv_length($method);
// $iv = openssl_random_pseudo_bytes($ivlen);

// av3DYGLkwBsErphcyYp+imUW4QKs19hUnFyyYcXwURU=
$encrypted = base64_encode(openssl_encrypt($plaintext, $method, $key, OPENSSL_RAW_DATA, $iv));

// My secret message 1234
$decrypted = openssl_decrypt(base64_decode($encrypted), $method, $key, OPENSSL_RAW_DATA, $iv);

echo 'plaintext=' . $plaintext . "\n";
echo 'cipher=' . $method . "\n";
echo 'encrypted to: ' . $encrypted . "\n";
echo 'decrypted to: ' . $decrypted . "\n\n";

C#

using System.Security.Cryptography;
using System.IO;
using System.Text;
using System;

public string EncryptString(string plainText, byte[] key, byte[] iv)
{
    // Instantiate a new Aes object to perform string symmetric encryption
    Aes encryptor = Aes.Create();

    encryptor.Mode = CipherMode.CBC;

    // Set key and IV
    byte[] aesKey = new byte[32];
    Array.Copy(key, 0, aesKey, 0, 32);
    encryptor.Key = aesKey;
    encryptor.IV = iv;

    // Instantiate a new MemoryStream object to contain the encrypted bytes
    MemoryStream memoryStream = new MemoryStream();

    // Instantiate a new encryptor from our Aes object
    ICryptoTransform aesEncryptor = encryptor.CreateEncryptor();

    // Instantiate a new CryptoStream object to process the data and write it to the 
    // memory stream
    CryptoStream cryptoStream = new CryptoStream(memoryStream, aesEncryptor, CryptoStreamMode.Write);

    // Convert the plainText string into a byte array
    byte[] plainBytes = Encoding.ASCII.GetBytes(plainText);

    // Encrypt the input plaintext string
    cryptoStream.Write(plainBytes, 0, plainBytes.Length);

    // Complete the encryption process
    cryptoStream.FlushFinalBlock();

    // Convert the encrypted data from a MemoryStream to a byte array
    byte[] cipherBytes = memoryStream.ToArray();

    // Close both the MemoryStream and the CryptoStream
    memoryStream.Close();
    cryptoStream.Close();

    // Convert the encrypted byte array to a base64 encoded string
    string cipherText = Convert.ToBase64String(cipherBytes, 0, cipherBytes.Length);

    // Return the encrypted data as a string
    return cipherText;
}

public string DecryptString(string cipherText, byte[] key, byte[] iv)
{
    // Instantiate a new Aes object to perform string symmetric encryption
    Aes encryptor = Aes.Create();

    encryptor.Mode = CipherMode.CBC;

    // Set key and IV
    byte[] aesKey = new byte[32];
    Array.Copy(key, 0, aesKey, 0, 32);
    encryptor.Key = aesKey;
    encryptor.IV = iv;

    // Instantiate a new MemoryStream object to contain the encrypted bytes
    MemoryStream memoryStream = new MemoryStream();

    // Instantiate a new encryptor from our Aes object
    ICryptoTransform aesDecryptor = encryptor.CreateDecryptor();

    // Instantiate a new CryptoStream object to process the data and write it to the 
    // memory stream
    CryptoStream cryptoStream = new CryptoStream(memoryStream, aesDecryptor, CryptoStreamMode.Write);

    // Will contain decrypted plaintext
    string plainText = String.Empty;

    try {
        // Convert the ciphertext string into a byte array
        byte[] cipherBytes = Convert.FromBase64String(cipherText);

        // Decrypt the input ciphertext string
        cryptoStream.Write(cipherBytes, 0, cipherBytes . Length);

        // Complete the decryption process
        cryptoStream.FlushFinalBlock();

        // Convert the decrypted data from a MemoryStream to a byte array
        byte[] plainBytes = memoryStream.ToArray();

        // Convert the decrypted byte array to string
        plainText = Encoding.ASCII.GetString(plainBytes, 0, plainBytes.Length);
    } finally {
        // Close both the MemoryStream and the CryptoStream
        memoryStream.Close();
        cryptoStream.Close();
    }

    // Return the decrypted data as a string
    return plainText;
}

Usage

string message = "My secret message 1234";
string password = "3sc3RLrpd17";

// Create sha256 hash
SHA256 mySHA256 = SHA256Managed.Create();
byte[] key = mySHA256.ComputeHash(Encoding.ASCII.GetBytes(password));

// Create secret IV
byte[] iv = new byte[16] { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };

string encrypted = this.EncryptString(message, key, iv);
string decrypted = this.DecryptString(encrypted, key, iv);

Console.WriteLine(encrypted);
Console.WriteLine(decrypted);

Source: https://stackoverflow.com/a/45574121/1461181