XOR a file with a given key.

XOR

using System;
using System.Text;
using System.IO;

namespace XOR
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 2)
                return;
            byte[] key = Encoding.UTF8.GetBytes(args[0]);
            for (int i = 1; i < args.Length; i++)
            {
                if (!File.Exists(args[i]))
                    continue;
                using (FileStream fs = new FileStream(args[i], FileMode.Open, FileAccess.ReadWrite))
                {
                    long offset = 0;
                    byte[] buffer = new byte[1024];
                    int count = buffer.Length;
                    while (count==buffer.Length)
                    {
                        count = fs.Read(buffer, 0, buffer.Length);
                        for (int j = 0; j < buffer.Length; j++)
                            buffer[j] ^= key[j % key.Length];
                        fs.Seek(offset, SeekOrigin.Begin);
                        fs.Write(buffer, 0, count);
                        offset += count;
                    }
                }
            }
        }
    }
}