Full writeup: [https://github.com/happysox/CTF_Writeups/tree/master/XMAS_CTF_2018/friedrich](https://github.com/happysox/CTF_Writeups/tree/master/XMAS_CTF_2018/friedrich)

### Summary

* XOR the first 200 bytes with 13 to repair the original elf
* Patch the anti-debug instructions
* Dynamic analysis
* Solve linear equation `Ax=b` where
* A is a 36*36 matrix containing scalars found in the runtime memory
* b is a vector with 36 sums found the same way
* x is the flag

Original writeup (https://github.com/happysox/CTF_Writeups/tree/master/XMAS_CTF_2018/friedrich).