1c42421380
- Generated screenshots for all 33 challenges (ESP, Hardware, IoT, OT, Misc, Web3) - Replaced all 123 placeholder lines with actual PNG image references - Cleaned duplicate images from previously partial updates - All write-ups now have full illustrated solutions
134 lines
3.0 KiB
Markdown
Executable File
134 lines
3.0 KiB
Markdown
Executable File
# Lain_Br34kC0r3
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| Field | Value |
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|-------|-------|
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| Category | IoT |
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| Difficulty | Medium |
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| Points | 500 |
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| Author | neverhack |
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| CTF | Espilon 2026 |
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---
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## Description
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This challenge emulates a UART interface on a Lain router.
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Open both connections, interact as if it was real hardware.
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- **TX**: Read only
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- **RX**: Write only
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Maybe Lain can help you?
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---
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## TL;DR
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Connect to the split UART interface. Use `settings` to get the XOR key, `dump_bin` to get
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the obfuscated firmware, de-obfuscate to extract the AES key and IV from `.rodata`, then
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use `flag` to get the ciphertext and AES-CBC decrypt it to recover the flag.
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---
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## Tools
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| Tool | Purpose |
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|------|---------|
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| `nc` | Split UART connection |
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| Python 3 + `pycryptodome` | XOR decoding and AES-CBC decryption |
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| `strings` / Ghidra | Static analysis of deobfuscated firmware |
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---
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## Solution
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### Step 1 — Connect
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```bash
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# Terminal 1 — TX (read output)
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nc <host> 1111
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# Terminal 2 — RX (send commands)
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nc <host> 2222
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```
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### Step 2 — List available commands
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```text
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help
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```
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Commands available: `help`, `flag`, `dump_bin`, `settings`, `whoami`, `show config`
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### Step 3 — Get the XOR key
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```text
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settings
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```
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Returns the XOR key used to obfuscate the firmware dump.
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### Step 4 — Dump and deobfuscate the firmware
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```text
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dump_bin
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```
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Save the hex output from TX, then deobfuscate:
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```python
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key = bytes.fromhex("<key_from_settings>")
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firmware_enc = bytes.fromhex("<dump_from_dump_bin>")
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firmware = bytes(b ^ key[i % len(key)] for i, b in enumerate(firmware_enc))
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with open("firmware.bin", "wb") as f:
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f.write(firmware)
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```
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### Step 5 — Extract AES key and IV from firmware
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Quick method:
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```bash
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strings -n 10 firmware.bin | grep -iE "key|iv|aes|lain"
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```
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Or open in Ghidra with Xtensa architecture, navigate to `app_main()` → AES setup
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functions → locate `therapy_aes_key` and associated IV in `.rodata`.
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### Step 6 — Get the encrypted flag
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```text
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flag
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```
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Returns the ciphertext as a hex string on TX.
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### Step 7 — Decrypt the flag
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```python
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from Crypto.Cipher import AES
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from Crypto.Util.Padding import unpad
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key = b"<key_from_firmware>" # 16 or 32 bytes
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iv = b"<iv_from_firmware>" # 16 bytes
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ciphertext = bytes.fromhex("<hex_from_flag_command>")
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cipher = AES.new(key, AES.MODE_CBC, iv)
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plaintext = unpad(cipher.decrypt(ciphertext), AES.block_size)
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print(plaintext.decode())
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```
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---
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## Flag
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`ECW{LAIN_Br34k_CryPT0}`
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