Walkthrough of Intro to GDB

What is dynamic analysis?

Dynamic analysis is a type of analysis that requires live execution of the code. If, for example, we encounter a code that decrypts or decompresses to a huge amount of data, and if we want to see the contents of the decoded data, then the fastest option would be to do dynamic analysis. We can run a debug session and let that area of code run for us.

What is GDB?

GDB (GNU DeBugger) is the bulit in Linux debugger on the GNU framework, basically everything Linux uses. It allows the reverser to step through a program and analyze in the inner working of that program in runtime. More advanced features included in GEF and other extensions allow for a more comprehensive view of the binary.

How do computers run a binary?

Computers use something called the stack to store information while a program is running by pushing and popping values off the top. This type of memory is different from the memory in hard drive/SSD as the stack uses data storages known as registers which can store 4 bytes in a 32-bit system and 8 bytes in a 64-bit system (EAX vs RAX). These registers interact with the RAM (Random Access Memory) using memory address pointers and constitute the major components of the CPU (central processing unit).

How do I use GDB?

Installing GDB: GDB should be built into any function Linux image but if you don't have a Linux image refer to here.

GDB disassembles a compiled binary into Assembly, a coding language based on simple opcodes and operants, a bit (a lot) different from what people traditionally think of as a "coding language", C, Python, Java, etc.. A quick guide for Assembly can be found here. GDB uses stepping to move through a live binary and allows the reverser to view all the moving components and their values, namely: registers and memory locations. A great tutorial for using GDB can be found here though it is slightly longer (but you can always watch at 2x speed ;)) or this article.

Examples:

Bomb Labs write up String compare write up Crackmes write up

Regards

If you still can't get the solution try Googling for other examples to understand theory and if you're still stuck just shoot me a ticket through the Discord Ticket bot. I wish you all the best on these challenges and I hope you enjoy your time trying to crack not only my challenges, but the rest of my team's. Good luck and have fun :D ~ BrokenAppendix