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Keep Your Code Hidden From Prying Eyes

Tecniche di back-jump nel codice

(by demogorgon)

Questa interessante tecnica consiste nell'eseguire dei back JMP per rendere difficile la lettura del codice.
This article is online from 4925 days and has been seen 5266 times

40HEX_6_002 SEGMENT PUBLIC 'code'
ORG 100H


Concealment: Keep Your Code Hidden From Prying Eyes
by <A HREF="v4.1.html#demogorgon">Demogorgon</A>/PHALCON/SKISM

Recently, I have been experimenting with a few new programming techniques
that should be of great interest to the virus writing community. It is always
our top priority to keep our code out of the hands of lamers in order to
prevent the dreaded 'text change' and above all, to cause the anti-virus
community as much grief as possible. In order to do this, we must put a great
deal of effort into concealing our code. That is the focus of this article.

This file is divided into two parts. The first part is devoted to developing
'debug resistant' code, and the second part deals with defeating disassemblers.
I will not cover encryption, because methods of encryption are commonly known
and there is really not much further I can go with that. For a complete review
of self encryption methods, take a look at Dark Angel's Funky Virus Writing
Guide (number three, the one that hasn't been released yet.)

Part_I: The debugger is NOT your friend

The basic idea behind writing debug ressistant code is finding a way to
make your code behave differently when it runs under a debugger. With a real
mode debugger, this is simplicity itself. All that is necessary is a little
knowledge of how a debugger works. A debugger, such as debug or TD traces
through a program by setting handlers to int 1 and int 3. These are called
after every instruction is executed. A virus that wishes to avoid being
debugged can simply replace the handlers for these interrupts, and the results
will be just about whatever you want. Here is some code to do this:

push cs
pop ds
mov dx, offset eat_int
mov ax,2501h
int 21h
mov al,03h
int 21h
... ; rest of code
eat_int: iret

As you can see, this requires minimal space in your code, and is certainly
worth the effort. You can experiment by placing something else at 'eat_int'.
Another commonly used tactic is to disable the keyboard interrupt while certain
parts of the code are being executed. This will surely keep lamers baffled,
though a pro would recognize what was going on immediately. I am sure McAfee's
programmer's scoff at code such as this. Also note that while this will defeat
the average real mode debugger, any protected mode debugger will step through
this as if it weren't there. Playing with interrupts will not help you when
your program will be running in a virtual cpu anyway. One method I found which
will work nicely against td386 is to throw in a hlt instruction. This will
give TD an exception 13 error, and terminate the program. Anyone who is aware
of this will just step over a hlt instruction, so therefore methods must be
used to conceal its presence, or to make it a necessary part of the code. This
will be covered in part II.
Another trick you can play is to call int3 within your program. If
someone tries to run your program under a debugger, it will stop each time int3
is called. It is possible to trace through it, but it will be annoying if
there are many int3's thrown in.

Part_2: Kill your disassembler

No matter how well you mess up debuggers, your program is entirely at the
mercy of a programmer armed with a good disassembler. Unless, of course, you
use techniques that will confuse disassemblers. My favorite method for
baffling them is to create code that overlaps. Overlapping code may seem a
little bit too complicated for most of us at first, but with the knowledge of a
few instruction hex translations, you too can make effective overlapping code
without sacrificing too much code size. Overlapping code can get as complex as
you would like, but this file will only deal with the simplest examples.

eat_sr: mov ax,02EBh
jmp $-2 ; huh?
... ; rest of code

This may confuse you at first, but it is fairly simple. The first instruction
moves a dummy value into ax. The second instruction jmps into the value that
was just moved into ax. '02EB' translates into 'jmp $+2' (remember that words
are stored in reverse). This jump goes past the first jmp, and continues on
with the code. This will probably not be sufficient to defeat a good
disassembler like Sourcer, but it does demonstrate the technique. The problem
with this is that Sourcer may or may not just pick up the code after commenting
out the 'jmp $-2'. It is difficult to predict how Sourcer will respond, and it
usually depends on the bytes that appear directly after the jmp. To severely
baffle Sourcer, it is necessary to do some stranger things. Take a look at
this example.

erp: mov ax,0FE05h
jmp $-2h
add ah,03Bh
... ; rest of code

This code is quite a bit more useful than the previous listing. Let us
simulate what would happen if we were to trace through this code, showing a hex
dump at each step to clarify things.

B8 05 FE EB FC 80 C4 3B mov ax,0FE05h ; ax=FE05h
^^ ^^ ^^
B8 05 FE EB FC 80 C4 3B jmp $-2 ; jmp into '05 FE'
^^ ^^
B8 05 FE EB FC 80 C4 3B add ax,0EBFEh ; 05 is 'add ax'
^^ ^^ ^^
B8 05 FE EB FC 80 C4 3B cld ; a dummy instruction
B8 05 FE EB FC 80 C4 3B add ah,3Bh ; ax=2503h
^^ ^^ ^^

The add ah,03Bh is there simply to put the value 2503h into ax. By adding
five bytes (as opposed to simply using 'mov ax,2503h') this code will confuse
disassemblers pretty well. Even if the instructions are disassembled properly,
the value of ax will not be known, so every int call after this point will not
be commented properly, as long as you never move a value into ax. You can
conceal the value from the disassembler by using 'add ax' or 'sub ax' whenever
If you examine this closely, you can see that any value can be put into
ax. Two of the values can be changed to whatever you want, namely the FE in
the first line, and the 3B in the last line. It is helpful to debug through
this chunk of code to determine what values should be placed here in order to
make ax what you would like it to be.
Back to the subject of killing debuggers, it is very sneaky to hide
something like a hlt instruction inside another instruction, such as a jmp.
For example, take a look at this:

glurb: mov cx,09EBh
mov ax,0FE05h ;-\
jmp $-2 ; >--this should look familiar to you
add ah,03Bh ;-/
jmp $-10
... ; rest of code

The three lines in the middle are a repeat of the previous example. The
important part of this code is the first line and the 'jmp $-10'. What happens
is, the jmp goes back into the 'mov cx' instruction. The '09EB' translates
into 'jmp $+9'. This lands in the '$-10' part of the first jmp. The $-10 just
happens to be stored as 0F4h, the hlt instruction. By making the hlt part of
another instruction, it is not visible when it is being traced through by
td386. It is also not possible to remove it without altering the code.

The purpose of this article is not to supply code to be thrown into your
own programs. The purpose is to get you to think about new ways to avoid
having your code looked at and modified by others. The most important thing is
to be original. It is pointless for you to simply duplicate this code, because
anyone else who has read this file will already know what you are trying to do.

code ENDS
END concealment

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