write-up: Hardware/Signal_Tap_Lain/README.md

Hardware/Signal_Tap_Lain/README.md

@@ -1,57 +1,150 @@
-# Signal Tap Lain — Solution
+# Signal Tap Lain
 
-## Overview
+| Field | Value |
+|-------|-------|
+| Category | Hardware |
+| Difficulty | Medium-Hard |
+| Points | 500 |
+| Author | Eun0us |
+| CTF | Espilon 2026 |
 
-A logic analyzer capture is streamed with 3 channels. Channel 1 (ch1) contains
-UART data at 9600 baud, 8N1 format. The player must identify the protocol from
-signal timing and decode the ASCII message.
+---
 
-## Steps
+## Description
 
-1. Connect and capture the data:
+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 ---`.
+Wait for `--- END OF CAPTURE ---` then press Ctrl+C.
 
-1. Analyze the capture. Use `info` command for metadata:
+### Step 2 — Get metadata
+
+Connect again and issue:
 
 ```text
 info
 ```
 
-Shows 3 channels: ch0 (reference), ch1 (data), ch2 (noise).
+Output:
 
-1. Focus on ch1. Look for patterns:
+```
+Channels: 3
+  ch0: reference clock (always HIGH)
+  ch1: data line
+  ch2: noise channel
+Sample rate: 1 MHz
+```
 
-   - Idle state is HIGH (1)
-   - Periodic falling edges = start bits
-   - Measure time between start bits to find character period
+> 📸 `[screenshot: info command output listing the three channels]`
 
-1. Calculate baud rate:
+### Step 3 — Analyze channel 1
 
-   - Bit period ≈ 104.17 μs → 9600 baud
-   - Character frame = 10 bits (1 start + 8 data + 1 stop) = ~1041.67 μs
+Focus on ch1. The key observations:
 
-1. Decode UART 8N1:
+- 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
 
-   - Start bit: falling edge (HIGH → LOW)
-   - Sample data bits at center of each bit period (1.5 × bit_period after start)
-   - 8 data bits, LSB first
-   - Stop bit: HIGH
+### Step 4 — Calculate baud rate
 
-1. Script or manually decode the ch1 data to ASCII. The message contains the flag
-   repeated 3 times.
+Measure the minimum pulse width in the ch1 column:
 
-## Key Concepts
+```
+Bit period ≈ 104.17 μs
+Baud rate = 1 / 0.00010417 ≈ 9600 baud
+```
 
-- **Logic analysis**: Reading digital signals and identifying protocols from timing patterns
-- **UART 8N1**: Universal Asynchronous Receiver/Transmitter — start bit, 8 data bits LSB-first, no parity, 1 stop bit
-- **Baud rate detection**: Measuring the shortest pulse width gives the bit period → baud rate
-- **Signal separation**: In a multi-channel capture, identifying which channel carries useful data
+A 10-bit UART frame (1 start + 8 data + 1 stop) = ~1041.67 μs.
+
+> 📸 `[screenshot: Python script measuring bit periods from ch1 transitions]`
+
+### 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.
+
+> 📸 `[screenshot: decoded UART output showing the flag repeated]`
+
+### 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_l41n}`
+`ESPILON{s1gn4l_t4p_d3c0d3d}`