So if you know anything about cryptography then you know that XOR is a pretty weak encryption method to use. Being susceptible to plain text attacks, knowing the plain text of one version of a file could deliver you your key! (EncryptedText ^ PlainText = KEY)
But here is the reason for this Job. Recently i have come across a custom XOR encryption that i cannot find out how to universally crack. I know its possible, as i have come close awhile back, But these days i just do not have time, and i cant get in that state of mind again. That being said, it bothers me that i can't quite figure this thing out still!
So here is how it goes down, I have the source code of the decrypter in Visual Studio's .SLN in either C++ or C#, which ever you prefer. In this solution is the source to a decrypter i wrote using the custom XOR encryption/decryption. This encryption/Decryption method uses a 20 byte key to encrypt/decrypt files. 4 bytes is used to generate a header to place at the top of the encrypted file so it can detect it later, and the other 16 bytes is used to encrypt 2048 bytes of the file.
The encrypter/decrypter doesn't just XOR the byte with the key though, It uses the key to generate 2 tables to encrypt/decrypt the file. Its kind of like a CFB XOR in a way.
Now that i have explained the process a bit, here is how the job will work. If you are interested in the job, message me and let me know. When you do, i will send you a rar file. In this rar file there will be a fully working C++ and C# solution of the decrypter with a hard coded key in it, A encrypted and decrypted file that goes with that key, and a encrypted file that was encrypted with a different key.
The solution is so you can read, understand, and use the decrypter and XOR method (Obviously :) )
The encrypted and decrypted files that go with the key in the solution/decrypter is so you can study the encryption, and also use the byte in the file for plain text attacks where needed (The method i was using involved using plain text bytes in the equations to break this thing universally, as i think urs will too.)
The separate encrypted file encrypted with a different key is used as your goal, This is the file you need to decrypt without having the key!
Brute force methods are allowed, though you wont be able to brute force a 20 byte key! But perhaps you will find a bottle neck in the code? :)
Any and all methods of cracking this is allowed. You may use plain text attack, as we are going to assume that anyone who gets there files encrypted by this will have at least one good version of there file to use.
Remember this crack must work on any files encrypted by this method universally!
When you are done, ill test the method on another encrypted file with a different key here and that will show if the method works, When this is done, payment will be released! Thanks!