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Forums > C64 Coding > EOR file coders
2008-02-22 04:42
The Shadow

Registered: Oct 2007
Posts: 304
EOR file coders

Someone once told me that it is impossible to open a file which was coded with an EOR coder. With todays machines, is there any conceivable way that an EOR coded file can be placed into a PC and descrambled?
 
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2008-03-10 15:20
Ymgve

Registered: May 2002
Posts: 84
Yeah, detecting changes to the error routine should work. He never actually uses a SYS command. He POKEs a small program into memory, hooks the error message vector, then executes a syntax error. There's also no numbers larger than 3 digits, all addresses are created through obfuscated math.
2008-03-10 17:05
MagerValp

Registered: Dec 2001
Posts: 1055
Nasty! :)
2008-03-10 19:51
tlr

Registered: Sep 2003
Posts: 1693
Quote: Nasty! :)


Thanks. :)

I've added the decrypted payload data for people to check out:
http://noname.c64.org/csdb/getinternalfile.php/55441/payload.prg

One attack vector I thought would be usable was statistics in some form. 6502 instruction statistics for instance.
I tried to make the basic stub hard to identify but the Ymgves digit statistics was a very clever idea... :)
2008-03-13 23:12
Burglar

Registered: Dec 2004
Posts: 1024
I wonder...

Seems the only successful cracks were done using statistics, but what if you make the final correct result look like complete garbage (somewhat even distribution of bytes), wouldn't that be a bitch to crack?
2008-03-14 03:30
The Shadow

Registered: Oct 2007
Posts: 304
Excellent idea!
2008-03-14 08:54
Mace

Registered: May 2002
Posts: 1799
I don't see the use of that.
Part of hacking is getting access to useful data.
It's more than finding the code alone.

What could be a nice solution in between is if the decoded data is raw crunched data that needs to be decrunched.
You can at least test if the data is correct, but you won't find it directly after finding the right code.
2008-03-14 12:43
Frantic

Registered: Mar 2003
Posts: 1624
I am not sure, but I think Burglar didn't seriously suggest that someone make a garbage sequence of bytes hard to crack, but rather than if you for example add series of trash bytes in between wellformed parts of code/data (and then pack it or so) to skew the statistical distribution in various ways, then this might be much harder to crack.

...or perhaps it was my turn to misunderstand now. :)
2008-03-14 13:08
Burglar

Registered: Dec 2004
Posts: 1024
Quote: I am not sure, but I think Burglar didn't seriously suggest that someone make a garbage sequence of bytes hard to crack, but rather than if you for example add series of trash bytes in between wellformed parts of code/data (and then pack it or so) to skew the statistical distribution in various ways, then this might be much harder to crack.

...or perhaps it was my turn to misunderstand now. :)


frantic, you understood it perfectly well ;)
2008-03-14 13:19
Mace

Registered: May 2002
Posts: 1799
Aaaah, ok, now I understand too :)
2008-03-14 19:49
tlr

Registered: Sep 2003
Posts: 1693
Quote: I am not sure, but I think Burglar didn't seriously suggest that someone make a garbage sequence of bytes hard to crack, but rather than if you for example add series of trash bytes in between wellformed parts of code/data (and then pack it or so) to skew the statistical distribution in various ways, then this might be much harder to crack.

...or perhaps it was my turn to misunderstand now. :)


That's the reason I only RLE packed the payload. Otherwise it might have gotten too hard.
As with all obfuscation exercises I left a wide hole somewhere else though, i.e leaving a rather large amount of digits in the basic stub.

A really hard challenge would use a full length password on a packed payload.

If you use a suitable chained cipher mode and shuffle the data (e.g start in the middle and wrap) it will be very hard to identify the plain text without decoding a lot of the data.
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