Ghosts in the Bulb

Category: Forensics (network / covert channels) Flag: isfcr{when_photons_and_seconds_both_carry_secrets_a7c3}

TL;DR

The capture is plain, unencrypted MQTT from 12 smart-home devices. Two devices hide data in covert channels:

Decode both, derive the key, decrypt the blob, get the flag.

1. Recon

Everything in capture.pcap is TCP to port 1883 (MQTT) toward the hub at 192.168.1.100. IP/TCP headers, MACs and sequence numbers are all clean and match topology.md, so nothing is hidden at the packet level.

Reassembling each device's TCP stream and parsing the MQTT PUBLISH packets gives JSON like {"on":true,"bri":..,"hue":..,"sat":..,"transition":..,"ts":..}. Message counts stand out:

• Bedroom bulb: 952 messages, far more than any other bulb of similar type.
• Studio bulb: 720 (a "music-sync" bulb, so noisy by design, a decoy).
• Robovac: 249, but it reports state: docked and battery: 100 in every message, so it should be boring and its timing should be regular.

2. Channel 1: bedroom bulb brightness LSB (the ciphertext)

Take the least significant bit of bri from each of the 952 messages, in order, and pack the bits MSB-first into bytes. That gives 119 bytes (952 / 8) with an obvious frame:

a5 5a a5 5a | 00 6d | <109 bytes ASCII> | f0 0d ca fe
 preamble     len=109     payload          trailer

The 109-byte payload uses only Base85 characters. Decoding it with base64.b85decode yields 87 bytes starting with the magic GITB, and the rest is high-entropy: encrypted. No simple XOR, compression or keys from topology.md opened it, so a key had to be hidden somewhere else.

(A trap I fell into: the 00 6d is a 16-bit length, not "a flag byte followed by data". Including the 6d in the Base85 payload gives a wrong decode that lacks the GITB magic.)

3. Channel 2: robovac inter-message timing (the key)

Other devices (thermostat, air-quality sensor) report at a fixed period with uniform random jitter, which is normal. The robovac is different: its gaps are not jitter around 60 s. After eight ordinary ~60 s gaps there is one long gap of about 137 s (about 9 x 15 s), followed by gaps clustered near 30, 45, 60 and 75 s, then ordinary ~60 s gaps again.

So the gaps are multiples of 15 s with a little jitter:

symbol = round(gap_seconds / 15)
9      -> start marker
2,3,4,5 -> data, 2 bits each (value = symbol - 2, MSB first)

Concatenate the 2-bit values after the marker and pack into bytes. At the right bit alignment (try all 8) the result is:

a5 | 4b 5a 37 57 4d 51 52 33 58 42 32 48 | aa aa aa ...
start   "KZ7WMQR3XB2H"                      idle fill (the repeating 60 s gaps = 0b10)

KZ7WMQR3XB2H is 12 characters from the Base32 alphabet (A-Z, 2-7), which is the key text.

4. Decrypting

The GITB container turned out to be AES-256-GCM:

GITB (4 bytes magic) | nonce (12) | ciphertext | tag (16)
key = SHA256("KZ7WMQR3XB2H")      # hash of the ASCII text, NOT of its Base32 decode
no AAD

The GCM tag verifies, which confirms the key and layout. Plaintext:

isfcr{when_photons_and_seconds_both_carry_secrets_a7c3}

The flag text refers to the two channels: light (the bulb) and time (the vacuum's reporting intervals).

Why it's hard to spot

• Every message is individually plausible: valid JSON, sensible values, nothing strange per packet.
• The studio "music-sync" bulb makes heavy, fast bulb traffic look normal.
• The data and the key are split across two unrelated devices and two unrelated carriers (a payload bit and timing), so neither is decodable alone.
• A docked robovac is the least interesting device on the network, which is why its timing is a good hiding place.

Dead ends worth knowing about

• Header-level channels (IP ID, TTL, TCP options, window): all clean.
• LSB streams of the other bulbs' fields and of the sensors: nothing structured.
• Brute-forcing the blob with keys derived from topology.md, MACs, hostnames and IPs: no hits.
• Thermostat and air-quality timing: uniform jitter, no information.

Reproduce

pip install scapy pycryptodome
python3 solve.py capture.pcap

Output:

[+] bedroom blob: 87 bytes, magic b'GITB'
[+] robovac covert key text: KZ7WMQR3XB2H
[+] FLAG: isfcr{when_photons_and_seconds_both_carry_secrets_a7c3}