PDA

View Full Version : simple rsa

Donan
February 10th, 2004, 23:05
Hi all! I was looking for a way to learn cripto, speacially RSA and found this crackme. I think its impossible to be more simple. But I have a few (lots) questions... First its TSCube RSA-24 crackme. Can be found at www.crackmes.de and some others site, just use google.
Ive also found a couple of tutorials, but none explain the crackme itself. Only RSA...

Code:


 00402871  CALL      004137E8       ;"GetDlgItemTextA" API

 00402876  LEA       ECX,[ESP+24]

 0040287A  PUSH      ECX

 0040287B  MOV       ECX,EBX

 0040287D  CALL      004029B0       ;the RSA function!

 00402882  TEST      EAX,EAX        ;check

 00402884  JZ        00402912       ;teh jump


Tracing it the call....

Code:


004029CD  |. 68 DC004200    PUSH crackme.004200DC                    ;  ASCII "9901"

004029D2  |. 8D8C24 E400000>LEA ECX,DWORD PTR SS:[ESP+E4]

004029D9  |. E8 52E7FFFF    CALL crackme.00401130

004029DE  |. 68 D0004200    PUSH crackme.004200D0                    ;  ASCII "12790891"

004029E3  |. 8D4C24 1C      LEA ECX,DWORD PTR SS:[ESP+1C]

004029E7  |. C78424 6406000>MOV DWORD PTR SS:[ESP+664],0

004029F2  |. E8 39E7FFFF    CALL crackme.00401130

004029F7  |. 68 C8004200    PUSH crackme.004200C8                    ;  ASCII "8483678"

004029FC  |. 8D8C24 7402000>LEA ECX,DWORD PTR SS:[ESP+274]

00402A03  |. C68424 6406000>MOV BYTE PTR SS:[ESP+664],1

00402A0B  |. E8 20E7FFFF    CALL crackme.00401130

00402A10  |. 68 C0004200    PUSH crackme.004200C0                    ;  ASCII "5666933"


I can see that 9901 is probably "e". All others numbers are harder to guess.
First question, whats CALL crackme.00401130? Its probably a bignum function but made me wonder at the beginning. The whole crypto stuff is done below and its quite easy to identify some calls such as 00402B1D CALL 00402310 ;RSA-Encrypt!

the rest I have just a little idea...

so how to identify the N and the messages?
Whats the best bignum lib for a beginner?
mike
February 11th, 2004, 00:22
Try http://www.alpertron.com.ar/ECM.HTM for a fast factoring program. That ought to make it clear which of the numbers is n!
ZaiRoN
February 11th, 2004, 06:04
Hi Donan,
this crackme has been discussed in an old thread inside the mini project area: http://www.woodmann.net/forum/showthread.php?t=3950
Maybe it helps you...

ZaiRoN
Donan
February 12th, 2004, 15:57
Hi Zairon and Mike! Thanks for the reply!

The other thread was good to understand what was going on...

Is there a way to find out wich cripto lib was used? Ive tried looking
at imports using IDA but had no luck... with this info I could compile an example and compare it to the program...

anyone got any hints?!

thanks

dONAN
ZaiRoN
February 12th, 2004, 18:37
Hi Donan,
from readme file: "This was coded to test my Bignum library"

ciao,
ZaiRoN