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11 October, 2010


Lession #1


By Buckaroo Banzai

Today I'm here to about is cracking a dos format (either
.EXE or .COM) file. This, in my mind is releativly the
simplest (in theory although pratice might say otherwise)
type of crack to do.

There are really 3 steps in cracking a dos file. Step
1, is finding where the protection routine is. How to go
about it, well, there are several diffrent methods. Here are
the steps that I often use.

First, I will run the program under PC-WATCH (PW)
trapping INT13 all functions and INT21 functions 3Dh and 3Fh.
Why trap the functions. This will give (hopefully) a
starting place to look for the protection. Once you have set
the breakpoints, press [F4] to execute and you will drop to
dos. When you do, PW should display several calls to INT13.
What closly at the CS:IP of these calls. Record it for later
because these are calls from dos. We will uses this data to
recognize what is a call to the protection and what is not.

Next, run the program to be cracked. As it executes,
PC-WATCH will show what files are opened (including the file
you just ran since DOS uses function 3Dh to open a file when
it executes one) and what (and more improtantly WHERE) data
is read to. Makes a list saying what data is read in from
what file. Here is an example. Lets say you ran the program
"XXX.COM". While running, "XXX.COM", you noticed that 2
other files "YYY.BIN" and "ZZZ.BIN" were also opened. So
make a list like this:


Now, lets say that after "XXX.COM" was opened, PW showed
that there were 2 reads from "XXX.COM" (the way to tell where
the data is being read from is by checking the BX register on
calls to 3Fh and the AX registers after calls to 3Dh. Yes,
you should select both INPUT REGISTERS and OUTPUT REGISTERS
from the PW menu) 1 at aaaa:bbbb and 1 as cccc:dddd. Right
after "YYY.BIN" was opened, PW showed data was read to
eeee:ffff and then after "ZZZ.BIN" was opened, data was
written to gggg:hhhh and iiii:jjjj. Now, our list looks like

aaaa:bbbb eeee:ffff gggg:hhhh
cccc:dddd iiii:jjjj

What we have just created in a program load map. This
map shows where to program to be cracked is loaded in memory.
Next, scan though the calls to INT13. Look for either calls
that return with errors, calls that have high values in the
CH ( > 28) or CL ( > 9) registers, or calls not made by dos
(those calls that have a CS:IP diffrent from the one we
copied down before we executed the program). Now, look at
the CS:IP of the call to INT13. Match the segment address
against the program load map. If only 1 match occurs, then
you now know what module the check is in so continue on to
step #2. If more than 1 match occurs, check the offset (IP).
Find the one that is closest to one of the write address's
offset. Once you find a match, then go on to step #2. If no
match occurs after both steps, it's time to track through the

Tracking your way though the program is a real bitch. I
do not like to do because it can just take to long. But here
is an overview on how it is done.

The object, is to keep narowing down calls until disk
access if found. How to do this. Well, load the program
under debug. Keep tracing through the program in till a
"CALL" instruction is found. Jot down you current IP and
PROCEED (using debug P command) over the instruction without
tracing in to it. If you end up at the next instruction
without access that disk, then you have not found the routine
you are looking for so keep going. Search for the next
"CALL" and then the next and then the next etc. At some
point, when you proceed over a call, the disk will either
check protection or load in a new module.

How to tell the diffrence, well PW is still active and
will tell you if it was a call to INT13 or INT21 or BOTH. If
it was the call to INT13 or a call to BOTH then you have
found a call to the protection routine (although the actual
call may be 100 levels deeper, you are on the right track).
Exit and restart but this time when you reach the call,
trace into it. Now do the same process until you get to the
call to the next level, then again for the next, etc.
Finally you should find where it is.

But hopefully, you won't have to do that. As I said, it
is very time consumming. Hopefully, you will know which
module to look in. If you do, here is how to find the call
to the protection. First, try the simple search method.
Load up the module using DEBUG and simply type:

S CS:100 FFFF CD 13 (use CS:100 if .EXE)

Debug will hopefully list 1 or more address. If not,
try the same command only using CS:0000. If again you are
not givin any address, you have some tricky debugging at hand
(an I suggest rereading the exercise in self-modifying code).

