Exetools - Help identify crypto

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Help identify crypto

Hi all.

How do you identify the cipher based on the assembly code of the encryption algorithm? I tried to use tools such as IDAScope, signsrch etc but none of them have found any constants (the program is not obfuscated).

I think the cipher in question is a common one but I am unable to tell which one. From what I see the ciphertext is a 256-byte block. There is also a static array in the code of which first 256 bytes are taken (the key?). Then I see both blocks are copied in memory in reverse order and then the magic is done.

Is anyone able to recognize it based on this description?

try cc or kanal for peid ?

Yes, I tried these too, unfortunately to no avail. Even by looking over the assembly code of the algo I couldn't locate any specific constants (except for 0x8000, 0xFFFD, 0xFFFE, 0xFFFF and 0x10000).

Some additional info: the resulting plaintext message is 256 bytes long as well. From what I see, during the decryption process the encrypted block undergoes many tranformations (at some point even doubles it's size).

Any guess?

Those constants are seen when computing CRC16. I would think the scanner would pick that up tho.
Perhaps its a compression instead?

you can use FindCrypt plugin for ida pro http://www.hexblog.com/?p=27

While analyzing a program quite often we want to know if it uses any crypto algorithm. Knowing the algorithm name would be useful too. Here is the plugin which can help us answer these questions.

The idea behind it pretty simple: since almost all crypto algorithms use magic constants, we will just look for these constants in the program body. Here is how we would do it manually:
http://www.sockpuppet.org/tqbf/log/2006/01/two-things.html
This approach will fail if the S-boxes have been altered but in most cases they are untouched (can you admit that you understand all consequences of modifying an S-box, say, in AES?)
The plugin supports virtually all crypto algorithms and hash functions. I also added the zlib library constants to the list just for the user convenience. Here is the full list:

PHP Code:


		
			
    Blowfish

    Camellia

    CAST

    CAST256

    CRC32

    DES

    GOST

    HAVAL

    MARS

    MD2

    MD4

    MD5

    PKCS_MD2 (byte sequence used in PKCS envelope)

    PKCS_MD5 (byte sequence used in PKCS envelope)

    PKCS_RIPEMD160 (byte sequence used in PKCS envelope)

    PKCS_SHA256 (byte sequence used in PKCS envelope)

    PKCS_SHA384 (byte sequence used in PKCS envelope)

    PKCS_SHA512 (byte sequence used in PKCS envelope)

    PKCS_Tiger (byte sequence used in PKCS envelope)

    RawDES

    RC2

    RC5

    RC6

    Rijndael

    SAFER

    SHA-1

    SHA-256

    SHA-512

    SHARK

    SKIPJACK

    Square

    Tiger

    Twofish

    WAKE

    Whirlpool

    zlib

Please note that the list does not contain the IDEA algorithm because it usually builds its tables on the fly. Other algorithms can be added if needed.
The plugin is very easy to use �C just select it from the plugins menu and it will do its job. At the end it will display a message box like this:
http://www.hexblog.com/ida_pro/pix/f...t_finished.gif
It also will rename all found arrays and put them in the marked location list:
http://www.hexblog.com/ida_pro/pix/findcrypt_marks.gif
The same approach can be used to find other magic constants and strings. The plugin can also be automated �C just hook to the ph.newfile processor module event and run the search.
The source code, as usual, comes with the plugin: findcrypt.zip
Have (cryptic) fun!

Findcrypt doesn't find anything. I can't find any constants similar to these from findcrypt source code in the disassembly of the algo. I also don't believe it's a compression since the output has the same length as input.