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BCTF CTF 2015: backdoor

Category: Crypto + Programming Points: 600 Solves: 1 Description:

tsh is my favorite backdoor in use. So I forked on github and add some my own functions.


I’ve setup a tsh client in cb mode, running at 7586, with my secret set, and repeatedly ping some important info out.

But very weird someone can steal my info, can you steal it?

You need exploit, crypto, coding, debugging skills, as well as luck and network speed, to solve this challenge.

tip: flag length less than 22( BCTF{} included)

tip2: challenge constant in pel.c not modified


The server running binary has alarm(3) uncommented in tsh.c. link here: https://github.com/zTrix/tsh/commit/f3ef65cbe8a42995ef9c183fb59de2e2d966a8d5#diff-245ad4e8eff2c0fa695986cd7e83dff6R177



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