I will explain in detail how to find self-modifying code
later but for now, lets assume we have found the protection
routine. Next, is to figure out what the copy protection is
trying to do. First, look to the printout from PW. Look
through it until you find the calls the INT13 from the
protection routine. Look at the AH register. If it is 00h
then the protection routine is probally reading in data from
the protected tracks. If not, then the protection is simply
looking for some KEY (in other words a damaged track or
sector) that DOS canno't duplicate.

The second choice is much eaiser to defeat. 2 quick
methods to defeating this type. First, you can fake the call
and simply set the registers. Take the follow check to a
protection routine:

1: mov AX,0201h ; Read 1 sectors using int 13h
2: mov CX,2909h ; Track 29h sector 09h
3: xor DX,DX ; Drive 0, head 0
4: int 13h ; Read sector
5: jnc Bad ; If no error then it's a copy
6: cmp AH,10h ; Was it a CRC error
7: jne Bad ; No, then it's a copy
8: mov AX,0h ; clear error flag
9: jmp Done ; we are done
Bad: mov AX,1 ; set error flag
Done: ret ; we are done

What is the above code trying to do. Well, it's
checking for a KEY on track 29h. That key is sector 09h.
Normally sector 09h would not have an error. On a read to
the original disk, after the int13 (line 4) is executed, the
carry flag (CF) would be set. The jnc in line 5 would jump
if CF is not set (indicating no error, which is bad since the
original disk would have an error there). The next line
checks AH to see if it is 10h. This is checking to see if
the error was a Bad CRC on the read (the error that should be
there). If it was not, then again it is not the protected
disk. Only after both of thoses conditions are met, will the
protection routine return a "GOOD" result.

The key here is the value returned in AX an possibly
CF. When the disk is the original, AX would return the value
of 0000h and CF = 1 but when it was a copy, it would return
0001h and CF = 0 or 1. Since on a bad return, CF can be 0 or
1 then it is preaty safe to assume CF is not used to signal
the return. So what must we do to beat the protection
routine, well, simply return from this CHECK with AX = 0000h.
Simple. Just change line 1 to "mov AX,0000h" and line 2 to
"RET". This will just bypass the check.

Now, this example is quite simple and would probally
never be used in a REAL protection routine. I kept it simple
to show the point, see the example on how to crack DRAWING
ASSISTANT for a better example.

The second and more perferd method is to simply bypass
the call to the protection routine and kill of the section of
code that test for the check position. Take the following

10: call 1 ; call the first example
11: cmp AX,0 ; Was it the original
12: jz Good2 ; Yes, then good
13: ... BAD it was a copy ; No, then bad

The above example, when used with the last example show
a typical call to a protection routine. The perfered method
to crack this protection would not be to simply fake the
return, but to remove the call to the protection. How to do
it, simple. Just jump over the check. Change line 10 to
"jmp Good2". This will bypass the protection routine.

Now, you might ask why would you want to take the extra
step of finding the call to the protection routine rather
than simply faking an int13 and returning with the proper
registers set. 2 reasons. First, What if there wasn't
enough room to setup the registers the way you needed them.
Then you would have to take the extra step. Second, what if
somewhere down the line, that routine is used for something
else (like the int13 is modified into an int10 in a game).
Since you have changed the bytes at that location, the
modifying routine would create code that wasn't exepcted.
But as always, if you can fake the return, and the program
works, leave it. After all, not to many people go around
look at other peoples cracks (do they???).

Now, what to do, if the program actually reads in
important data from the disk. Well, there are 2 ways to go
about this (possibly more). First, you could patch the
program so that when it calls it's protection routine, it
jumps to your user routine that opens a file and reads in the
data to the right place. This method is preaty simple to add
to a .COM file but a much harder to patch on to the back of a
.EXE. I won't really go in to this method much more than to
say use your brains. It's not a difficult concept.

