PicoCTF_2017: A Thing Called The Stack

Category: Reverse Engineering Points: 60 Description:

A friend was stacking dinner plates, and handed you this, saying something about a "stack". Can you find the difference between the value of esp at the end of the code, and the location of the saved return address? Assume a 32 bit system. Submit the answer as a hexidecimal number, with no extraneous 0s. For example, the decimal number 2015 would be submitted as 0x7df, not 0x000007df

Hint:

Where is the return address saved on the stack? Which commands actually affect the stack?

Write-up

Stacks are a common thing in the computerverse, simply because it's a highly efficient way of storing data by appending. To solve this challenge, you need to visualize the stack after the assembly code.

[stack]
ebp:        [old ebp]
ebp-4:      [ebp]
ebp-8:      [edi]
ebp-12:     [esi]
ebp-16:     [ebx]
ebp-264:    0x1     <- esp
ebp-268:    0x2
ebp-272:    0x3
ebp-276:    0x4

It so happens that we only need to find the difference, which is 264 or 0x108 in hexadecimal.

Therefore, the flag is 0x108.

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