C# - Serializing/Deserializing a DES encrypted file from a stream

Question

Does anyone have any examples of how to encrypt serialized data to a file and then read it back using DES?

I've written some code already that isn't working, but I'd rather see a fresh attempt instead of pursuing my code.

EDIT: Sorry, forgot to mention I need an example using XmlSerializer.Serialize/Deserialize.

Whitmer

By the way, I'm using the CF, so memory is a constraint. – Whitmer

Can you elaborate? Do you want to encrypt something and then serialize it to the XML format, or do you want to encrypt the serialized data? – Cortie

Whichever is less time consuming. I've got a collection of customer information that needs to be encrypted to a file. The way I saw it working was to serialize through a cryptostream to a file (which works) and then deserialize through a cryptostream from a file (which doesn't work). – Whitmer

Answer 1

Encryption

public static void EncryptAndSerialize(string filename, MyObject obj, SymmetricAlgorithm key)
{
    using(FileStream fs = File.Open(filename, FileMode.Create))
    {
        using(CryptoStream cs = new CryptoStream(fs, key.CreateEncryptor(), CryptoStreamMode.Write))
        {
            XmlSerializer xmlser = new XmlSerializer(typeof(MyObject));
            xmlser.Serialize(cs, obj); 
        }
    }
}

Decryption:

public static MyObject DecryptAndDeserialize(string filename, SymmetricAlgorithm key)    
{
    using(FileStream fs = File.Open(filename, FileMode.Open))
    {
        using(CryptoStream cs = new CryptoStream(fs, key.CreateDecryptor(), CryptoStreamMode.Read))
        {
            XmlSerializer xmlser = new XmlSerializer(typeof(MyObject));
            return (MyObject) xmlser.Deserialize(cs);
        }
    }
}

Usage:

DESCryptoServiceProvider key = new DESCryptoServiceProvider();
MyObject obj = new MyObject();
EncryptAndSerialize("testfile.xml", obj, key);
MyObject deobj = DecryptAndDeserialize("testfile.xml", key);

You need to change MyObject to whatever the type of your object is that you are serializing, but this is the general idea. The trick is to use the same SymmetricAlgorithm instance to encrypt and decrypt.

Elyse

Looks like we posted about the same time, I'll accept as it's near enough what I actually wanted! Thanks Bryce. – Whitmer

But how would I make key from a known string? Like "this_is_a_password_to_unlock_the_file"? This works in your "Usage" case, but if you want to save the key, and then use it to unlock the encrypted data, how would I do that? :/ – Burlie

Got it! To encrypt with a certain password: key.CreateEncryptor(Encoding.ASCII.GetBytes("64bitPas"), Encoding.ASCII.GetBytes("InitVector")), to decrypt with the same password: key.CreateDecryptor(Encoding.ASCII.GetBytes("64bitPas"), Encoding.ASCII.GetBytes("InitVector")) – Burlie

Answer 2

This thread gave the basic idea. Here's a version that makes the functions generic, and also allows you to pass an encryption key so it's reversible.

public static void EncryptAndSerialize<T>(string filename, T obj, string encryptionKey) {
  var key = new DESCryptoServiceProvider();
  var e = key.CreateEncryptor(Encoding.ASCII.GetBytes("64bitPas"), Encoding.ASCII.GetBytes(encryptionKey));
  using (var fs = File.Open(filename, FileMode.Create))
  using (var cs = new CryptoStream(fs, e, CryptoStreamMode.Write))
      (new XmlSerializer(typeof (T))).Serialize(cs, obj);
}

public static T DecryptAndDeserialize<T>(string filename, string encryptionKey) {
  var key = new DESCryptoServiceProvider();
  var d = key.CreateDecryptor(Encoding.ASCII.GetBytes("64bitPas"), Encoding.ASCII.GetBytes(encryptionKey));
  using (var fs = File.Open(filename, FileMode.Open))
  using (var cs = new CryptoStream(fs, d, CryptoStreamMode.Read))
      return (T) (new XmlSerializer(typeof (T))).Deserialize(cs);
}

Overthecounter

-1 for wrong ordering of params in CreateEncryptor & invalid initialization vector for DES and that that it wont decrypt the stream properly (due to unflushed cryptostream). – Arliearliene

Answer 3

Here is an example of DES encryption/decription for a string.

Cortie

Sorry, I need an example using XmlSerializer. I'll amend the main question. – Whitmer