The other method, is to create a LOADER or a TSR. I
suggest creating an Interrupt Service Routine (ISR) that
handles loading in the data. For example, let say you wrote
a routine to read in the needed data for a file. It is not
to difficult to convert that routine into an ISR. (For notes
on ISR and TSRs, try reading The Waite Group's "MS-DOS
PAPERS". It has one of the best sections on the subject).

Consider this following example:

A: call 1 ; test protection
B: jnc Good ; was it successfull
C: ... BAD load ; no then it's a copy
... EXIT TO DOS ; so exit to dos
Good: ... Good load ; yes then it original
call 7C00:0000 ; then jump of protection
; data

1: mov ax,0209 ; Read 9 sectors starting from
2: mov cx,290a ; Track 29h Sector 0Ah (10)
3: xor dx,dx ; for drive A: head 0
4: mov bx,7c00 ; read to 7c00:0
5: mov es,bx ;
6: mov bx,0 ;
7: int 13h ;
8: ret

What the above example dos. Lines 1-8 try to read in
sectors 0Ah - 12h for track 29h on drive A:. This is the
protection check routine. Lines A - Good attempt to check
the protection, and then if the check is good (CF = 0) then
a call to the loaded in code (the data loaded in by the
protection check) is made.

What we want to do, is somehow when INT 13h is called,
load in the needed data for disk. Well, here is my
suggestion. First, I would change line 7 from "int 13h" to
"int BBh". Next, I would create a small .COM loader that
would execute the main program as a child process (read the
DOS TECH REF on the EXEC function). But before I did that, I
would write an ISR (interrupt service routine) for INT BBh.
Here is the general outline for the ISR

þ Use dos to open the file containing the needed data
þ Read in the data to ES:BX (where int 13h would put it)
þ Close the file
þ set AX to 0000 and clear CF
þ iret

The loader would go like this :

þ Get current int BBh address (DOS func. 35h)
þ Set int BBh address to ours (DOS func. 25h)
þ use DOS to EXECUTE (Dos func. 4Bh) the program to be
þ Restore the address of BBh

Well, that about all I have to say about cracking a dos
file. I hope this section has been usefull to you. Next I
will show by example the techinques in this section while
cracking I.B.M. Drawing Assistance (1.00).

One last thing. After you have cracked the program, try
running it from a hard drive with PW set to trap calls to INT
21h function 1Bh (get fat byte). If the program make a call
to here, get the address and find that section of code. What
the program is trying to do is check to see if you are
running from a hard drive (most programs have diffrent
protection routines for hard drives). When you find it,
simply replace the "INT 21h" with a "MOV DS:[BX],FDh". This
will fake the program in to thinking you are working on a
floppy disk.

Ok, for our example we will be removing the code from
IBM's Drawing assistant. Now now, I know it's not the best
program out there, but shit, It's hard to find shit with on
disk copy protection anymore. So here we go...

I needed 3 programs in cracking the assist. series.
Locksmith by Alph Logic, Periscope debugger, and DEBUG
(supplied with DOS). By using these three programs together,
I was able to figure out and remove the copy protection in
under 30 minutes.

Drawing Assistant (DA) is IBM's answer to colorpaint for
the Jr. It is a simple drawing program (a more advanced
version is included in StoryBoard Deluxe) but easy to learn
and use. So far, I have only seen version 1.00 of this

DA made calls to the copy protection routine in 3
diffrent modules. The files "SETDRAW.EXE", "DRAWASST.EXE"
and "DRAWASST.TWO" all contained calls to the copy
protection. Also, "DRAWASST.TWO" and "DRAWASST.EXE" are for
all intensive puporses then same file.

I first started off by loading DRAWASST.EXE with debug
and searched for any int 13's by executing the debug command

s CS:0 FFFF CD 13 Search CS:0 - CS:FFFF for CD
13 (int 13)

I located 2 diffrent calls to int 13h, so I then listed
them. Here is what I found...

