Question

I’ve been trying to find out the correct way to encrypt a byte[16] variable using DES algorithm. Here’s the scenario:

• The data should be encrypted in 8-byte parts. the key for encryption is: byte[] {11, 11, 11, 11, 11, 11, 11, 11}
• First 8 bytes is encrypted using Instance Vector = new byte[8] (8 bytes with 0 value).
• the encrypted result will be the IV for the next 8 bytes. (is this CBC?)
• that last 8 byte is the result I should send.

With this information, I have implemented the Encrypt method as follows:

public static byte[] Encrypt(byte[] data)
{
    var dataChunk = new byte[8];
    var IV = new byte[8];
    var result = new byte[8];
    var key = new byte[] { 11, 11, 11, 11, 11, 11, 11, 11 };

    for (int counter = 0; counter < data.Length / 8; counter++)
    {
        // Copy the next 8-byte chunk.
        Array.Copy(data, counter * 8, dataChunk, 0, 8);
        var des = System.Security.Cryptography.DES.Create();
        des.Key = key;
        des.IV = IV;
        des.Padding = PaddingMode.None;
        ICryptoTransform cryptoTransform = des.CreateEncryptor(key, IV);

        // Encrypt the datra chunk.
        cryptoTransform.TransformBlock(dataChunk, 0, 8, result, 0);

        // Set the new IV.
        Array.Copy(result, IV, 8);
    } 
return result;
}

Is this the correct way of encrypting data using DES cryptography?

Answer 1

You don’t need to create an encryptor for each block. The encryptor itself implements the operation modes (CBC, …) and padding.
So you need something like this:

byte[] input = ...
byte[] key = ...
byte[] iv = ...

using (DES des = DES.Create())
{
    des.Mode = CipherMode.CBC;
    des.Padding = PaddingMode.None;

    using (ICryptoTransform encryptor = des.CreateEncryptor(key , iv))
    {
        byte[] output = encryptor.TransformFinalBlock(input, 0, input.Length);
    }
}

I’m omitting the warning of using DES here. You might also want to take a look at CryptoStream afterwards for an even easier en- and decryption …