Because if that happens, I don’t have a one-to-one relationship, and my hypothesis that this is a variant of hexadecimal, is wrong. If the letter that I add to the dictionary is already present in the dictionary, I compare the stored hexadecimal digit for that letter with the one I looked up, and if they are different, I generate an exception.
#010 editor lz77 script code
I will now add code that does the following: for each letter of the encoded string, I will lookup the corresponding hexadecimal digit in the hexadecimal representation of the unencoded string, and add this decoding pair to the dictionary. That’s at the beginning of the payload, so it’s possible that I have found the location of the encoded string “!This program cannot be run in DOS mode”. I add code to find the position of the first occurrence of string uy inside the encoded payload: Hexadecimal digits 5 and 4 are part of the digits we already decoded. Notice that the letter T is represented as 54 in hexadecimal. I add the following lines to print out string “!This program cannot be run in DOS mode” in hexadecimal: I will know use this string to try to match more letters with hexadecimal digits (I’m assuming the PE file contains this string). DO is 444F in hexadecimal, and is part of the well-known string found at the beginning of (most) PE files: !This program cannot be run in DOS mode Notice that apart from MZ, letters DO also appear. I will now add a small dictionary (dSubstitute) with this translation, and add code to do a search and replace for each of these letters (that’s the for loop): So let’s assume that this represents MZ (4D5A in hexadecimal), thus y is 4, d is d, u is 5 and a is a. Since I know this is a PE file, I know the file has to start with the letters MZ. Let’s make a small modification to the program, and represent each pair of characters that couldn’t be decoded as hexadecimal, by a NULL byte (data.append(0): So I don’t know which letter represents digit 0. That is not the case here for letters d and q. It often has a frequency of 20% or higher. Character 0 is by far the most frequent when we do a frequency analysis of the hexadecimal representation of a “classic” PE file. PE files that are not packed, contain a lot of NULL bytes.
#010 editor lz77 script windows
I do know that the payload is a Windows executable (PE file). Looking at the statistics produced by byte-stats.py, I see that there are 2 letters that appear most frequently, around 9% of the time: d and q. If that fails (ValueError), that pair of characters is just ignored.Īnd then, the last statement, I do an hexadecimal/ascii dump of the data that I was able to convert. The next piece of code, starting with “data = ” and ending with “data = bytes(data)”, will read two characters from the encodedpayload, and try to convert them from an hexadecimal byte to a byte. Since I’m actually dealing with letters only, I’m converting these bytes to characters and store this into variable encodedpayload. The content of the file is in variable data. I am replacing this default code with the following code (I will post a link to the complete program at the end of this blog post): You will find this default processing code in the template: To analyze and try to decode this, I’m making a custom Python program based on my Python template for processing binary files. This is likely some form of variant of hexadecimal encoding (16 characters) with an extra character (17 in total).
There are 17 unique bytes used to encode this payload. Next I use my tools byte-stats.py to produce statistics for the bytes found inside the payload: That makes it very unlikely that it is indeed base64 or base85. Plus, for the last 2 decodings, only 17 unique characters were found. Only base64 and 2 variants of base85, but that doesn’t decode to anything I recognize. That failed: no netbios encoding was found. Since my tool base64dump.py can handle netbios name encoding, I let it try all encodings: That is an encoding where each byte is represented by 2 hexadecimal characters, but the characters are all letters, in stead of digits and letters. Seeing all these letters, I thought: this is lowercase Netbios Name encoding. NET assembly), but here I’m going to show how you can try to decode a payload like this without having the decoder.
#010 editor lz77 script how to
In this blog post, I will show how to decode a payload encoded in a variation of hexadecimal encoding, by performing statistical analysis and guessing some of the “plaintext”. I also recorded a video for this blog post.