Dare I suggest this, but have you studied the PIC manuals regarding
how code protection is actually IMPLEMENTED?

  My recollection of the description is that a code-protected part will
read back "data" from its EPROM which is "scrambled" according to a
pre-determined and un-revealed algorithm.  What this means is that by
successfully programming a part and code-protecting it, you may "read"
back a file from it.  The ONLY thing the file is now useful for, is to
COMPARE with the same file read back from any other device likewise
programmed, to check whether programming is successful, or has been
maintained (i.e., not hit by cosmic rays as so often seems to happen to
the chip in $2 watches).

  I rather suspect the "scrambling" involves a CRC.

  RTFM?

  Paul B.