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
58 lines
1.5 KiB
Markdown
58 lines
1.5 KiB
Markdown
# Signal Tap Lain — Solution
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## Overview
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A logic analyzer capture is streamed with 3 channels. Channel 1 (ch1) contains
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UART data at 9600 baud, 8N1 format. The player must identify the protocol from
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signal timing and decode the ASCII message.
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## Steps
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1. Connect and capture the data:
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```bash
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nc <host> 3800 > capture.csv
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```
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Wait for `--- END OF CAPTURE ---`.
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1. Analyze the capture. Use `info` command for metadata:
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```text
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info
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```
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Shows 3 channels: ch0 (reference), ch1 (data), ch2 (noise).
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1. Focus on ch1. Look for patterns:
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- Idle state is HIGH (1)
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- Periodic falling edges = start bits
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- Measure time between start bits to find character period
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1. Calculate baud rate:
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- Bit period ≈ 104.17 μs → 9600 baud
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- Character frame = 10 bits (1 start + 8 data + 1 stop) = ~1041.67 μs
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1. Decode UART 8N1:
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- Start bit: falling edge (HIGH → LOW)
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- Sample data bits at center of each bit period (1.5 × bit_period after start)
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- 8 data bits, LSB first
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- Stop bit: HIGH
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1. Script or manually decode the ch1 data to ASCII. The message contains the flag
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repeated 3 times.
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## Key Concepts
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- **Logic analysis**: Reading digital signals and identifying protocols from timing patterns
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- **UART 8N1**: Universal Asynchronous Receiver/Transmitter — start bit, 8 data bits LSB-first, no parity, 1 stop bit
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- **Baud rate detection**: Measuring the shortest pulse width gives the bit period → baud rate
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- **Signal separation**: In a multi-channel capture, identifying which channel carries useful data
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## Flag
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`ESPILON{s1gn4l_t4p_l41n}`
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