Oldies but Goodies A Dos Game CD-check with Sourcer 7	Not Assigned
04 March 1998	by FootSteps.
	Courtesy of Fravia's page of reverse engineering	slightly edited by fravia+
fra_00F1 980304 FootSteps 1000 P4 PC	Great essay! The come back of Sourcer, the importance of DOS, CD-ROM faking... a lot of very important things and an interesting message for beginners and protectors alike: dos protections are FAR from easy to circumvene and dos protection are not dead (and should by all means NOT be considered dead :-) I agree totally with FootSteps: cracking is the big game: your brain, your wit and your intuition against clever opponents (and you'll note that FootSteps divides his essay using the messages of the old game)... that's LIFE! Yes... the old proverb by +ORC helds true: if you teach a man how to reverse he will never be thirsty again... because of the fun he'll have in the reversing itself! I'm like FootSteps, I regularly peruse my old 'big black' floppies searching for 'forgotten' mighty protections... and you would be amazed to see how easily you can port these old dos protections to windoze!
	There is a crack, a crack in everything That's how the light gets in
Rating	(x)Beginner ( )Intermediate ( )Advanced ( )Expert

YOU ARE STANDING IN A OPEN FIELD WEST OF A WHITE HOUSE...

Hello!
Well, I don't know about you, but I'm a guy who play gladly
oldies; I mean old games. Do you remember Zork? You know: "You
are standing in a open field, ..." then, "open mailbox", etc...
Oldies are interesting, because:

First, you must *find* them on the web. Beyond the Warez, where you
can find a few, you can use here what we've learned in the Fravia
site to search them. Like zork.zip <- This is a easy and ridiculous
example! :-) Try to find "The Guild of Thieves"... A good one...
theguildofthieves.zip ? No, no... Search better!

Second, when you've found them, enjoy to play them.
You can find jewels. Remember "Bard's Tale"? Marvelous to
play this 8086's one with a Pentium!

Third, even if found in the WareZ scene, you're playing for a moment
when you see something like that: "Please enter the word of the page..."!
Well, this is where the oldies is very funny to play:
cracking this old protection scheme. Because they are DOS-based, and sometimes,
very difficult.

Well, follow me for a easy one, a 1994 DOS game (not so much oldie...) 
with two CD-checks, whose the second is interesting,
interesting cause if you are like me, unable to run
Softice DOS due to some video card problems, you'll run this
game with WindoZ95, using Winice, but our beloved and young Winnie
hang running this oldie!
Better, IDA Pro 3.7 doesn't reverse correctly it!
Then, welcome to Sourcer 7, cracked by Frog's Print, somewhere on our
Fravia WebSite! 

THE GREAT UNDERGROUND EMPIRE (was called Zork itself)


Well, we say that you've done a good research and found the target,
the game DOMINUS, 13.9Mo.
I know, you're hurry to play with it. Ok, run it.
Well, an amazing "Please insert the Dominus CD" screen come in front
of our honest eyes; sure, we'd like buy and insert this CD :-)
but we cannot find it nowhere in any store... it's a DEAD program!

Then, since on the contrary we can find Winnie on the net, we run it
nevertheless...

You want to know how? Did you read the good essay of Animadei on the 
DOS CD checks?

Well, we use Winnie to do a

BPINT 2F IF (AH==15)

Remember? The DOS CD-ROM resident MSCDEX is called with this
interruption, 2F, and AH set to 15. We don't care AL. We want
just to know where is the *first* CD-check that this game does.

Run Dom.bat. Winnie stops; good.
We are in LOADER.EXE, that is in the batch DOM.BAT we've started.

