ESPILON-CTF-2026-Writeups/Hardware/Signal_Tap_Lain

Signal Tap Lain — Solution

Overview

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

1. Connect and capture the data:

nc <host> 3800 > capture.csv

Wait for --- END OF CAPTURE ---.

1. Analyze the capture. Use info command for metadata:

info

Shows 3 channels: ch0 (reference), ch1 (data), ch2 (noise).

1. Focus on ch1. Look for patterns:

   • Idle state is HIGH (1)
   • Periodic falling edges = start bits
   • Measure time between start bits to find character period

2. Calculate baud rate:

   • Bit period ≈ 104.17 μs → 9600 baud
   • Character frame = 10 bits (1 start + 8 data + 1 stop) = ~1041.67 μs

3. Decode UART 8N1:

   • 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

4. Script or manually decode the ch1 data to ASCII. The message contains the flag repeated 3 times.

Key Concepts

• 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

Flag

ESPILON{s1gn4l_t4p_l41n}