Cracking On the IBMpc - Part I
****************************************
* 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 *
* *
****************************************
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 illegally formatted track/sector or to write/format a track/sector that has been damaged in some way.
INT-13 is called like any normal interrupt 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 important 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 interrupt.
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]nassemble 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 corresponding 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 illegally 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 information on INT-13 see the IBM Technical Reference Manuals.
Coming Soon
In part II, I will cover CALLs to INT-13 and INT-13 that is located in differents 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
