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* B U C K A R O O B A N Z A I *
* aka the Reset Vector *
* *
* presents *
* *
* Cracking On the IBMpc *
* Part I *
* *
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Introduction
------------
For years, I have seen cracking
tutorials for the APPLE computers, but
never have I seen one for the PC. I
have decided to try to write this series
to help that pirate move up a level to a
crackest.
In this part, I will cover what
happens with INT 13 and how most copy
protection schemes will use it. I
strongly suggest a knowledge of
Assembler (M/L) and how to use DEBUG.
These will be an important figure in
cracking anything.
INT-13 - An overview
--------------------
Many copy protection schemes use the
disk interrupt (INT-13). INT-13 is
often use to either try to read in a
illegaly formated track/sector or to
write/format a track/sector that has
been damaged in some way.
INT-13 is called like any normal
interupt with the assembler command
INT 13 (CD 13). [AH] is used to select
which command to be used, with most of
the other registers used for data.
INT-13 Cracking Collage
-----------------------
Although, INT-13 is used in almost all
protection schemes, the easiest to crack
is the DOS file. Now the protected
program might use INT-13 to load some
other data from a normal track/sector on
a disk, so it is important to determine
which tracks/sectors are inportant to
the protection scheme. I have found the
best way to do this is to use
LOCKSMITH/pc (what, you don't have LS.
Contact your local pirate for it.)
Use LS to to analyze the diskette.
Write down any track/sector that seems
abnormal. These track are must likely
are part of the protection routine.
Now, we must enter debug. Load in the
file execute a search for CD 13. Record
any address show. If no address are
picked up, this mean 1 or 2 things, the
program is not copy protected (bullshit)
or that the check is in an other part of
the program not yet loaded. The latter
being a real bitch to find, so I'll
cover it in part II. There is another
choice. The CD 13 might be hidden in
self changing code. Here is what a
sector of hidden code might look like
-U CS:0000
1B00:0000 31DB XOR BX,BX
1B00:0002 8EDB MOV DS,BX
1B00:0004 BB0D00 MOV BX,000D
1B00:0007 8A07 MOV AL,[BX]
1B00:0009 3412 XOR AL,12
1B00:000B 8807 MOV [BX],AL
1B00:000D DF13 FIST WORD...
In this section of code, [AL] is set
to DF at location 1B00:0007. When you
XOR DF and 12, you would get a CD(hex)
for the INT opcode which is placed right
next to a 13 ie, giving you CD13 or INT-
13. This type of code cann't and will
not be found using debug's [S]earch
command.
Finding Hidden INT-13s
----------------------
The way I find best to find hidden
INT-13s, is to use a program called
PC-WATCH (TRAP13 works well also). This
program traps the interrupts and will
print where they were called from. Once
running this, you can just disassemble
around the address until you find code
that look like it is setting up the disk
interupt.
An other way to decode the INT-13 is
to use debug's [G]o command. Just set a
breakpoint at the address give by
PC-WATCH (both programs give the return
address). Ie, -G CS:000F (see code
above). When debug stops, you will have
encoded not only the INT-13 but anything
else leading up to it.
What to do once you find INT-13
-------------------------------
Once you find the INT-13, the hard
part for the most part is over. All
that is left to do is to fool the
computer in to thinking the protection
has been found. To find out what the
computer is looking for, examine the
code right after the INT-13. Look for
any branches having to do with the CARRY
FLAG or any CMP to the AH register.
If a JNE or JC (etc) occurs, then
[U]nassembe the address listed with the
jump. If it is a CMP then just read on.
Here you must decide if the program
was looking for a protected track or
just a normal track. If it has a
CMP AH,0 and it has read in a protected
track, it can be assumed that it was
looking to see if the program had
successfully complete the READ/FORMAT of
that track and that the disk had been
copied thus JMPing back to DOS
(usually). If this is the case, Just
NOP the bytes for the CMP and the
corrisponding JMP.
If the program just checked for the
carry flag to be set, and it isn't, then
the program usually assumes that the
disk has been copied. Examine the
following code
INT 13 <-- Read in the Sector
JC 1B00 <-- Protection found
INT 19 <-- Reboot
1B00 (rest of program)
The program carries out the INT and
find an error (the illegaly formatted
sector) so the carry flag is set. The
computer, at the next instruction, see
that the carry flag is set and know that
the protection has not been breached.
In this case, to fool the computer, just
change the "JC 1B00" to a "JMP 1B00"
thus defeating the protection scheme.
NOTE: the PROTECTION ROUTINE might be
found in more than just 1 part of
the program
Handling EXE files
------------------
As we all know, Debug can read .EXE
files but cannot write them. To get
around this, load and go about cracking
the program as usual. When the
protection scheme has been found and
tested, record (use the debug [D]ump
command) to save + & - 10 bytes of the
code around the INT 13.
Exit back to dos and rename the file
to a .ZAP (any extention but .EXE will
do) and reloading with debug.
Search the program for the 20+ bytes
surrounding the code and record the
address found. Then just load this
section and edit it like normal.
Save the file and exit back to dos.
Rename it back to the .EXE file and it
should be cracked. ***NOTE: Sometimes
you have to fuck around for a while to
make it work.
DISK I/O (INT-13)
-----------------
This interrupt uses the AH resister to
select the function to be used. Here is
a chart describing the interrupt.
AH=0 Reset Disk
AH=1 Read the Status of the Disk
system in to AL
AL Error
----------------------------
00 - Successful
01 - Bad command given to INT
*02 - Address mark not found
03 - write attempted on write prot
*04 - request sector not found
08 - DMA overrun
09 - attempt to cross DMA boundry
*10 - bad CRC on disk read
20 - controller has failed
40 - seek operation failed
80 - attachment failed
(* denotes most used in copy protection)
AH=2 Read Sectors
input
DL = Drive number (0-3)
DH = Head number (0or1)
CH = Track number
CL = Sector number
AL = # of sectors to read
ES:BX = load address
output
AH =error number (see above)
[Carry Flag Set]
AL = # of sectors read
AH=3 Write (params. as above)
AH=4 Verify (params. as above -ES:BX)
AH=5 Format (params. as above -CL,AL
ES:BX points to format
Table)
For more infomation on INT-13 see the
IBM Techinal Reference Manuals.
Comming Soon
------------
In part II, I will cover CALLs to
INT-13 and INT-13 that is located in
diffrents overlays of the program
Happy Cracking.....
Buckaroo Banzai
<-------+------->
PS: This Phile can be Upload in it's
unmodified FORM ONLY.
PPS: Any suggestion, corrections,
comment on this Phile are accepted and
encouraged.....
ht
next to a 13 ie, giving you CD13 or INT-
13. This type of code cann't and will
not