Rating:
# __Timisoara CTF__
## _SSS Part 2_
## Information
**Category:** | **Points:** | **Writeup Author**
--- | --- | ---
Crypto | 250 | MiKHalyCH
**Description:**
> [files](sss_part2.zip)
## Solution
This task is all about [bad realization](sss.py) of SSS algo. It uses same coefficients for different [splits](shares.txt).
If `f(x)` is [lagrange polynomial](https://en.wikipedia.org/wiki/Lagrange_polynomial), then each `y = f(x) + hex(flag_substring)`.
It means that `y_21 - y_11 = y_22 - y_21`. We can compute `y_i` for each chunk, if we know `y_i` for just one chunk and difference between any results.
So let's calculate `y_1` for each chunk.
Yet we need to predict `f(x)` to decrypt each part of flag.
We know the first 7 bytes of flag: `timctf{`. We need to predict only 3 another.
[After several attempts](solver.py) I got the flag