3AB7:0099  B8 1500     mov ax,1500h
3AB7:009C  CD 2F       int 2Fh                 ; Yes!
3AB7:009E  89 0E 29CA  mov WORD PTR [29CA],cx
3AB7:00A2  8B C1       mov ax,cx
3AB7:00A4  07          pop es
...
3AB7:00AC  CB          retf

We see an INT 2F called with AX set to 1500. Look at your Interrupt list.
This is done to know how many CD-ROM drives you've got on your machine.
Just hit F12, to land in this classic protection scheme you can trace 
for the pleasure (we land in 350D:0147 after hiting F12):


350D:0140  59            pop cx
350D:0141  59            pop cx
350D:0142  9A 3AB7:008F  call   3AB7:008F               ; how many CD drives?
350D:0147  A3 017A       mov	word ptr 017A,ax
350D:014A  83 3E 017A 00 cmp	word ptr 017A,0         ; no CD drives?
350D:014F  74 54         je	350D:01A5               ; then jump to "insert CD..."
350D:0151  1E            push	ds
350D:0152  B8 0227       mov	ax,227h
350D:0155  50            push	ax
350D:0156  16            push	ss
350D:0157  8D 46 96      lea	ax,[bp-6Ah]
350D:015A  50            push	ax
350D:015B  9A 3067:41EF  call   3067:41EF               ; stringcopy "A:\DOMINUS.ZIP"
350D:0160  83 C4 08      add	sp,8
350D:0163  A0 017A       mov	al,byte ptr 017A        ; the number of your CD drive
350D:0166  04 41         add	al,41h                  ; change to "X:DOMINUS.ZIP"
350D:0168  88 46 96      mov	[bp-6Ah],al
350D:016B  1E            push	ds
350D:016C  B8 0236       mov	ax,236h
350D:016F  50            push	ax
350D:0170  16            push	ss
350D:0171  8D 46 96      lea	ax,[bp-6Ah]
350D:0174  50            push	ax
350D:0175  9A 3067:2F38  call	3067:2F38               ; open "X:DOMINUS.ZIP"
350D:017A  83 C4 08      add	sp,8
350D:017D  89 56 FE      mov	[bp-02],dx              ; Was
350D:0180  89 46 FC      mov	[bp-04],ax              ; the good file DOMINUS.ZIP
350D:0183  0B 46 FE      or	ax,[bp-02]              ; on the CD?
350D:0186  75 2F         jnz    350D:01B7               ; ax!=0 then jump to good user!
.... else, ax==0 and "Insert CD" and no game... how sad...

Well, it's not too difficult, in fact, it's like the classic
Win32 GetDriveTypeA we all know when reversing WindoZ applications.

The program, here LOADER.EXE, look first for the number of CD drives you got
using the interruptions of MSCDEX. No matter if you haven't loaded it
because WindoZ95 emulate it (at least one thing it can do).
LOADER.EXE find your drive, elsewhere you'll got the message "Insert CD".
Then, it copies in memory the string "A:DOMINUS.ZIP". Uh? Strange?
Next, it copies the letter of your CD drive, let say 'X' in this string,
which becomes "X:DOMINUS".
Then, using again some 2F interruptions we'll not describe here because
it has been already done by Animadei, it just check if the file "X:DOMINUS.ZIP" 
is on your CD-drive, how fun!
If ax!=0, you're a good gamer, since you've bought the right CD.

Since we are also good crackers ;-), we just change the 350D:0140 bytes with:

350D:0140 EB 75         jmp 01B7                      ; jmp good user!

avoiding with this all the boring CD and comparison checks.
Not our fault, as I said: the right CD isn't in any shop.

Well, we run the game, see a pretty US-GOLD presentation screen,
(of 1994 of course), and we run a new game.

I DON'T KNOW THE WORD "SOFTICE"

Err... we won't play a new game, because SoftIce has hanged.
We just can reboot our machine.
Reset.
Bios check.
Presentation screen.
Yes, I say presentation screen because we've got to boot in DOS.
I'm sure you've got a boot manager, haven't you? hehehe...
Then, we boot DOS. And we run the game.
The same that under W95: The PC hangs.

