Question

I’m just about to build a simple chat application using One Time Pad.
I’ve already made the algorithm, and to encrypt the messages, I need some sort of key material that is the same on both sides. The distribution of the key material is supposed to happen with physical contact (e.g. USB dongle). So I would like to make some very large random key files, that the two clients can use to communicate. So my questions are:

• I need a very secure random number/string generator, do you know any good ones that I can use in C#?
• And how do I, when I use such big files, avoid to load the whole file into memory, as I plan to read a chunk of the key material (e.g. 1 MB), and remove it from the file afterwards when read, so the same key won’t be used twice.

Answer 1

I Should probably start with this: I assume this is a for fun or for exercise project – not an attempt to create something truly secure.

1. As owlstead says: Use RNGCryptoServiceProvider.
2. Removing the used key material from the file is much easier if you use it in reverse. If you need to encrypt 1024 bytes, read the last 1024 bytes from the file and truncate it. Simplified:

 

byte[] Encrypt(byte[] plain){
  using (FileStream keyFile = new FileStream(FileName, FileMode.Open))
  {
    keyFile.Seek(-plain.Length, SeekOrigin.End);
    byte[] key = new byte[plain.Length];
    keyFile.Read(key, 0, plain.Length);    
    byte[] encrypted = new byte[plain.Length];
    for(int i=0;i<plain.Length;i++){
      encrypted[i] = (byte) (plain[i] ^ key[plain.Length - 1 - i]);
    } 
    keyFile.SetLength(keyFile.Length - plain.Length);   
    return encrypted;
  }      
}