Game Vault

The vault sat behind an entire game, but winning the game was only the first lock. A stale enemy marker produced an accepted 88-action victory replay; a hidden console then led through employee accounts to a production 2FA test code.

Win the replay

The client opens a fresh TCP connection for each request. Every packet begins with an eight-byte random preamble that seeds a four-word XOR stream; the ordered pair of session IDs binds requests together. A successful handshake returns session IDs, a map seed, an attestation challenge, and a resource key. Resource and replay requests include a deterministic 90-byte client attestation, so the solver reimplements that client function rather than searching for a secret.

Replay actions are little-endian u32s encrypted byte by byte. Starting with r = map_seed and delta = XOR(all 16 session-ID bytes) (or 0x7a when zero):

c[i] = p[i] XOR ((r + i) & 0xff)
r    = ((r XOR p[i]) + delta) & 0xffffffff

The replay checksum is DJB2 over the session pair, action count, and ciphertext. The map also changes each session: MT19937 seeded by map_seed performs 100 swaps for each of 64 visits, so solve.py rebuilds each permutation before choosing a world entry.

The actual game bug is simpler than this transport. Enemies cycle through a four-slot ring. Enemy1 sets a marker byte, but a later clone fails to clear it reliably. Reuse that slot for a weak enemy, kill it, and the 88-action route earns:

You have received the key!

World-map selector 209 opens a developer console without adding a normal replay action.

Follow the console breadcrumbs

help reveals login, chat, hints, and credits. A decoded hint points toward the secret shop and four rests used by the replay. credits exposes XOR-obfuscated employee labels; chat includes the reminder that employees should change default passwords. Testing likely credentials with check_console_logins.py finds Sarah Chen's password123.

From there the challenge becomes a deliberately silly office escalation chain:

login sarah.chen password123
lookup devon.park
lookup priya.nandakumar
lookup elena.vasquez

login devon.park bubbletea
manual                         # reveals reset_passwrd

login priya.nandakumar elena.vasquez
reset_passwrd adrian.osei chosenpassword
login adrian.osei chosenpassword
source_list
source_dump sentinel_auth.cpp

The dumped sentinel_auth.cpp contains an unfinished production 2FA check with a hardcoded test code, 125721. Use Priya's unrestricted reset again for vault administrator Marcus Webb, then submit it:

login priya.nandakumar elena.vasquez
reset_passwrd marcus.webb chosenpassword
login marcus.webb chosenpassword
vault 125721

TISC{wh@t_rE_m0r3_Lyk3_LLM_ctf}

Reproduce it

solve.py contains the packet stream, attestation, seeded map route, and console chain. check_console_logins.py covers the credential search. The precise packet format matters when writing a new harness: the successful handshake is an unframed 37-byte record, while most other replies use u32_le length | body framing.