help me with xor encryption in c#

Question

I wrote this code in c# to encrypt a text with a key :

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text;
namespace ENCRYPT
{
    class XORENC
    {
        private static int Bin2Dec(string num)
        {
            int _num = 0;

            for (int i = 0; i < num.Length; i++)
            {
                _num += (int)Math.Pow(2, num.Length - i - 1) * int.Parse(num[i].ToString());
            }

            return _num;
        }

        private static string Dec2Bin(int num)
        {
            if (num < 2) return num.ToString();
            return Dec2Bin(num / 2) + (num % 2).ToString();
        }

        public static string StrXor(string str, string key)
        {
            string _str = "";
            string _key = "";
            string _xorStr = "";
            string _temp = "";

            for (int i = 0; i < str.Length; i++)
            {
                _temp = Dec2Bin(str[i]);

                for (int j = 0; j < 8 - _temp.Length + 1; j++)
                {
                    _temp = '0' + _temp;
                }

                _str += _temp;
            }

            for (int i = 0; i < key.Length; i++)
            {
                _temp = Dec2Bin(key[i]);

                for (int j = 0; j < 8 - _temp.Length + 1; j++)
                {
                    _temp = '0' + _temp;
                }

                _key += _temp;
            }

            while (_key.Length < _str.Length)
            {
                _key += _key;
            }

            if (_key.Length > _str.Length) _key = _key.Substring(0, _str.Length);

            for (int i = 0; i < _str.Length; i++)
            {
                if (_str[i] == _key[i]) { _xorStr += '0'; } else { _xorStr += '1'; }
            }
            _str = "";

            for (int i = 0; i < _xorStr.Length; i += 8)
            {
                char _chr = (char)0;
                _chr = (char)Bin2Dec(_xorStr.Substring(i, 8)); //ERROR : (Index and length must refer to a location within the string. Parameter name: length)
                _str += _chr;
            }

            return _str;
        }
    }
}

the problem is that I always get error when I want to decrypt an encryted text with this code. see the example below for more info :

string enc_text = ENCRYPT.XORENC("abc","a"); //enc_text = " ♥☻"
string dec_text = ENCRYPT.XORENC(enc_text,"a"); //ERROR : ArgumentOutOfRangeException

any one can help ?

Answer 1

If you have a character, a char, you can convert it to an integer, an int.

And then you can use the ^ operator to perform XOR on it. You don't appear to be using that operator at the moment, which might be the source of your problem.

string EncryptOrDecrypt(string text, string key)
{
    var result = new StringBuilder();

    for (int c = 0; c < text.Length; c++)
        result.Append((char)((uint)text[c] ^ (uint)key[c % key.Length]));

    return result.ToString();
}

That kind of thing. Here's a longer version with comments that does the same thing in steps, to make it easier to learn from:

string EncryptOrDecrypt(string text, string key)
{
    var result = new StringBuilder();

    for (int c = 0; c < text.Length; c++)
    {
        // take next character from string
        char character = text[c];

        // cast to a uint
        uint charCode = (uint)character;

        // figure out which character to take from the key
        int keyPosition = c % key.Length; // use modulo to "wrap round"

        // take the key character
        char keyChar = key[keyPosition];

        // cast it to a uint also
        uint keyCode = (uint)keyChar;

        // perform XOR on the two character codes
        uint combinedCode = charCode ^ keyCode;

        // cast back to a char
        char combinedChar = (char)combinedCode;

        // add to the result
        result.Append(combineChar);
    }

    return result.ToString();
}

The short version is the same but with the intermediate variables removed, substituting expressions directly into where they're used.