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