(the "dummy code check" trick)
by sPIRIT and HellRaiser
(12 February 1998, slightly edited by Fravia+)
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of reverse engineeringHow to generate RealPlayer Plus 4.0 installation keys By sPIRIT - sOFTPROJECT 98, and HellRaiser Tools needed: SoftICE (Winsuxx 95 or NT version) Any ASM compiler (TASM 3.2 or greater will do) Hello World, this is HellRaiser writing! I've been searching for some time for a code/keygen for RealAudio Player Plus, but it looks like I couldn't find anything so far... so I decided to do it on my own :) Too bad I discovered AFTERWARDS that the record function does not convert audio streams to RA files, but only allows to save streams from netradios on the hard disk... bah, who cares, I did this and I'm going to tell you everything. Thanks to sPIRIT^SPJ who helped me a bit, expecially teaching SiCE commands, messing up the debug phase and writing this article. As a newbie to NT cracking (I just installed SoftIce/NT yesterday and never had experiences about NT cracking) I played around a lot... sorry if it doesn't sound clear O:) There has to be an easier way to crack this target, changing a flag in one memory location or such, but I thought a keygen was more userful and funnier. Okay, at first I expanded the installation package to my HDD and hooked the GetWindowTextA, which the installation program seems to use intensively. Even if it calls it when you press a key, don't think about it, it's just to write out the new code. The first thing that comes to mind is that just after you press "NEXT>" it reads the 3 codes, copies them to another memory area and then checks for some kind of flag (or at least I thought so) which will probably go wrong (CAC instead of CAB). Setting the register and/or the memory to CAB will not show the warning message, but it will also NOT install the target correcly. Something better had to be done... BPM to the new-written memory area containing the codes (funny, it writes them in 3 different lines, with an offset of 10h between each beginning) and some skipping. It had to read them, or else they were useless... and indeed, SiCE soon told me that it was. Even more funnier, it was copying them to another memory area. New BPM to this new area (why was it coping them? not for fun, of course :). When SiCE popped up again, the installation program was checking for the numbers to be digits. Some skip of these routines, then I came to some other call, which I traced... what a real mess!!! A lookup table, a lot of memory writes... ARGH... I'll tell you... this is probably the most messed up ASCIIZ -> BIN code snippet I've ever seen... they have for sure spent some days rewriting this part in assembly ;))) After some like 1 hour trying to understand it, it just returned a number in EAX, and it was what I wrote sooner. BPC, and BPX to next call, since it was pushing the code... The next call contained some interesting code like I've rarely seen elsewhere... it had to be the encrypt/test routine. It just checks for a number to be inserted (1000-00-1000, which I first thought was some magic register-all code, but instead it was the 'fake magic number' he was putting in when the input code was wrong 8))), does some cripples with the codes (the routine reads the 10 digits... I did not spend almost any time at all understanding it, I didn't care at all). A couple of lines below was the interesting part. Some codes were written in memory and especially in EAX there was some interesting code... the program was putting 2 different values from 2 quite different tables with values ranging from 0 to 8 with some checks for equality... this had to be the code check. It took just some patience to copy it down in a text file and make it callable via assembly :))) just a loop to check if the output from the routine (with 3 lines from the function which originally got original result) was 0 or 1... at the 1st 1 (duh...) I found I tried the code... it worked, among with all other codes it said to be good 8) Now