{ First, some initialization routines }

18FD:0343 1E PUSH DS
18FD:0344 B80000 MOV AX,0000
18FD:0347 50 PUSH AX
18FD:0348 B89724 MOV AX,2497
18FD:034B 8ED8 MOV DS,AX
18FD:034D BB1000 MOV BX,0010
18FD:0350 2E CS:
18FD:0351 8A07 MOV AL,[BX]
18FD:0353 3C00 CMP AL,00
18FD:0355 7418 JZ 036F

{ This set is called if DA has been installed }
{ on the hard drive }
{ When cracked, this will NEVER be called }

18FD:0357 B419 MOV AH,19
18FD:0359 CD21 INT 21
18FD:035B 8AD0 MOV DL,AL
18FD:035F B41C MOV AH,1C
18FD:0361 CD21 INT 21
18FD:0363 8A07 MOV AL,[BX]
18FD:0365 BB9724 MOV BX,2497
18FD:0368 8EDB MOV DS,BX
18FD:036A 3CF8 CMP AL,F8
18FD:036C 7475 JZ 03E3

{ This set is called if DA is running from the floppy }

18FD:036F B419 MOV AH,19
18FD:0371 CD21 INT 21
18FD:0373 FEC0 INC AL
18FD:0375 B400 MOV AH,00
18FD:0377 A320C6 MOV [C620],AX
18FD:037A 8AD0 MOV DL,AL
18FD:037C B41C MOV AH,1C
18FD:037E CD21 INT 21
18FD:0380 8A07 MOV AL,[BX]
18FD:0382 BB9724 MOV BX,2497
18FD:0385 8EDB MOV DS,BX
18FD:0387 3CF8 CMP AL,F8
18FD:0389 7408 JZ 0393

{ Here is the called to read in the key disk }

18FD:038B E8A675 CALL 7934
18FD:038E 3D0100 CMP AX,0001
18FD:0391 7450 JZ 03E3

Let's take these code segments 1 at a time. The fist, is
some simple initialization routines. Here is the code, only
this time full comments are added.

{ First, some initialization routines }
; Setup for return to DOS

18FD:0343 1E PUSH DS
18FD:0344 B80000 MOV AX,0000
18FD:0347 50 PUSH AX

; Setup DS to point to the data segment

18FD:0348 B89724 MOV AX,2497
18FD:034B 8ED8 MOV DS,AX

18FD:034D BB1000 MOV BX,0010 ; CS:10 points to
; installed flag
18FD:0350 2E CS:
18FD:0351 8A07 MOV AL,[BX]

18FD:0353 3C00 CMP AL,00 ; If not installed,
; jump to diskette
18FD:0355 7418 JZ 036F ; routines

What we are want to do, is fool DA in to thinking that it
is stilling loading from diskette. Nothing really needs to
be changed in this segment, but, just to be safe, we will
force the jump at 355. To change the current values, use
DEBUG's [A]ssembler command.

A CS:355
18FD:355 JMP 36F

Now, we have forced the jump, we can move on to the third
code segment skipping over the second since it will never be
called again. The 3rd code segment checks to see if you are
using a hard drive. It does so by first getting the logical
drive letter, then reading in the FAT descriptor byte for
that drive. Here is the commented code.

{ This set is called if DA is running from the floppy }

; First, get the current drive

18FD:036F B419 MOV AH,19 ; DOS function 19h -
; Get Current Drive
18FD:0371 CD21 INT 21

18FD:0373 FEC0 INC AL ; Add 1 for BIOS
18FD:0375 B400 MOV AH,00 ; Clear AH
18FD:0377 A320C6 MOV [C620],AX ; Store it at C620
18FD:037A 8AD0 MOV DL,AL ; Store it in DL
18FD:037C B41C MOV AH,1C ; DOS function 1Ch
; Get Fat desc.
18FD:037E CD21 INT 21 ; returns pointer
; in DS:BX
18FD:0380 8A07 MOV AL,[BX] ; Get the actual
; byte
18FD:0382 BB9724 MOV BX,2497 ; Restore DS
18FD:0385 8EDB MOV DS,BX

18FD:0387 3CF8 CMP AL,F8 ; Check to see if
; it is a H/D
18FD:0389 7408 JZ 0393 ; Yes, then jump

{ Fall in to the check for the key disk }

As you can see, this section of code is quite straigth
forward. It just checks to see if you are using a hard
drive. What we want to do is to fake an DOS function 1Ch and
return the value for a floppy. This is done by putting the
value of FDh in AL then NOPing the rest. Again use the
Debug's [A] command.

