[LilacCTF] Bootstrap

Introduction

Well, this is the second CTF that we were doing at 2026 as Project Sekai. And actually this is big ctf since this means that I dont have to look into chinese ctfs until xCTF and Mimic which is end of this year. so huge dub!

Before talking about the challenge, I was quite surprised how solving curve was very steep since there was 510+ teams in the ctf and only 2 solves for this challenge. and also most of the cryptography in 1 ~ 2 digit solves due to the reason that It wasn’t llmable so that was interesting to see.

Challenge Introduction

The challenge is about SEAL CKKS ciphertext. Running the solver requires the same SEAL serialization format and compression mode used to generate the ciphertext. The provided chall.cpp shows how the ciphertext was created and reveals the encryption parameters. The solver then exploits a sparse secret key structure to decrypt and recover the embedded flag.