02 Jun

Enigma’s EP_CryptDecryptBuffer internals

For one of my private projects, I needed to decrypt some data. Original executable uses Enigma's EP_CryptDecryptBuffer function but I needed to implement the same thing using .NET. Much to my surprise, there was no information about what encryption algorithm is used by Enigma or how it works internally. So, I started by compiling Enigma's sample project CryptBuffer and debugging the executable.

TL;DR - IDEA cipher in CBC mode, password = MD5(user_password), IV for CBC comes from IDEA.Encrypt(8-zeroes, password).

Research

After protecting my executable with Enigma, few assembly lines

got changed into something horrible that looked like an Enigma's VM:

I didn't feel like spending my time on analyzing that. Much easier solution was to put hardware breakpoints on my data and password and wait for them to trigger. In a few seconds I landed into code that looked like MD5 (notice the constants!):

and few hits later I landed into code that I didn't recognize at first:

Delphi compiler had helpfully included RTTI information telling me that this second piece of code belongs to a class called TIdeaCipher. Now I had all the information I needed, I just had to put it all together. smile

Implementing decryption in C#

There aren't many IDEA implementations in C#. In fact, there are 2: by BouncyCastle and by LexBritvin. Since I'm not a big fan of huge and complex libraries like BouncyCastle, so I took the simplest possible code: from Github and modified it a bit.

First, we need change password generation algorithm to use MD5 to generate real password:

Second, we need to add support for CBC mode. It requires both calculating correct IV value and pre/post-processing each block we're encrypting or decrypting.

Calculating IV is quite simple:

Processing each block is slightly harder but not a rocket science either. As explained in image borrowed from Wikipedia:
.

Before decrypting the data block, we need to store the encrypted values as IV for the next block. After decrypting the block, we need to XOR each byte of data with a corresponding byte of IV. Unfortunately, during encryption the data flow is different, so the simple crypt function has to be replaced with 2 functions: encrypt and decrypt.

Few more cosmetic changes and that's it! We got clean and nice implementation of EP_CryptDecryptBuffer in pure C#. smile
Download link: https://bitbucket.org/kao/ep_cryptdecryptbuffer/

Have fun!

P.S. My implementation only supports cases where data length is divisible by 8, adding support for other lengths is left as an exercise for the reader.

28 Jun

Bugs in Enigma Virtual Box

While working on a new version of my static EnigmaVB unpacker, I tried to generate test files to cover most of the Enigma Virtual Box features. In the process, I ran into quite a few bugs in Enigma Virtual Box v7.40.

So, here's a short list:

Registry virtualization

1. Importing REG file with wrapped lines:

Data get truncated at the end of first line.

2. Importing REG file with entry type REG_NONE:

It gets virtualized as a string value "hex(0):"

File virtualization

1. If size of any embedded file > 4GB: creates invalid x86 executable;
2. If total size of all embedded files > 4GB: creates invalid x86 executable;
3. If size of main EXE > 2 GB: creates executable that seems to be valid but won't run;
..and that's only for x86 executables. I wonder how many more issue will surface when I start testing x64 executables. wink

TLS callbacks

Since Enigma Virtual Box uses TLS callbacks to initialize its hooks and handlers, it will (accidentally?) break any executable that also uses TLS callbacks. However, it preserves TLS StartAddressOfRawData, EndAddressOfRawData and AddressofIndex fields. Very weird.. smile

Have fun (and remember to test your software properly)!

27 May

Static Enigma Virtual Box unpacker, part 2

Here comes a new version. smile This time I added support for unpacking external packages. "External packages" are data files that can be loaded by Enigma Virtual Box and can contain both embedded files and registry entries.

I also made my unpacker 100% Unicode-aware - there should not be any more problems with non-english filenames. But I had to switch to Delphi 2009 compiler to do this, so there might be some unexpected bugs lurking around.

And, of course, lots of internal bugs had to be fixed. My code is not perfect, you know! wink

EnigmaVB Unpacker v0.30

Download link: Please get latest version from this post

P.S. Thanks to Manofwar for giving me few example files for development & testing!

05 Mar

Improved static Enigma Virtual Box unpacker

Last few weeks have been really hectic. I moved to a new apartment, so lots of time was spent on packing, unpacking, cleaning, and other non-computer related chores. Finally it's done, I got a great new place to live and I'm happy. smile

To relax and get back into shape, I spent an evening with one of my old projects - Enigma Virtual Box unpacker. I fixed few little bugs and added support for x64 executables.

EnigmaVB unpacker

Get it here: Please get latest version from this post