That is here we gonna use this sympathic freeware, X-RAY 1.5, a good
spy utility. We must know what's going on just before the PC hangs.
Just use:

XRAY /J /F

To say the program to Jump instead of calling the original
interrupt address (seems to work better with that.), and to
write a log File on our hard drive, named c:\xray.log
Then, run dom.bat. The game hangs when you choose a new game.

Well, reboot with WindoZ 95. Open XRAY.LOG with your favorite text editor.
Xray just show the Interrupts 21 calls. But it's enough. Cause if
you're not tired, you will see at the line 5113 the following Int 21:

34h, GET BUSY FLAG
30h, GET DOS VERSION
3Dh, OPEN FILE, F:\DOMINUS.ZIP           ;line 5113
3Fh, READ DEVICE, Handle=5, 0Ah bytes
3Eh, CLOSE FILE, Handle=5

Ha ha! Seems like the program search a second time for his
favorite file on his favorite CD!

Mmmm... If you disassemble LOADER.EXE with IDA Pro, all will be great,
but you'll not see others CD-checks references in it.
Then, the bytes we've changed don't appear any more in this
executable. This is not this program which checks for the second
time the CD.
Who checks? Mmm...
Just hit control+f in WindoZ95. Search the string "DOMINUS.ZIP"
in all your \DOMINUS directory.
Haha! It is present in LOADER.EXE, we knew it, and in
TEMP.EXE
But Softice hang, we couldn't trace this second CD check,
then we run in laughing IDA Pro 3.7, asking it to reverse
this ridiculous TEMP.EXE of little 210Ko.

Argh again! IDA Pro 3.7 doesn't reverse correctly this little
executable. We see a lot of DB, you know all that.
Curious. Coz IDA recognize well this program compiled with
Borland C, like the precedent LOADER.EXE, marking sometimes
some call _fileopen and other call _strcpy. But the rest of the
listing is a pretty garbage. What happens to you, beloved IDA? ;-)

You can try to reverse TEMP.EXE with W32Dasm, it does it well,
but you know how W32Dasm reverse Dos based applications, we get
no Data X-Ref at all, and it's not funny at all to follow such a 
dead listing...

At this point, I was really bored, without my Winice and IDA,
frightened to the idea to follow a long W32Dasm listing with no
help at all, lost in these large and dark codewoods for many hours,..

IT'S PITCH DARK

I began to follow this "Data Xref crippled" listing of TEMP.EXE
reversed by W32Dasm, and it took me a lot of hours to understand
very little. It made me think to all these old text adventures games
we played, when we were lost in giant labyrinths. Darker and darker...

I thought that reversing was exactly the same thing that playing
text adventure games. A lot of places, you're obliged to write a map,
some of them were very difficult, some were easy, some were fabulous,
some were boring...
And like in all this kind of game, come the point where you're stuck.
Really stuck. With no way out, no solution...

Reversing is really like playing a big huge ZORK!

Cause at a given moment, you find the right thing to do, you remember 
the right tool to use, you devise the right sentence to enter.

LIGHT LAMP

Well, I've remembered a nice soft I've let in a dark and sad place
on my HD, forgetting it since using IDA. It was Sourcer 7, reversed
by Frog's Print.

Hope came in my mind. I used the version 6.51 of it before passing 
to (welcome) IDA in order to debug Dos applications, and it was already 
a nice one.

Put TEMP.EXE in your Sourcer directory, 
be sure to have enough low Dos memory left, and just type:

SR TEMP.EXE

setting the Passes to 9; the more analysys you got, the better is 
the result.

And yeepee! We got a pretty listing, with all the Data Xref we want!
Sorry to have almost forgotten you, Sourcer! Welcome again!