A CS:37C

18FD:0380 NOP
18FD:0381 NOP
18FD:0382 NOP
18FD:0383 NOP

Now, you might ask why I didn't simple force a jump over
the code. The answer is what if DA uses the value at C620 at
a later time (which it doesn't but let's pretend). If I had
forced the jump then the value might not have been
initialized and the crack might not work. Now that we have
simulated running from diskette, we must deal for the check
for the key disk.

This is where Periscope came in to play. Using
periscope, I made the above corrections and ran the program
up till CS:038B (the call to the check). Here is the code,
including the actual check. I have indented the check to
make it easier to read.

{ Here is the called to read in the key disk }

18FD:038B E8A675 CALL 7934 ; Check key on disk
; (track 27h side 0)

18FD:7934 A120C6 MOV AX,[C620] ; Get drive
; letter
18FD:7937 FEC8 DEC AL
18FD:7939 A23BC6 MOV [C63B],AL ; Store it for
; later
18FD:793C 1E PUSH DS
18FD:793D 07 POP ES
; Setup pointers to what sectors to try to read

18FD:793E BB30C6 MOV BX,C630
18FD:7941 891E39C6 MOV [C639],BX
18FD:7945 C6063CC603 MOV BYTE PTR [C63C],03
18FD:794A C6063DC601 MOV BYTE PTR [C63D],01

; Reset the disk

18FD:794F B400 MOV AH,00
18FD:7951 CD13 INT 13

; Get address of sector to read an put it in CL

18FD:7953 8B1E39C6 MOV BX,[C639]
18FD:7957 8A0F MOV CL,[BX]

; Setup the rest of the read information

18FD:795C 81C3D007 ADD BX,07D0
18FD:7960 B001 MOV AL,01
18FD:7962 B527 MOV CH,27
18FD:7964 8A163BC6 MOV DL,[C63B]
18FD:7968 B600 MOV DH,00
18FD:796A B402 MOV AH,02
18FD:796C CD13 INT 13

; Test for an error and jump if none is present (ie: the
; copy protection has been removed)

18FD:796E 80FC00 CMP AH,00
18FD:7971 740C JZ 797F

; test the bad read (protection is missing) 3 times

18FD:7973 FE0E3CC6 DEC BYTE PTR [C63C]
18FD:7977 75D6 JNZ 794F
18FD:7979 B80000 MOV AX,0000
18FD:797C EB13 JMP 7991

; Get next sector to check. If finished, set the flag and
; return.

18FD:797F FF0639C6 INC WORD PTR [C639]
18FD:7983 FE063DC6 INC BYTE PTR [C63D]
18FD:7987 803E3DC603 CMP BYTE PTR [C63D],03
18FD:798C 75C1 JNZ 794F
18FD:798E B80100 MOV AX,0001
18FD:7991 C3 RET

; Check to see if the OK flag was set (ax = 0001h means check
; was good)
18FD:038E 3D0100 CMP AX,0001
18FD:0391 7450 JZ 03E3

The key check used in DA is quite simple. It simple tries
to read in the illegaly numbered sectors on Track 27h side
0h. If they are missing, it assumes that it is running a
pirated copy. What we must do, is to fool the scheme in to
thinking a good read happened. I choses to fake the read
using the easiest method. Since the protection scheme only
check to see if AX returns the value > 0000h, I simply
modified the routine at 1BFD:7934 to set AX to 0000h and then
return. Here is the new code (enter using debug's A

A 1BFD:7934
1BFD:7934 MOV AX,0000
1BFD:7936 RET

Now, this file is unprotected and if you type "G" at
debug's command prompt, the program will execute, sort-of.
See DRAWASST.EXE calls DRAWASST.TWO which also has the
protection scheme. So both must be changed. To make to
changes perement in DRAWASST.EXE, rename the file to
DRAWASST.DEB and edit it. To find the address of the start
of the protection code, use debug's search command...