it's sPIRIT writing. I know that this program is rather old, but it's always interesting to see how a GOOD utility is protected, just for learning' sake. Here are some code snippets from the PNSETUP.EXE diassembled listing. Take a look at this code: * Referenced by a Jump at Address:00404736(C) :004047AB 8D4C2410 lea ecx, dword ptr [esp+10] ;Here is our key :004047AF E8BCD4FFFF call 00401C70 ;Let's check it!! :004047B4 85C0 test eax, eax ;Is EAX=00000001 ? :004047B6 7420 je 004047D8 ;Too bad! It's a WRONG key :004047B8 8D4C2410 lea ecx, dword ptr [esp+10] :004047BC E89F430000 call 00408B60 ;Let's check it another time :004047C1 85C0 test eax, eax :004047C3 7513 jne 004047D8 ;Uhm... BAD key! :004047C5 8D4C2410 lea ecx, dword ptr [esp+10] :004047C9 E8D2000000 call 004048A0 ;Good key, you honest guy! :004047CE 33C0 xor eax, eax :004047D0 5E pop esi :004047D1 81C45C010000 add esp, 0000015C :004047D7 C3 ret * Referenced by a Jump at Addresses:004047B6(C), :004047C3(C) :004047D8 8D442434 lea eax, dword ptr [esp+34] ;Jumps here when a BAD key is :004047DC 682C010000 push 0000012C ;entered :004047E1 50 push eax * Possible Reference to String Resource ID=03271: "Warning: This is an invalid serial number! " And the program goes on asking for a new key, or installing the lite version. Now the question is, what does the "call 00401C70" do? * Referenced by a Jump at Address:00401C65(U) :00401C70 E84B6E0000 call 00408AC0 ;ASCIIZ -> HEX conversion :00401C75 50 push eax ;the code now is in EAX :00401C76 E8656D0000 call 004089E0 ;Let's check it :00401C7B 83C404 add esp, 00000004 :00401C7E C3 ret Here is the code that checks the keycode, as HellRaiser explained: * Referenced by a CALL at Addresses:004089E5 , :00408B71 :00408BA0 83EC14 sub esp, 00000014 :00408BA3 53 push ebx :00408BA4 56 push esi :00408BA5 8B742420 mov esi, dword ptr [esp+20] :00408BA9 57 push edi :00408BAA 55 push ebp :00408BAB 81FEE8CD9A3B cmp esi, 3B9ACDE8 ;This checks for a dummy code :00408BB1 750D jne 00408BC0 ;(1000-00-1000) :00408BB3 B801000000 mov eax, 00000001 :00408BB8 5D pop ebp :00408BB9 5F pop edi :00408BBA 5E pop esi :00408BBB 5B pop ebx :00408BBC 83C414 add esp, 00000014 :00408BBF C3 ret The dummy code check was a bitch to understand. It sees that that key is something similar to a *magic* one, as it also sets the flag in eax (00000001), but the code that followed doesn't fill all the flags. In fact, a later call checks for HEXcode = 3B9ACDE8, and reports a BAD KEY status. Let's go on. * Referenced by a Jump at Address:00408BB1(C) :00408BC0 85F6 test esi, esi ;The key can't be all 0's :00408BC2 750A jne 00408BCE ;The key isn't all 0's: real check :00408BC4 33C0 xor eax, eax ;or the fake "magic" :00408BC6 5D pop ebp :00408BC7 5F pop edi :00408BC8 5E pop esi :00408BC9 5B pop ebx :00408BCA 83C414 add esp, 00000014 :00408BCD C3 ret The "jne 00408BCE" brings to the REAL key check, it begins this way: * Referenced by a Jump at Address:00408BC2(C) :00408BCE 33C9 xor ecx, ecx :00408BD0 BF01000000 mov edi, 00000001 ... ... :00408C7E 89442420 mov dword ptr [esp+20], eax EAX contains the flag, 00000000 if the key is not valid, 00000001 otherwise (as we want!) * Jump from Addresses:00408C08(C), :00408C68(C), :00408C6E(C), :00408C78(C) :00408C82 8B442420 mov eax, dword ptr [esp+20] :00408C86 5D pop ebp :00408C87 5F pop edi :00408C88 5E pop esi :00408C89 5B pop ebx :00408C8A 83C414 add esp, 00000014 :00408C8D C3 ret Ok. This check is over, but later on, at :004047BC call 00408B60, it checks the code another time, to see if the first time we entered the fake magic I mentioned above (in that case the flag in eax was set), and if it was done, fails the installation. Ok, the real key check was ripped, along with the two tables it used to check if the entered code was valid, located at 0040B5A8 and 0040B6C8. Simply take a look at the source code of the keygen to see how they are used.
Keep up the good work sPIRIT - sOFTPROJECT 98(c) 1998 sPIRIT and HellRaiser All rights reversed
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