Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!ames!ncar!boulder!pikes!udenva!isis!nbires!maa
From: maa@nbires.nbi.com (Mark Armbrust)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Unprotecting BASIC programs
Summary: via a bug in GWBASIC
Message-ID: <365@nbires.nbi.com>
Date: 17 Mar 89 04:48:12 GMT
References: <3251@ihuxv.ATT.COM>
Reply-To: maa@nbires.UUCP (Mark Armbrust)
Organization: NBI Inc, Boulder CO
Lines: 46

In article <3251@ihuxv.ATT.COM> riley@ihuxv.ATT.COM (d. riley) writes:
>Could the person (or anyone who may have such information) who sent Matt
>Pressly (mattp@oakhill) the unprotect for GWBASIC programs using degug either
>post that procedure here or at least email it to me (att!ihuxv!riley) also?
>
>					Thanks in advance.

Think that was me, so...

There is a feature (bug) of the GWBASIC interpreter that causes it to work a
bit strangely when it reads a null program (as opposed to an empty file).  This
null program is the two-byte sequence FF, 1A.  Create this file with the
debugger and call it U.BAS:

>debug
-e 100 ff 1a
-n u.bas
-r cx
CX 0000
:2
-w
-q
>

Now start basic and load the protected program and then load U.BAS.  The
protected program should now be unprotected:

LOAD "prot-prg
LIST
Illegal function call
LOAD "u
LIST
10 rem I used to be protected
20 rem but now I am not

(As a side note, it you make a file containing FE 1A, it will act as a 
protector!)

There is an algorithmic way to do this, and I have source to a .ASM program
to do it.  If there is enough interest, I'll post it.

Enjoy,
Mark

maa@nbires.nbi.com
maa@nbires.UUCP