6a0877384d
- Remove undeployed challenges: Phantom_Byte, Cr4cK_w1f1, Lain_Br34kC0r3 V1, Lain_VS_Knights, Lets_All_Love_UART, AETHER_NET, Last_Train_451, Web3/ - Sync 24 solve/ files from main CTF-Espilon repo - Update all READMEs with real CTFd final scores at freeze - Add git-header.png banner - Rewrite README: scoreboard top 10, edition stats (1410 users, 264 boards, 1344 solves), correct freeze date March 26 2026
177 lines
4.9 KiB
Markdown
177 lines
4.9 KiB
Markdown
# Accela Signal
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| Field | Value |
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|-------|-------|
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| Category | Misc |
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| Difficulty | Hard |
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| Points | 536 |
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| Author | Eun0us |
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| CTF | Espilon 2026 |
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---
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## Description
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During the KIDS experiment, Tachibana Labs deployed a covert LoRa-like mesh network
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throughout the hospital infrastructure. Code-named "Accela Signal", these transmissions
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use Chirp Spread Spectrum modulation to carry data fragments below the detection threshold
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of conventional receivers.
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Your NAVI has intercepted the baseband IQ stream from one of these nodes. The signal
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contains chirp-modulated frames with encoded data. Analyze the modulation, demodulate
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the chirps, decode the protocol, and extract the hidden message.
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Hint: The stream banner tells you the basics. The rest is signal processing.
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- IQ Stream: `tcp/<host>:9002`
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Format: **ESPILON{flag}**
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---
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## TL;DR
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Capture an 8 kSps int16 LE IQ stream. Identify Chirp Spread Spectrum (LoRa-like) modulation
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(N=128, SF=7). Implement dechirp + FFT demodulation. Detect frames by preamble (8x symbol-0
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chirps) and sync (2x downchirps). Gray-decode symbols. Unpack 7-bit symbols to bytes. Find
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the data frame (type=0x02), XOR with key `L41N`, decode 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` | Capture IQ stream |
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| Python 3 + numpy + scipy | CSS demodulation pipeline |
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| `inspectrum` / matplotlib | Visual spectrogram analysis |
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---
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## Solution
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### Step 1 — Capture and read the banner
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```bash
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nc <HOST> 9002 > capture.raw
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```
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First bytes are a text banner before the binary IQ data:
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```
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IQ baseband, 8000 sps, int16 LE interleaved
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Chirp Spread Spectrum detected. N=128.
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```
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### Step 2 — Analyze the spectrogram
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Load the IQ data in inspectrum or plot with matplotlib. You see:
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- Characteristic **chirps**: frequency sweeps from low to high
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- Repeating groups of identical chirps (preamble)
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- Occasional downchirps (sync)
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This is **Chirp Spread Spectrum (CSS)**, identical in principle to LoRa.
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### Step 3 — Determine parameters
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- Each chirp spans 128 samples → **N = 128**
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- `N = 2^SF` → **SF = 7** (spreading factor)
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- Each symbol encodes 7 bits → 128 possible symbols
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- Baseband sample rate = 8000 Hz
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### Step 4 — Implement dechirping
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```python
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import numpy as np
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# Read raw int16 LE interleaved IQ
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raw = np.fromfile("capture.raw", dtype="<i2").astype(float) / 32768.0
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iq = raw[0::2] + 1j * raw[1::2]
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N = 128
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# Base upchirp (symbol 0): exp(j * pi * n^2 / N)
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n = np.arange(N)
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upchirp0 = np.exp(1j * np.pi * n**2 / N)
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downchirp = np.conj(upchirp0)
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def decode_symbol(chirp_samples):
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"""Dechirp then find FFT peak = symbol value"""
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dechirped = chirp_samples * np.conj(upchirp0)
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spectrum = np.abs(np.fft.fft(dechirped))
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return int(np.argmax(spectrum))
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```
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### Step 5 — Detect frames
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Frame structure:
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```
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[Preamble: 8x symbol 0] [Sync: 2x downchirp] [Header: 1 symbol = length] [Payload: L symbols]
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```
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Scan the IQ stream for runs of 8 consecutive symbols decoding to 0.
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### Step 6 — Gray decode and symbol-to-byte unpacking
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```python
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def gray_decode(val):
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mask = val
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while mask:
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mask >>= 1
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val ^= mask
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return val
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def symbols_to_bytes(symbols):
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"""Pack 7-bit symbols (SF=7) into 8-bit bytes"""
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bits = ""
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for s in symbols:
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bits += f"{s:07b}"
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return bytes(int(bits[i:i+8], 2) for i in range(0, len(bits) - 7, 8))
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```
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### Step 7 — Parse frame payload and decrypt
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Payload format: `[type:1] [data:L] [crc16:2]`
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- Type `0x01` = beacon (cleartext ASCII, for verification)
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- Type `0x02` = data frame (XOR-encrypted flag)
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```python
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import crcmod
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crc16_fn = crcmod.predefined.mkCrcFun('crc-ccitt-false')
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for frame_type, data, crc in decoded_frames:
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if crc16_fn(bytes([frame_type]) + data) != crc:
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continue # invalid frame
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if frame_type == 0x02:
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key = b"L41N"
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flag = bytes(b ^ key[i % 4] for i, b in enumerate(data))
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print(flag.rstrip(b'\x00').decode())
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```
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### Key insights
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- CSS encodes data as a cyclic frequency shift of a chirp signal
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- The dechirp + FFT converts frequency offset into a bin index (symbol value)
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- Gray coding ensures adjacent symbols differ by 1 bit, reducing BER on noisy channels
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- The beacon frame (type 0x01) provides a known-plaintext verification step
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- The stream banner hint `"N=128"` directly gives the spreading factor
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---
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## Flag
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`ESPILON{4cc3l4_ch1rp_spr34d_w1r3d}`
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