mrf_steg.htm How to reverse engineer Steganos (First step)

	How to reverse engineer Steganos (First step) Speed up brute force cracking	Steganography
11 February 1998	by mrf
	Courtesy of Fravia's page of reverse engineering
fra_00XX 980213 mrf 0100 NA PC	Great! It's a first step, but in the CORRECT direction! We are finally beginning to get into Steganos' guts... now the big question is: will we go deeper inside Steganos' encryption routines immediately next or would it be (may be :-) more useful to examine a little the inner working of the OTHER applications from the stego page: S-tools, before delving inside Steganos? Any path you choose, this essay by mrf will help you a lot... Enjoy!
	There is a crack, a crack in everything That's how the light gets in
Rating	( )Beginner (x)Intermediate ( )Advanced ( )Expert

This essay is quite easy to understand. It gives some basics about how to reverse engineer Steganos in general. If you know how to work with Wdasm89 and SoftIce, you'll not have any problems to comprehend. It is recommended to know a little bit of Pascal, though.

How to reverse engineer Steganos (First Step)
Some basic ideas to speed up brute force cracking
Written by mrf

Introduction

Jean Flynn wrote a great essay about how he reached the advanced stego
page, but he didn't mention some details about Steganos that are very 
useful to know. 
The program calculates a checksum of the password. This'll be the target 
of my brute force attack.

Tools required

W32Dasm
SoftIce
Steganos Decryptor 1.0 (Steganos itself should work, too)
Turbo Pascal (any version should do)

Targe= t's URL/FTP

http://www.steganography.com

Progr= am History

Essay

You should read Jean Flinn's essay about how
he reached the advanced stego page. 
He explains the encryption routine itself, so i don't want to do this 
again. 
Finally, he used a dictionary derived from +Fravia's page. 
If we want to reverse engineer Steganos images in general, we'll 
not have the knowledge to build such a dictionary of probable words.
Thus, we have to think about a brute force attack.

I was not able to find a serious weakness in Steganos, as well, but i
found something that helps a little bit. Jean Flinn did not mention that 
Steganos calculates a checksum of the password, which is stored within 
the image. 
You should find the algorithm easily: Enter a password, bpx hmemcpy and
notice what's going on...



:00407053 8A0439                  mov al, byte ptr [ecx+edi]	- store char in al
:00407056 8AD1                    mov dl, cl			- store counter in dl
:00407058 FEC2                    inc dl			- +1
:0040705A F6EA                    imul dl			- * al (char)
:0040705C 32D8                    xor bl, al			- xor checksum with al
:0040705E 41                      inc ecx			- inc counter
:0040705F 3BCE                    cmp ecx, esi			- finished ?
:00407061 7EF0                    jle 00407053			- no, do it again

:00407063 80FB2A                  cmp bl, 2A
:00407066 7502                    jne 0040706A
:00407068 B301                    mov bl, 01

:0040706A A1041B4400              mov eax, dword ptr [00441B04]
:0040706F 85C0                    test eax, eax
:00407071 7502                    jne 00407075
:00407073 B32A                    mov bl, 2A
	
:00407075 8D4C2410                lea ecx, dword ptr [esp+10]
:00407079 E8BED50100              call 0042463C
:0040707E 5F                      pop edi
:0040707F 8AC3                    mov al, bl
:00407081 5E                      pop esi
:00407082 5B                      pop ebx
:00407083 C3                      ret
 
 This is the code translated into Pascal.
 
  	function Checksum (passw: string): byte;
 	var i: integer;
 	    b: byte;
 	begin
 	 b:=0;
 	 for i:=1 to length (passw) do
 	  b:=b xor (ord(passw[i])*i);
 	 if b=42 then b:=1; 
 	 Checksum:=b;
 	end;
 
 Steganos Decryptor compares the original checksum with the checksum of
the password at 402bf0.
 
