Lion City Layover

A tourist maze led to three postcard keys, a curious WebAssembly cartridge, and finally a boarding pass. The key twist: the server checked one copy of a cartridge section, then ran another.

Earn the ticket

The page bundle exposed the route through the challenge:

/api/harbour/start → /stamp → /tide → /run → /claim
                    ↘ /postcard

/start returns a session and three maze grids. Each needs stamps A, B, and C before the exit. The browser shows the moves, but /stamp replays them server-side. A breadth-first search over (x, y, collected-stamp-mask) finds valid direction strings for all three grids. Submitting them earns a short-lived signed ticket for /run and /claim.

Collect the postcards

/tide?format=a reveals three back halves; any present format value also unlocks last_stamp. Match each clue to an image from /start:

Clue	Image trick	Full postcard key
Artificial steel trees	RGB least-significant-bit stream	Q7M2-9KP4-4L8N-TW3R
Chicken with fragrant rice	A second PNG appended after IEND	A5ZD-91HJ-K7C2-P9QM
Purple Vanda Miss Joaquim	Plaintext after IEND	N8YF-R4T6-WQ1B-7VL3

For the Supertree image, read each RGB channel's low bit, pack groups of eight, and search the resulting bytes for postcard{...}. The other two images hide their front halves after the PNG end marker. Sending the completed keys to /postcard unlocks cartridge documentation, including this line:

Two stamps went through immigration. The officer checked the first. The replay kiosk performed the last.

That turns out to be a remarkably literal hint.

Make the validator see a different cartridge

The supplied singa_legacy.wasm is a valid WebAssembly v1 container. Its useful data lives in a custom section named singa: MERLION\0v1.6.1\0, a little-endian program length, and program bytes. The reference program 01 2a fe ff is accepted and returns chroma: [42].

The postcard documentation calls 0x61 a legacy diagnostic reader. A cartridge containing just 61 00 00 20 ff is blocked as an unsafe opcode. But when a safe singa section comes first and a second singa section holds the diagnostic program, /run accepts the module and executes the second section. The two phases disagree:

immigration validator → first singa section
replay interpreter     → last singa section

The diagnostic output begins SINGA-PASSPORT-v61. Each reader instruction takes a big-endian offset and byte count, so two 32-byte reads reach the service's 64-byte output cap:

program = bytes([
    0x61, 0x00, 0x60, 0x20,
    0x61, 0x00, 0x80, 0x20,
    0xff,
])

Every /run request also needs proof of work: find a nonce whose SHA256(ticket + "." + module_b64 + "." + nonce) starts with 0000. The resulting chroma bytes disclose rsa_n, rsa_e = 65537, rsa_c, and claim_sha256.

Open the boarding pass

The diagnostic jokes that “Fermat liked neighbours on the same HDB landing.” The RSA primes really are close: starting at ceil(sqrt(n)), find a for which a² - n is a square, then set p = a - b and q = a + b.

Decryption yields:

boarding-pass:katong-1965-to-marina-2026:kopi-o-kosong

Its SHA-256 matches the leaked claim_sha256. Submit the boarding pass with the ticket to /api/harbour/claim:

TISC{w3lc0m3_70_51ng4p0r3_l4h_61}

The included solve.py performs the maze, postcard, cartridge, proof-of-work, and claim steps.