make a crypter

[taos]

Hi!!
I'm making a simple packer but I need your help.

When I encrypt a file, the decrypter code is in the EXE but I'm coding a Dll that decrypt the EXE.

EXE program are encrypted with key A.
EXE OEP is changed to make a call to a function (B) in a dll pasing a parameter.
The parameters are all the encrypted bytes of the code section in a loop.
If you have registered the APP then the function B use this unlock code to return the right decrypted byte.

But can be possible?
How can I determine wich could be the unlock code in the PC to previously encrypt the app?
In EXECryptor if you don't have the right unlock code you can not unpack the EXE. how they do it?.
Ideas???
Regards