Finding a way to encrypt messages in C# and decrypting them in PHP or vice versa seems to be a “challenge” for many users. I wrote this tutorial to provide some help with this: below, you can find how to encrypt / decrypt messages in C# / PHP using AES256 with CBC mode.

1.Basic Information

AES 256 with CBC mode requires 3 values: the message, a key (32 bytes long) and an initialization vector (IV). Note that you must use the same IV when encrypting / decrypting a message: otherwise the message is lost. Sending the IV with the message is perfectly safe but it always has to be a random value. Since it has a fixed size, I always place the IV at the end of the encrypted text.

The encrypted messages should be encoded using base64 before being sent.

structure

Encryption steps:
  • encrypt the text
  • add the IV at the end
  • encode everything (base64)
Decryption steps:
  • decode the message
  • get & remove the IV
  • proceed to decrypt

Ok, enough talking, let’s see some code…

2.PHP Encryption/Decryption Code

PHP accepts keys that are not 32 bytes long and simply extends them to the correct length. Well…C# doesn’t, so you’ll have to use a key that is 32 bytes long.

Encryption
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function encrypt($text, $pkey)
{
	$key = $pkey;  
	$IV = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_CBC), MCRYPT_RAND); 

	return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_CBC, $IV)."-[--IV-[-".$IV); 
}
Decryption
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function decrypt($text, $pkey)
{
	$key = $pkey;   
	$text = base64_decode($text); 
	$IV = substr($text, strrpos($text, "-[--IV-[-") + 9);
	$text = str_replace("-[--IV-[-".$IV, "", $text);

	return rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $key, $text, MCRYPT_MODE_CBC, $IV), "\0");
}

3.C# Encryption/Decryption Code

As I said before, C# doesn’t accept keys that aren’t 32 bytes long - it will throw an error. Also, many people get tricked here because of the encoding (most of the times you have to use Encoding.Default).

Encryption
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public static string EncryptMessage(byte[] text, string key)
{
    RijndaelManaged aes = new RijndaelManaged();
    aes.KeySize = 256;  
    aes.BlockSize = 256;
    aes.Padding = PaddingMode.Zeros;
    aes.Mode = CipherMode.CBC;

    aes.Key = Encoding.Default.GetBytes(key);
    aes.GenerateIV();  

    string IV = ("-[--IV-[-" + Encoding.Default.GetString(aes.IV));

    ICryptoTransform AESEncrypt = aes.CreateEncryptor(aes.Key, aes.IV);
    byte[] buffer = text;

    return
Convert.ToBase64String(Encoding.Default.GetBytes(Encoding.Default.GetString(AESEncrypt.TransformFinalBlock(buffer, 0, buffer.Length)) + IV));

}
Decryption
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public static string DecryptMessage(string text, string key)
{
    RijndaelManaged aes = new RijndaelManaged();
    aes.KeySize = 256;
    aes.BlockSize = 256;
    aes.Padding = PaddingMode.Zeros;
    aes.Mode = CipherMode.CBC;

    aes.Key = Encoding.Default.GetBytes(key);

    text = Encoding.Default.GetString(Convert.FromBase64String(text));

    string IV = text;
    IV = IV.Substring(IV.IndexOf("-[--IV-[-") + 9);
    text = text.Replace("-[--IV-[-" + IV, "");

    text = Convert.ToBase64String(Encoding.Default.GetBytes(text));
    aes.IV = Encoding.Default.GetBytes(IV);

    ICryptoTransform AESDecrypt = aes.CreateDecryptor(aes.Key, aes.IV);
    byte[] buffer = Convert.FromBase64String(text);

    return Encoding.Default.GetString(AESDecrypt.TransformFinalBlock(buffer, 0, buffer.Length));
}