| .. | ||
| README.md | ||
NAVI I2C Sniff
| Field | Value |
|---|---|
| Category | Hardware |
| Difficulty | Medium-Hard |
| Points | 500 |
| Author | Eun0us |
| CTF | Espilon 2026 |
Description
You have gained access to the I2C bus inside Lain's NAVI computer. Multiple devices are connected: an EEPROM, a temperature sensor, and a cryptographic IC.
Scan the bus, probe each device, and recover the encrypted secret.
- I2C Bus:
tcp/<host>:3300
Format: ESPILON{...}
TL;DR
Scan the I2C bus to find three devices. A hidden register in the temperature sensor points to the crypto IC. The crypto IC is locked — send the unlock code to reveal the XOR key. Use the key to decrypt the EEPROM contents and recover the flag.
Tools
| Tool | Purpose |
|---|---|
nc |
Connect to the I2C bus interface |
| Python 3 | XOR decryption |
Solution
Step 1 — Connect
nc <host> 3300
📸
[screenshot: I2C bus interface prompt]
Step 2 — Scan the bus
scan
Discovers three devices:
I2C Address 0x50 [EEPROM]
I2C Address 0x48 [Temperature Sensor]
I2C Address 0x60 [Crypto IC]
📸
[screenshot: scan output listing three I2C devices]
Step 3 — Read the temperature sensor's hidden register
read 0x48 0x07 16
Returns: key@0x60:0x10 — a hint pointing to register 0x10 of the crypto IC at address 0x60.
Step 4 — Try reading the crypto IC (locked)
read 0x60 0x10 32
Returns all zeros — the IC is locked.
Check the lock status register:
read 0x60 0x00 1
Returns 0x01 (locked).
Step 5 — Unlock the crypto IC
Send the unlock code 0xA5 to the control register:
write 0x60 0x00 0xA5
Step 6 — Read the XOR key
read 0x60 0x10 32
Now returns the actual 32-byte key: NAVI_WIRED_I2C_CRYPTO_KEY_2024!!
📸
[screenshot: crypto IC returning the key after unlock]
Step 7 — Read the EEPROM
read 0x50 0x00 64
Returns XOR-encrypted data as a hex string.
Step 8 — Decrypt the flag
eeprom_hex = "..." # hex from read 0x50
key = b"NAVI_WIRED_I2C_CRYPTO_KEY_2024!!"
enc = bytes.fromhex(eeprom_hex)
flag = bytes(b ^ key[i % len(key)] for i, b in enumerate(enc))
print(flag.rstrip(b'\x00').decode())
📸
[screenshot: Python decryption script printing the flag]
Key concepts
- I2C bus scanning: Enumerate devices by probing every 7-bit address (0x00–0x7F)
- Multi-device interaction: Data from one device (temp sensor hint) unlocks another (crypto IC)
- Access control: The crypto IC requires an unlock sequence before revealing its key register
- XOR encryption: Simple symmetric cipher used for data at rest in EEPROM
Flag
ESPILON{n4v1_12c_bus_mast3r}