:00402A28 E813460000              call calcPasswdChksum (00407040)
:00402A2D 8A4C2418                mov cl, byte ptr [esp+18]
:00402A31 83C404                  add esp, 00000004
:00402A34 3AC1                    cmp al, cl
:00402A36 0F84A1000000            je 00402ADD
 
 

 By setting a bpx 402a34, you'll find the original checksum in cl (8a for 
 +Fravia's image). 
This is a good target for our attack, because it will sort out lots of 
wrong passwords. +Fravia told us that his password has 8 chars and i 
guessed that all of them were lower case (very probable for +Fravia's
passwords :-). 
Thus, I was able to sped up the algorithm a lot.
 
 
  	 checksum:=0;
  	 for a:=1 to 26 do begin
	  chksum:=fe[a] xor chksum;
	  for b:=1 to 26 do begin
	   chksum:=(fe[b] shl 1) xor chksum;
	   for c:=1 to 26 do begin
	    chksum:=(fe[c]*3) xor chksum;
	    for d:=1 to 26 do begin
	     chksum:=(fe[d] shl 2) xor chksum;
	     for e:=1 to 26 do begin
	      chksum:=(fe[e]*5) xor chksum;
	      for f:=1 to 26 do begin
	       chksum:=(fe[f]*6) xor chksum;
	       for g:=1 to 26 do begin
	        chksum:=(fe[g]*7) xor chksum;
	        for h:=1 to 26 do begin
	         chksum:=(fe[h] shl 3) xor chksum;
	         if (chksum=$8a) then ChkIfRightPwd;
	         chksum:=(fe[h] shl 3) xor chksum;
	        end;
	        chksum:=(fe[g]*7) xor chksum;
	       end;
	       chksum:=(fe[f]*6) xor chksum;
	      end;
	      chksum:=(fe[e]*5) xor chksum;
	     end;
	     chksum:=(fe[d] shl 2) xor chksum;
	    end;
	    chksum:=(fe[c]*3) xor chksum;
	   end;
	   chksum:=(fe[b] shl 1) xor chksum;
	  end;
	  chksum:=fe[a] xor chksum;
	 end;
 Note: fe[1..26] contains 'a'..'z'

 

 It looks complicated, but is really fast. But there are still several
thousand "right" passwords.

 A serious weakness is that Steganos hides and encrypts the 
 filename (+00h) of the text. 
Thus, I wrote a procedure which ckecks if the first part of the decrypted 
text is a filename (very similar to Jean Flinn's approach). 
My whole program could check all possibilities in about 80 hours (but i 
guessed the first letter was 's' for something like 'stego'... it took not more 
than two hours :-)

Here are my procedures for Steganos in Pascal:

 
	procedure makeCodeTable (passw: string;var codetable: array of byte);
	var i,old,temp: byte;
	    ba: array [0..255] of byte;
	begin
	 for i:=0 to 255 do codetable[i]:=i;
	 for i:=0 to 255 do begin
	  ba[i]:=ord (passw[1+(i mod length(passw))]);
	 end;
	 old:=0;
	 for i:=0 to 255 do begin
	  old:=(codetable[i]+ba[i]+old) and 255;
	  temp:=codetable[i];                           
	  codetable[i]:=codetable[old];               
	  codetable[old]:=temp;
	 end;
	end;
 
 
  Notes

```
passw is the password...
```

codetable[0..255] is used for decryption later on

the whole procedure is located at :00407090

<pre>{ function decode (b:byte; var codetable:array of byte;var count:byte;var co:byte; var old: byte):byte; var temp: byte; begin inc (count); old:=codetable[count]+old; temp:=codetable[count]; codetable[count]:=codetable[old]; codetable[old]:=temp; temp:=codetable[count]+temp; temp:=codetable[temp]; co:=co+$0d; b:=b xor temp xor co; decode:=b; end; Notes

```
b is the byte to decrypt
```
```
codetable[0..255] (see above)
```
```
count is a counter (starts at 0)
```
```
co is a constant (starts at 28h)
```
```
old starts at 0
 
```

the whole procedure is located at :00406fa0

Final Notes

 This is only the first step to decrypt steganography. It helps a little
 bit, but is far from being good. It makes brute force attacks faster, 
 but if someone uses numbers, lower and upper case characters, a brute 
 force attack is still too slow.

 Note for all steganography authors: Don't encrypt additional information
 such as a filename. This helps us a lot...

Ob Duh

Hey, Ob duh does not apply: Steganos Decryptor is freeware :-) If you just want to strip files out of images, you don't have to pay a penny.

You are deep inside Fravia's page of reverse engineering, choose your way out: