>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)