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
1.5 KiB
1.5 KiB
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
- Connect and capture the data:
nc <host> 3800 > capture.csv
Wait for --- END OF CAPTURE ---.
- Analyze the capture. Use
infocommand for metadata:
info
Shows 3 channels: ch0 (reference), ch1 (data), ch2 (noise).
-
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
-
Calculate baud rate:
- Bit period ≈ 104.17 μs → 9600 baud
- Character frame = 10 bits (1 start + 8 data + 1 stop) = ~1041.67 μs
-
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
-
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}