C# encryption (AesManaged) to PHP equivalent

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February 2019

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1

I have some data provided by our third party which is encrypted using a C# algorithm. Please see below:

using System.Security.Cryptography;

private static int KEY_SIZE = 32;
private static byte[] IV = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

public static string EncryptString(string plaintext, string password)
{
    byte[] key = new byte[KEY_SIZE];
    byte[] passwordbytes = Encoding.UTF8.GetBytes(password);

    for (int i = 0; i < KEY_SIZE; i++)
    {
        if (i >= passwordbytes.Length)
            key[i] = 0;
        else
            key[i] = passwordbytes[i];
    }

    byte[] encrypted;

    // Create an AesCryptoServiceProvider object
    // with the specified key and IV.
    using (AesManaged aesAlg = new AesManaged())
    {
        aesAlg.Mode = CipherMode.CBC;
        aesAlg.KeySize = KEY_SIZE * 8;

        // Create a decrytor to perform the stream transform.
        ICryptoTransform encryptor = aesAlg.CreateEncryptor(key, IV);

        // Create the streams used for encryption.
        using (MemoryStream msEncrypt = new MemoryStream())
        {
            using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
            {
                using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                {
                    //Write all data to the stream.
                    swEncrypt.Write(plaintext);
                }
                encrypted = msEncrypt.ToArray();
            }
        }
    }

    return Convert.ToBase64String(encrypted);
}

I would like to write the decryption for this in PHP. I was also provided just a key they called it "AES256 Encryption key".

Also the decryption algorithm they provided me in C# is as follows:

public static string DecryptString(string cipherText, string password)
{
    byte[] key = new byte[KEY_SIZE];
    byte[] passwordbytes = Encoding.UTF8.GetBytes(password);

    for (int i = 0; i < KEY_SIZE; i++)
    {
        if (i >= passwordbytes.Length)
            key[i] = 0;
        else
            key[i] = passwordbytes[i];
    }

    byte[] CipherTextBytes = Convert.FromBase64String(cipherText);

    // Declare the string used to hold
    // the decrypted text.
    string plaintext = null;

    // Create an AesCryptoServiceProvider object
    // with the specified key and IV.
    using (AesManaged aesAlg = new AesManaged())
    {
        aesAlg.Mode = CipherMode.CBC;
        aesAlg.KeySize = KEY_SIZE * 8;

        // Create a decrytor to perform the stream transform.
        ICryptoTransform decryptor = aesAlg.CreateDecryptor(key, IV);

        // Create the streams used for decryption.
        using (MemoryStream msDecrypt = new MemoryStream(CipherTextBytes))
        {
            using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
            {
                using (StreamReader srDecrypt = new StreamReader(csDecrypt))
                {
                    // Read the decrypted bytes from the decrypting stream
                    // and place them in a string.
                    plaintext = srDecrypt.ReadToEnd();
                }
            }
        }

    }
    return plaintext;
}

But I want to write the decryption in PHP. As I am very new to encryption stuff please guide me in the right direction.

Many thanks....

1 answers

4

We have figured out as below. I hope it would be useful for someone.

function encrypt($string = '', $key = '') {
    $key = utf8_encode($key);

    //make it 32 chars long. pad with \0 for shorter keys
    $key = str_pad($key, 32, "\0");

    //make the input string length multiples of 16. This is necessary
    $padding = 16 - (strlen($string) % 16);
    $string .= str_repeat(chr($padding), $padding);

    //emtpy IV - initialization vector
    $iv = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";

    $encrypted = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $string, MCRYPT_MODE_CBC, $iv));
    return rtrim($encrypted);
}

function decrypt($string = '', $key = '') {
    $key = $key . "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
    $iv = "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0";
    $string = base64_decode($string);

    return mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $string, MCRYPT_MODE_CBC, $iv);
}