>>>>>I know my code works because I tested it and it is in production at several customers. However, I will double check that I am not missing something important, based on your finding. Thank you.
>>>>
>>>>My slightly modified version of the Encrypt function:

        public string Encrypt(string tcStr, string tcPassword)
>>>>        {
>>>>            byte[] tcStrBytes = Encoding.ASCII.GetBytes(tcStr);
>>>>            int tcStrLength = tcStrBytes.Length;
>>>>
>>>>            string lcPassword = tcPassword + ((char) 0).ToString();
>>>>            byte[] lcPasswordBytes = Encoding.ASCII.GetBytes(lcPassword);
>>>>
>>>>            int lnPassLen = lcPasswordBytes.Length;
>>>>            int lnPassPos = 0;
>>>> 
>>>>            string lcStrOut = string.Empty;
>>>>
>>>>            int lnPassNum = ((CipherGetPnum(tcPassword)/997 - 1)%254) + 1;
>>>>
>>>>            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]);
>>>>                lcStrOut = lcStrOut + Convert.ToChar(lnByte==0 ? tcStrBytes[i] : lnByte & 0xFF);
>>>>                lnPassPos = lnPassPos > lnPassLen ? 0 : lnPassPos + 1;
>>>>            }
>>>>            return lcStrOut;
>>>>        }

Quick test comparing results with the VFP version using string 'Test' and password 'Viv' shows identical results in VFP (returned asc values: 255,246,253,131) i.e. string length is 4. Your C# only returns first three values (i.e. string length 3)
>>>
>>>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 :-} )