S CS:0 FFFF B4 19 CD 21 8A D0

Now, just uses the modified address to change the program
(the code will still be the same, just all calls and jumps
will be to diffrent addresses). Use the same process to stip
DRAWASST.TWO (it uses the exact same code). When you have
both of those files unprotected, you can move on to
unprotecting the setup program "SETDRAW.EXE"

DRAWASST.EXE AND .TWO are not the only programs that
make calls to the protection routine. SETDRAW.EXE also makes
the above calls. Although the check here is much easier to
bypass. Here is the asm listing of SETDRAW with all of the
calls to the protection. This time, I will not go in to
quite as much detail as I did for the other two version.

I will tell you this. When SETDRAW checks the key disk,
first it checks to see if the protection exists and then to
see if the track hasn't been modified. It again uses AX to
determine what happeded. I used Periscope to trace through
the original version to find out what the correct values are.
Here is the asm code...

; Initialization - checks the current mode and saves it.

18FD:0000 1E PUSH DS
18FD:0001 B80000 MOV AX,0000
18FD:0004 50 PUSH AX
18FD:0005 B8321A MOV AX,1A32
18FD:0008 8ED8 MOV DS,AX
18FD:000A B40F MOV AH,0F
18FD:000C CD10 INT 10
18FD:000E 3C02 CMP AL,02
18FD:0010 740D JZ 001F
18FD:0012 3C03 CMP AL,03
18FD:0014 7409 JZ 001F
18FD:0016 A28900 MOV [0089],AL
18FD:0019 B002 MOV AL,02
18FD:001B B400 MOV AH,00
18FD:001D CD10 INT 10

; Gets the current drive

18FD:001F B400 MOV AH,00
18FD:0021 B419 MOV AH,19
18FD:0023 CD21 INT 21
18FD:0025 A28700 MOV [0087],AL
18FD:0028 8AD0 MOV DL,AL

; Checks the FAT descriptor

18FD:002C B41C MOV AH,1C
18FD:002E CD21 INT 21
18FD:0030 8A07 MOV AL,[BX]
18FD:0032 BB321A MOV BX,1A32
18FD:0035 8EDB MOV DS,BX
18FD:0037 C606880000 MOV BYTE PTR [0088],00
18FD:003C 3CF8 CMP AL,F8
18FD:003E 742A JZ 006A

; Read in protection scheme

18FD:0040 8A168700 MOV DL,[0087]
18FD:0044 E87E0A CALL 0AC5
18FD:0047 C606880000 MOV BYTE PTR [0088],00
18FD:004C 3D0000 CMP AX,0000
18FD:004F 7419 JZ 006A

; Read in the dummy scheme

18FD:0051 C606880001 MOV BYTE PTR [0088],01
18FD:0056 8A168700 MOV DL,[0087]
18FD:005A B84500 MOV AX,0045
18FD:005D E8BD0A CALL 0B1D
18FD:0060 3D0000 CMP AX,0000
18FD:0063 7405 JZ 006A

; Start of actual routine.

18FD:0065 C606880000 MOV BYTE PTR [0088],00

There is isn't much to say about the above code. To bypass
it, we will change the hard drive check (int 21 function 1c).
Do the same thing we did for DRAWASST.EXE

A 18FD:2C
18FD:002C mov AL,FD
18FD:002E nop
18FD:002F nop
18FD:0030 nop
18FD:0031 nop

Now, just jump over the check to the key disk.

A 18FD:40

18FD:0040 jmp 0065

And thats it. Now SETDRAW is unprotected. Drawing
Assistant may be used, copied or backed up at your pleasure.

As you can see, this was a good example although the
fact that if you only made the changes in DRAWASST.EXE and
not in DRAWASST.TWO then the program would copy DRAWASST.TWO
to DRAWASST.EXE to restore the protection was a bit strange.

Well, I hope you are proud. But be warned, next we take
on DOC checks, so get a good nights sleep. Till then, play
lots of SMASH T.V.

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