>>>>>>>>I am testing your code and mine code and neither one works :). I am sure it is me. Here is how I test it, please let me know if this is the correct approach:
>>>>>>>>

>>>>>>>>string cTest1 = "ABC123";
>>>>>>>>string cTest1Encr;
>>>>>>>>string cTest1Decr;
>>>>>>>>cTest1Encr = Encrypt( cTest1, "123abc");
>>>>>>>>cTest1Decr = Encrypt( cTest1Encr, "123abc");
>>>>>>>>

>>>>>>>>
>>>>>>>>Shouldn't cTest1Decr show "ABC123"?
>>>>>>>
>>>>>>>Aren't you encrypting the values instead of decrypting the 2nd?
>>>>>>
>>>>>>The second parameter is the password string. The first one is the value.
>>>>>
>>>>>Too late to play tonight - I'll get back to you in the morning (my morning :-} )
>>>
>>>Encoding problem when char values > 128. This should work. Didn't look at your code I'm afraid:

using System;
>>>using System.Collections.Generic;
>>>using System.Linq;
>>>using System.Text;
>>>
>>>namespace Cipher
>>>{
>>>    public class Cipher
>>>    {
>>>        private const int PwMinNum = 1000;
>>>
>>>        public string Encrypt(string tcStr, string tcPassword)
>>>        {
>>>            byte[] tcStrBytes = Encoding. GetEncoding(1252).GetBytes(tcStr);
>>>            int tcStrLength = tcStrBytes.Length;
>>>
>>>            string lcPassword = tcPassword + ((char) 0).ToString();
>>>            byte[] lcPasswordBytes = Encoding.GetEncoding(1252).GetBytes(lcPassword);
>>>
>>>            int lnPassLen = lcPasswordBytes.Length;
>>>            int lnPassPos = 0;
>>>            int lnPassNum = ((CipherGetPnum(lcPasswordBytes)/997 - 1)%254) + 1;
>>>
>>>            byte[] bytes = new byte[tcStr.Length];
>>>
>>>            for (int i = 0; i < tcStrLength; i++)
>>>            {
>>>                int lnNum01 = ((lnPassNum + (i - tcStrLength)) - 1);
>>>                lnPassNum = (Math.Abs(lnNum01)%254)*Math.Sign(lnNum01) + 1;
>>>                var lnByte = tcStrBytes[i] ^ (lnPassNum ^ lcPasswordBytes[lnPassPos]);
>>>                bytes[i] = lnByte==0 ? tcStrBytes[i] : (byte) (lnByte & 0xFF);
>>>                lnPassPos = lnPassPos > lnPassLen ? 0 : lnPassPos + 1;
>>>            }
>>>            return Encoding.GetEncoding(1252).GetString(bytes);
>>>        }
>>>
>>>        private int CipherGetPnum(IEnumerable<byte> ascii)
>>>        {
>>>            int liRet = 1 + ascii.Select((t, i) => t + i).Sum();
>>>            while (liRet < PwMinNum)
>>>            {
>>>                liRet = liRet << 1;
>>>            }
>>>            return liRet;
>>>        }
>>>    }
>>>}

>>
>>Thank you very much. Your code works; almost 100%. Except when a string that you are trying to encrypt has more than 2 spaces
>>(e.g."ABC123 ", 3 spaces after '3'), it produces error. Btw, if the string has only 2 empty spaces, no error. E.g. "ABC123 " (two spaces after 3), no error.
>
>Duh. Can't count - I was exceeding password length :-{ .
>Two changes should fix:

int lnPassLen = lcPasswordBytes.Length - 1;
>//and
>lnPassPos = lnPassPos == lnPassLen ? 0 : lnPassPos + 1;

Increment lnPassPos in the for clause ?

 for (int i = 0; i < tcStrLength; i++, lnPassPos++ )