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PicoCTF 2018 – Reverse Engineering - assembly 0


PicoCTF 2018 – Reverse Engineering  - assembly 0

Objective:
What does asm0(0xb6,0xc6) return? Submit the flag as a hexadecimal value (starting with '0x'). NOTE: Your submission for this question will NOT be in the normal flag format. Source [1]  located in the directory at /problems/assembly-0_0_5a220faedfaf4fbf26e6771960d4a359.
Hints:
(1)    basical assembly tutorial [2]  (2) assembly registers [3]

Source:

.intel_syntax noprefix
.bits 32
               
.global asm0

asm0:
                push      ebp                                                       
                mov       ebp,esp
                mov       eax,DWORD PTR [ebp+0x8]
                mov       ebx,DWORD PTR [ebp+0xc]
                mov       eax,ebx
                mov       esp,ebp
                pop        ebp       
                ret


Solution:

So  I spent about an hour watching youtube videos to try and understand assembly language to get his one.

Here is what I learned


This part
push      ebp                                                       
            mov       ebp,esp

has to do with the stack pointers getting setup for the program and really don’t mean a whole lot other than the stack is being created an this is where the pointers are set.

The parts that actually matter to this solution are
 mov       eax,DWORD PTR [ebp+0x8]  - This is putting the 1st variable in EAX which was 0xb6
 mov       ebx,DWORD PTR [ebp+0xc] –  This is putting the 2nd variable in to EBX which is 0xc6
 mov       eax,ebx                                                - this overwrites eax with ebx so now EAX is 0xc6

the next part of the assembly just kind of tears the program down and this particular assembly will always return what is in EAX

so it should return 0xc6

Here is a great little youtube video that help break it down that John Hammond already made

https://www.youtube.com/watch?v=sLsgSC6ViS8


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