Rating:
# __X-MAS CTF 2018__
## _Dataflow Programming Christmas Edition_
## Information
**Category:** | **Points:** | **Writeup Author**
--- | --- | ---
Reverse | 454 | MiKHalyCH
**Description:**
> Santa's MechaGnomes have invented some new DataFlow language that they code the factory robots' instructions. You found a weird string checker, you need to interpret the code in order to recover the string! The flag obtained from the challenge must be wrapped up like: X-MAS{flag}
>
>[chall](src/chall.zip)
>
>Authors: Gabies + littlewho
## Solution
We have a good [manual](src/ChristmasFlow_programming_language_manual.pdf) of programming language and the [code](src/code.txt).
It was to hard to write a good interpreter for this language (yeah, I spend a lot of time for it) because of treads (a.k.a. _iteration levels_).
_Get ready for the real_ __EYESOLUTION__!
I noticed that we can split the full code by `EQU 158L -` into blocks like this:
```
17 CLN 19L 27L
18 SND_2 19R -
19 EQU 20R -
20 BRB 26L 30L
21 SND_10 22L -
22 CLN 24L 25R
23 SND_1 24R -
24 ADD 25L -
25 MUL 26R -
26 EQU 158L -
```
and
```
90 CLN 92L 98L
91 SND_9 92R -
92 EQU 93R -
93 BRB 97L 101L
94 SND_204 96L -
95 SND_2 96R -
96 SHR 97R -
97 EQU 158L -
```
Each block contains two parts:
* The first block compairs some constant with some number from [0, 18] (from 0-1 lines of code). Looks like an index of flag character.
* The second block calculates some integer with differend arithmetic and bitwise operations. All the values are ASCII characters.
The examples above would calculate `flag[2] = chr((10 + 1) * 10) = 'n'` and `flag[9] = chr(204 >> 2) = '3'`.
Now we need to calculate all characters and collect the flag!
