Assembly is just shell code that hasn't been commented. If you know how to write assembly, you can change it. Also, it is possible the authors of this software

Assembly is just shell code that hasn't been commented. If you know how to write assembly, you can change it. Also, it is possible the authors of this software have the source for Petya, and modified it there (easier). From what people who have taken the time to reverse engineer this code claim, NotPetya is a wrapper. The modified Petya code is stored in a buffer and is then called when it's time to encrypt the filesystem. To get started reverse engineering, modifying binaries, linking, etc. you need to start with learning x86_64 assembly. There are some pretty good courses and tutorials online. Since there are only a few opcodes that are used frequently, it might help to search for "reverse engineering bomb lab" and go from there. Here is the first search result, maybe it is good, maybe not: http://zpalexander.com/binary-bomb-lab-phase-1/ http://zpalexander.com/binary-bomb-lab-phase-1/