Help!!!

[na_th_an]

It can't be decoded 'cause it fails on something completely necessary: encoding using a biyective function. Right now you are encoding several source characters with the very same destination character, for example you are encoding ".", "R" and j with a "P" (not actual, just an example). When deciphering, you find a "P" and... what to choose, ".", "R" or "j"?

It's like if I coded this encryption program:

Code:

For i% = 1 To Len (text$)
   If Asc(Mid$(text$, i%, 1)) < 100 Then
      Mid$(text$, i%, 1) = "A"
   Else
      Mid$(text$, i%, 1) = "B"
   End If
Next

Which would turn any sentence into something like "AAABBBABBABABBABABBABAAABABABBBBABABAAABBA". Now... how to decrypt? Impossible: you are losing information with the encryption.