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BEGGIN2.TXT
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1997-11-21
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Cracking for the beginner - Part two
by Y0SHi
06/12/97
mm/dd/yy
In part two of this beginners cracking tutorial, I examine one of Lord Caligo's own creations; try_me.com. This file was
altogether not too difficult, and can be found on Lord Caligo's website.
And now, the cracking!
Ok, the first thing I did was to dissassemble the program with Sourcer. The listing looks like this:
in al,21h ; port 21h, 8259-1 int
IMR
xor cx,cx ; Zero register
mov bx,12Ah
mov dx,16Ch
call sub_1
call sub_3
call sub_4 ; Sub does not return
here
loc_1:
inc ch
loc_2:
int 21h ; DOS Services
ah=function 00h
; terminate, cs=progm
seg prefx
jmpn loc_3
db 31h, 32h, 33h
sub_1 proc near
or al,2
retn
sub_1 endp
sub_2 proc near
and al,0FDh
retn
sub_2 endp
sub_3 proc near
cmp al,0AAh
jne loc_4 ; Jump if not equal
retn
loc_3:
inc cl
inc bx
cmp ch,3
je loc_5 ; Jump if equal
cmp al,[bx]
je loc_1 ; Jump if equal
call sub_5
jmpn loc_7
loc_4:
mov dx,164h
jmpn loc_6
loc_5:
mov dx,offset data_2 ; ('')
loc_6:
mov ah,9
int 21h ; DOS Services
ah=function 09h
; display char string at
ds:dx
loc_7:
int 20h ; DOS program terminate
sub_3 endp
sub_4:
out 21h,al ; port 21h, 8259-1 int
comands
sub_5 proc near
cmp al,0Dh
jne loc_2 ; Jump if not equal
jmp short loc_4
sub_5 endp
db 0C3h
db 0Ah, 0Dh, 'false$'
db ' - TRY*ME v1.00 '
copyright db '(c) by Lord Caligo -
In the above code, basically we see that in loc_3, ch is compared with 3, and
if they are equal, data_2 ("right") is put into dx and then is printed, after,
the program is terminated with int 20. So, all we really have to do is change
loc_3 to read jmp loc_5 (actually, we'd have to chane loc_5 to some sort of
memory location). So.... the crack for try_me.com:
search for: the mnemonics for je loc_5 (7516)
replace with: the mnemonics for jmp loc_5 (EB1C)
WTF???
It seems that when we modified the program, no longer is the prompt for the
password shown; the password is automatically right, you don't have to enter
anything! Tres bizzare! (I'm not french; this is appropriate though. I am
the only American cracker I know)
Why I chose to accept this crack
The reason I didn't go back and alter this crack to ask for a password is
that if you've ever downloaded a cracked password crippled game, you notice
that most of the time you don't have to enter a password anyway, the cracker
did a good job. So I thought "What the hell, no one wants to enter a password anyway".
Contacting the Author
To contact me on IRC, DON'T look for Y0SHi, this is some stupid bot for
y0shi.paradise.com (no, that's not my web page). Look for YOSHi on EfNet.
Email: gargos@juno.com
Contacting Lord Caligo: caligo@lords.com