Because you now just need to search the dead-listing TEMP.LST
for "DOMINUS.ZIP" with your favorite Text-Editor!
And look, it's here, just in front of your eyes:

;Indexed Entry Point

31A9:00A1		loc_233:
31A9:00A1  74 2B	jz	loc_235			; Jump if zero
31A9:00A3  04 41	add	al,41h			; 'A'
			assume	ds:seg_j
31A9:00A5  A2 01A7	mov	byte ptr data_845,al	; (54AC:01A7='A:\DOMINUS.ZIP')
31A9:00A8  9A 4872:01EA	call	far ptr sub_593		; (4872:01EA)
31A9:00AD  B4 3D	mov	ah,3Dh			; '='
31A9:00AF  B0 00	mov	al,0
31A9:00B1  BA 01A7	mov	dx,offset data_845	; (54AC:01A7='A:\DOMINUS.ZIP')
31A9:00B4  CD 21	int	21h			; DOS Services  ah=function 3Dh
										;  open file, al=mode,name@ds:dx
31A9:00B6  72 16	jc	loc_235			; Jump if carry Set
31A9:00B8  8B D8	mov	bx,ax
31A9:00BA  B4 3F	mov	ah,3Fh			; '?'
31A9:00BC  B9 000A	mov	cx,0Ah
31A9:00BF  BA 01A7	mov	dx,offset data_845	; (54AC:01A7='A:\DOMINUS.ZIP')
31A9:00C2  CD 21	int	21h			; DOS Services  ah=function 3Fh
							;  read file, bx=file handle
							;   cx=bytes to ds:dx buffer
31A9:00C4  72 08	jc	loc_235			; Jump if carry Set
31A9:00C6  B4 3E	mov	ah,3Eh
31A9:00C8  CD 21	int	21h			; DOS Services  ah=function 3Eh
							;  close file, bx=file handle
31A9:00CA  72 02	jc	loc_235			; Jump if carry Set
31A9:00CC  EB 05	jmp	short loc_236		; (00D3)
31A9:00CE       loc_235:				;  xref 31A9:00A1, 00B6, 00C4, 00CA
31A9:00CE  B8 4C01	mov	ax,4C01h
31A9:00D1  CD 21	int	21h			; DOS Services  ah=function 4Ch
							;  terminate with al=return code
31A9:00D3	loc_236:				;  xref 31A9:00CC
GOOD USER...


Should I really comment better this part of code Sourcer already reversed?
Really self-explanatory, no?

Then... this second crack to the CD check is quickly made:
The program came at 31A9:00A1 which is the protection check.
The "jz loc_235" is here to see if you got at least one CD drive.
You got no one? Or you got, like you see in the following code, not
the right CD? Then this program eject you immediately to the terrific
int 21 with ah set to 4C, the DOS termination... Oh, no...

But, we are very clever. Remark the "jmp short loc_236" at offset
31A9:00CC, just at the end of the protection routine and just before the
horrible Dos termination.
Then we just need to jump to this location.

What about a sympathic:
31A9:00A1 JMP loc_236
instead of the original sad:
31A9:00A1 jz loc_235
which say the program we got obviously the right CD rom in our drive?

Yes?
We just need to change the jz loc_235 74 2B in our clever JMP loc_236.
First, replace the jz opcode "74" by the jmp opcode "EB".
Second, we need to jmp location 236 instead of the 235 one,
which is placed 5 octets after. Then, jmp loc_236 is jmp loc_235 + 5,
is EB (2B + 5), is finally EB 30. Yeah!

Then, just four bytes more to change, for esthetic you know,
to avoid the light of our CD drive twinkle when TEMP.EXE ask
MSCDEX to look how many CD drives you got.
If you haven't no CD drive on your machine, just don't care. But if
you are Cresus or Bill Gates, you'd like avoid all of your 56 CD light drives
to light! :-)

Remember that it was INT 2F with ah set to 15 that check your CD drive?
Then, we've got just to replace in our code the opcode CD 2F, present at
two locations, with some 40 48 opcodes, for example.

Then, exit your hexeditor and run a new game.
Well, it works! And now we can play!

We won't play a long time finally, coz this game isn't a wonderful oldie...
Cracking it was more fun... than playing it...


All right, back to the Net, looking for a new Dos oldie to crack!

And soon we realize that Cracking is the best adventure 
game we've ever played...

--FootSteps.

Back to Project4

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