Vernam Cipher XOR not always working

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Here's the encryption method I create,

C#

public string Encrypt(string Text, string Key)
        {
            string newText = Text;
            string newKey = Key;
            string newResult = string.Empty;

            //EQUATE TEXT AND KEY LENGTH
            if (Text.Length > Key.Length)
            {
                int left = 0;

                for (int i = 0; i < Math.Abs(Text.Length - Key.Length); i++)
                {
                    newKey += Key.Substring(left, 1);
                    left += 1;

                    if (left == Key.Length) { left = 0; }
                }
            }
            else if (Text.Length < Key.Length)
            {
                newKey = Key.Substring(0, Text.Length);
            }
            
            //GET ASCII CODE > CONVERT TO BINARY > MERGE BINARY
            for (int i = 0; i < Text.Length; i++)
            {
                byte txtASC = Encoding.Default.GetBytes(newText)[i]; 
                byte keyASC = Encoding.Default.GetBytes(newKey)[i];

                string txtBIN = Convert.ToString(Convert.ToInt32(txtASC), 2);
                string keyBIN = Convert.ToString(Convert.ToInt32(keyASC), 2);
                string newBIN = string.Empty;

                newBIN = Convert.ToString(Convert.ToInt32(txtBIN, 2) + Convert.ToInt32(keyBIN, 2), 2); //CHANGE + TO - IN DECRYPT METHOD

                newResult += ((char)Convert.ToInt32(newBIN, 2)).ToString();

            }

            return newResult;
        }

When I use a simple 'a' key, and a single 'apple' word, both encrypt and decrypt works.
But when I put sentences and a longer key, I can't read the result of decryption anymore.
Can someone explain what's wrong in my code?

Posted 28-Nov-13 22:21pm

Midnight Ahri

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OriginalGriff · Answer 1 · 2013-11-29T00:22:00

Would you like a list?
It's hard to spot the problem you are complaining of - your code doesn't work with a string "Apple" and a key of "1234" so your testing is a little suspect...For a Vernam Cipher, I would expect to get an encrypted string the same length as the input string, and your code doesn't do that. It also doesn't implement a Vernam Cipher: the lack of an XOR in there is a bit of a clue.
http://www.cryptomuseum.com/crypto/vernam.htm[^]

I think you need to sit down, read the link, and start all over again.
BTW: You don't need to create a new key that is long enough to cover the whole message:
Just create an index starting at zero, and check it against the length of the key:

C#

int keyIndex = 0;
foreach (char c in Text)
    {
    if (keyIndex >= key.Length) keyIndex = 0;
    char keyValue = key[keyIndex];
    ...
    keyIndex++;
    }