PicoCTF_2017: Forest

Category: Reverse Engineering Points: 200 Description:

I was wandering the forest and found a file. It came with some string

Hint:

A number of disassemblers have tools to help view structs

Write-up

This challenge was a fun one, given that it had a varying level of requirements in it. For the most part, IDA Pro will be very useful but for us freebies out there, we can use RetDec. We get a file decompiled like this.

This is an example of binary tree sorting, with each letter sorted with the smaller character going to the left and the larger character going to the right. The string we have to sort in this case is yuoteavpxqgrlsdhwfjkzi_cmbn.

Next, the string we are given refers to the ways we should select characters. L means go down the left branch, R goes down the right branch while D means this character. If we compare the string we got and the top-down tree we graphed, we get the flag.

Therefore, the flag is you_could_see_the_forest_for_the_trees_ckyljfxyfmsw.

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