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SW0:00401762 mov [ebp+bad_boy_counter], 0 SW0:00401769 push offset aLordcarder; 'LordCarder' SW0:0040176E push offset entered_name SW0:00401773 call compare_strings SW0:00401778 add [ebp+bad_boy_counter], eax SW0:0040177B push offset a5ec9; '5EC909D6050ACDFE0674C0B745B8F'. SW0:00401780 push offset entered_sn SW0:00401785 call compare_strings SW0:0040178A add [ebp+bad_boy_counter], eax SW0:0040178D push offset aProthief; 'ProThief' SW0:00401792 push offset entered_name SW0:00401797 call compare_strings SW0:0040179C add [ebp+bad_boy_counter], eax SW0:0040179F push offset a5e_0; '5EC909D6050ACDFE0674C0B745B8F'. SW0:004017A4 push offset entered_sn SW0:004017A9 call compare_strings SW0:004017AE add [ebp+bad_boy_counter], eax SW0:004017B1 cmp [ebp+bad_boy_counter], 0 SW0:004017B5 jnz blacklisted. SW0: 00405215 db 'C814338D071D56578B2D5CB176E08B56DE1A2AE32D4D83B46475CF', 0 First part of the serial is exactly the same! Let's go back to ElGamal basics and think a bit. Revisiting ElGamal signing algorithm I'm sure you already remember the algorithm from my previous blog post. But here it is again.

Smile To sign a message M, one would: • Make a hash of message, H(M). In this crackme, it's SHA1 of the username • Generate a random number K where K. Some weeks ago I found a nice keygenme on. The description looked interesting enough: Yet another company is making wild claims!

Your mission: prove that people shouldn't trust companies promoting 'revolutionary' crypto algos. Keygen this son of a crypto nightmare and write a DETAILED tutorial! Rules: 1) The only acceptable solution is a keygen 2) No patching of course It was not solved for few weeks, so I decided to take a look at it. Smile Crash-course in ElGamal signature scheme I hate cryptography. It's complex, it's confusing and unless you're prepared to study this field for years, you can't really understand why stuff works this or that way.

So, here's a short version, just enough to solve this keygenme. It's based on the explanation in InfoSec Institute's. Key generation • Generate a random prime number P with chosen length. • Generate two random numbers, G and X, with G. Y = CC945009A3E4215D042284F4FE567DFDAAEB906E8A620597FAF4953935F217EC Private key X is.

Well, private. Smile Without it we can't generate correct keys for a name of our choice.

So, the challenge would be to recover the private key somehow. Signing To sign a message M, one would: • Make a hash of message, H(M). In this crackme, it's SHA1 of the username • Generate a random number K where K. SW0: 00401092 xor random_seed, eax; original seed value = 0x37333331 Hmm, that's weird. Normally PRNG is initialized using rdtsc instruction or something even than that. And what exactly is arg_0? It's a handle of the DialogBox - not random at all!

Finally, we've found a reason why this ElGamal implementation is broken! Smile Bruteforcing the private key Now that we know the weakness, we can write a bruteforcer that will go through all possibilities of random seeds and generate all possible ElGamal keys.

Once we generate a key that has the same P, G and Y as in the keygenme, we will also know the correct private key X. But generating all these numbers is a slow process! Let's look back at the code and see what we can optimize. 1) We don't need to generate all the numbers. It's enough if we find correct P - the rest of numbers will match automatically. So, let's remove the rest of the code.