# Signal Tap Lain

| Field | Value |
|-------|-------|
| Category | Hardware |
| Difficulty | Medium-Hard |
| Points | 500 |
| Author | Eun0us |
| CTF | Espilon 2026 |

---

## Description

A debug probe is capturing signals from Lain's NAVI.
Three channels are being recorded, but what protocol is in use?

- Signal Tap: `tcp/<host>:3800`

Capture the data, identify the protocol, and decode the message.

Format: **ESPILON{...}**

---

## TL;DR

Download a multi-channel logic analyzer CSV capture. Identify that channel 1 carries UART
at 9600 baud by measuring the bit period. Implement UART 8N1 decoding (LSB-first) and
extract the ASCII flag, which is repeated 3 times in the stream.

---

## Tools

| Tool | Purpose |
|------|---------|
| `nc` | Capture the signal tap data |
| Python 3 / numpy | Logic analysis and UART decoding |
| `info` command | Get channel metadata from the service |

---

## Solution

### Step 1 — Connect and capture

```bash
nc <host> 3800 > capture.csv
```

Wait for `--- END OF CAPTURE ---` then press Ctrl+C.

### Step 2 — Get metadata

Connect again and issue:

```text
info
```

Output:

```
Channels: 3
  ch0: reference clock (always HIGH)
  ch1: data line
  ch2: noise channel
Sample rate: 1 MHz
```

![info command output listing the three channels](https://git.espilon.net/Eun0us/ESPILON-CTF-2026-Writeups/raw/branch/main/screens/signal_tap_info.png)

### Step 3 — Analyze channel 1

Focus on ch1. The key observations:

- Idle state is **HIGH** (logic 1) — consistent with UART
- Periodic **falling edges** = start bits
- Measure the time between the start bit falling edge and the next bit transition

### Step 4 — Calculate baud rate

Measure the minimum pulse width in the ch1 column:

```
Bit period ≈ 104.17 μs
Baud rate = 1 / 0.00010417 ≈ 9600 baud
```

A 10-bit UART frame (1 start + 8 data + 1 stop) = ~1041.67 μs.

![Python script measuring bit periods from ch1 transitions](https://git.espilon.net/Eun0us/ESPILON-CTF-2026-Writeups/raw/branch/main/screens/signal_tap_measure.png)

### Step 5 — Decode UART 8N1

```python
import csv

# Load capture
with open("capture.csv") as f:
    reader = csv.DictReader(f)
    rows = list(reader)

timestamps = [float(r["timestamp_us"]) for r in rows]
ch1 = [int(r["ch1"]) for r in rows]

BIT_PERIOD = 104.17  # microseconds at 9600 baud
chars = []
i = 0
while i < len(ch1) - 1:
    # Find start bit (falling edge: HIGH → LOW)
    if ch1[i] == 1 and ch1[i+1] == 0:
        start_time = timestamps[i+1]
        # Sample 8 data bits at center of each bit period
        bits = []
        for b in range(8):
            sample_time = start_time + BIT_PERIOD * (1.5 + b)
            # Find closest sample
            j = min(range(len(timestamps)),
                    key=lambda k: abs(timestamps[k] - sample_time))
            bits.append(ch1[j])
        # LSB first
        byte = int("".join(map(str, reversed(bits))), 2)
        if 32 <= byte <= 126:
            chars.append(chr(byte))
        i += int(BIT_PERIOD * 10 / (timestamps[1] - timestamps[0]))
    else:
        i += 1

print("".join(chars))
```

The decoded message contains the flag repeated three times.

![decoded UART output showing the flag repeated](https://git.espilon.net/Eun0us/ESPILON-CTF-2026-Writeups/raw/branch/main/screens/signal_tap_decode.png)

### Key concepts

- **Logic analysis**: Reading digital waveforms and identifying protocols from timing patterns
- **UART 8N1**: Start bit (HIGH→LOW), 8 data bits LSB-first, no parity bit, 1 stop bit (HIGH)
- **Baud rate detection**: The shortest pulse width in the signal equals one bit period
- **Signal separation**: Channel 0 is a reference clock, channel 1 carries data, channel 2 is noise;
  only ch1 has meaningful transitions

---

## Flag

`ESPILON{s1gn4l_t4p_d3c0d3d}`
