🚀 go-pugleaf

frasertw@newton.ccs.tuns.ca (Terrance W Fraser) writes:

>kocherp@leland.Stanford.EDU (Paul Carl Kocher) writes:

>>In article <1992Dec8.022439.24913@newton.ccs.tuns.ca> frasertw@newton.ccs.tuns.ca (Terrance W Fraser) writes:
>>>Does anyone know what GWBASIC for msdos uses to 'encrypt' a program

>>It's a substitution cipher with period 143, probably meaning that there
>>is an 11-byte key and a 13-byte key that get xored/added/etc in some
>>combintation, though after a ten-minute look at it wasn't immediately
>>obvious what steps were used.  It also looks possible that the plaintext
>>might be multiplied by an odd number as part of the encryption.

>I played with it for a while. Your right about the 143 byte period. It
>appears that
>  1) each byte gets encypted separately.
>  2) it's not self-reversing.
>  3) the substitution table changes from byte to byte but repeats
>     every 143 bytes.
>Probably the easiest thing to do is find the code that does it, but
>BASIC is a *big* program.

>Anyone know of one of those PC books that goes into this?

The "back door" approach is *much* faster. You create a short binary file.
(I'd have to look up the exact contents, but it's something like 3 bytes
long). You name it UNPROT.BAS.

Then you load *any* "protected" BASIC file. It won't be LISTable nor can you
SAVE it. 

Now load UNPROT.BAS. 

Voila! You can now LIST the file, or SAVE it or whatever.

UNPROT is the "header" for an *unprotected* BASIC program. But there's
no program attached. BASIC cheerfully loads it into RAM over the header
for the current program....


-- 
Leonard Erickson		      leonard@qiclab.scn.rain.com
CIS: [70465,203]			 70465.203@compuserve.com
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