>>>>>>>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;