# XMASSpirit

Now that elves have taken over Santa has lost so many letters from kids all
over the world. However, there is one kid who managed to locate Santa and sent
him a letter. It seems like the XMAS spirit is so strong within this kid. He
was so smart that thought of encrypting the letter in case elves captured it.
Unfortunately, Santa has no idea about cryptography. Can you help him read the
letter?

## Flag

HTB{4ff1n3_c1ph3r_15_51mpl3_m47h5}

- See letter.pdf

## How to solve

See `decrypt.py`

## Progress

### Code analysis:
  - Choose a `randint` `a` from `1` to `mod`, that is not divisible by mod.
  - Choose a `randint` `b` from `1` to `mod`.
  - for each byte in `dt`
    - calculate `(a * byte + b) % mod`
    - append to output bytes
  - return bytes when done

### Notes

- `s_next = (a * s_prev + b) mod p` is a linear congruential generator