如何在C＃中对两个字符串进行XOR？

Question

I am making a programme to encrypt a message using a vegnier cypher. But i am not sure what is wrong with it as when I xor the two variables it just outputs a question mark in a box any help will be appreciated.

    static string xor(string plaintext, string pad)
    {
        string cyphertext = "";
        for (int i = 0; i < (plaintext.Length); i++)
        {
            char p1 = Convert.ToChar(plaintext.Substring(i, 1));
            char c1 = Convert.ToChar(pad.Substring(i, 1));
            char ct1 = (char)(p1 ^ c1);
            cyphertext = cyphertext + (Convert.ToString(ct1));

        }
        return cyphertext;
    }

Answer 1

The code itself is likely working, the problem you are having is not all valid values for a char are printable characters. When you XOR the values you end up with a char that represents character that is unprintable for the console.

Answer 2

Since resultant characters can well be unprintable (eg 'A' ^ 'A' == '\\0' - LB example in the comments) I suggest encoding when outputing the encrypted string on the console. The routing itself:

static string xor(string plaintext, string pad) {
  // check for null, empty strings etc.
  if (String.IsNullOrEmpty(plaintext) || String.IsNullOrEmpty(pad))
    return plaintext;

  // Do not append string in the loop; use StringBuilder for this
  StringBuilder cyphertext = new StringBuilder(plaintext.Length);

  // You have no need in SubString and other stuff: just char xor char
  // i % pad.Length - if pad is shorter than plaintext
  for (int i = 0; i < plaintext.Length; ++i) 
    cyphertext.Append((char) (plaintext[i] ^ pad[i % pad.Length]));

  return cyphertext.ToString();
}

To show the text on the console let's represent each character as its unicode value c => ((int) c).ToString("x4") :

string test = "Quick brown fox";
string pad = "Pad";

Console.Write(string.Join(" ", xor(test, pad).Select(c => ((int) c).ToString("x4"))));

The output is

0001 0014 000d 0033 000a 0044 0032 0013 000b 0027 000f

Please, notice command characters

Answer 3

A single xor function would be enough when working with byte arrays, but if you want get a printable result you can use more intelligent codes....

var encstr = Enc("MyTextToHide", "MyKey");
var decstr = Dec(encstr, "MyKey");

---

static string Enc(string plaintext, string pad)
{
    var data = Encoding.UTF8.GetBytes(plaintext);
    var key = Encoding.UTF8.GetBytes(pad);

    return Convert.ToBase64String(data.Select((b, i) => (byte)(b ^ key[i % key.Length])).ToArray());
}

static string Dec(string enctext, string pad)
{
    var data = Convert.FromBase64String(enctext);
    var key = Encoding.UTF8.GetBytes(pad);

    return Encoding.UTF8.GetString(data.Select((b, i) => (byte)(b ^ key[i % key.Length])).ToArray());
}