ESPILON-CTF-2026-Writeups/ESP/ESP_Start

ESP Start

Field	Value
Category	ESP
Difficulty	Easy
Points	50
Author	Eun0us
CTF	Espilon 2026

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Description

Welcome to the ESP category.

Flash this firmware onto an ESP32 and connect to the UART console at 115200 baud.

The device outputs something on boot. Decode it.

Format: ESPILON{flag}

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TL;DR

Flash the provided firmware, read the XOR-encrypted flag and its key from the UART boot output, apply XOR, get the flag.

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Tools

Tool	Purpose
esptool.py	Flash firmware to ESP32
screen / minicom	Read UART serial output
Python 3	XOR decryption

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Solution

Step 1 — Flash the firmware

Install esptool and flash all three binary files at their correct offsets:

pip install esptool

esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 460800 write_flash -z \
    0x1000  bootloader.bin \
    0x8000  partition-table.bin \
    0x10000 hello-espilon.bin

On Linux, add your user to the dialout group first if you get a permission error:

sudo usermod -a -G dialout $USER

📸 [screenshot: esptool.py flashing — progress bar reaching 100%]

Step 2 — Connect to the UART console

Open a serial terminal at 115200 baud:

screen /dev/ttyUSB0 115200
# or
minicom -D /dev/ttyUSB0 -b 115200

Press the RESET button on the ESP32. The boot output appears:

=== Hello ESP ===
System ready.

Encrypted flag: 09 12 19 07 00 0E 07 35 3F 35 7D 3C 38 1E 3D 26 7F 1E 3E 7F 3E 72 34
XOR Key: 4C 41 49 4E

📸 [screenshot: serial terminal showing the encrypted flag and XOR key on boot]

Step 3 — Identify the XOR key

Convert the key bytes 4C 41 49 4E to ASCII:

0x4C = 'L'
0x41 = 'A'
0x49 = 'I'
0x4E = 'N'

Key = LAIN (the protagonist of the series).

Step 4 — Decrypt the flag

Apply the key cyclically to each byte of the encrypted output:

enc = bytes([
    0x09, 0x12, 0x19, 0x07, 0x00, 0x0E, 0x07, 0x35,
    0x3F, 0x35, 0x7D, 0x3C, 0x38, 0x1E, 0x3D, 0x26,
    0x7F, 0x1E, 0x3E, 0x7F, 0x3E, 0x72, 0x34
])
key = b"LAIN"
flag = bytes(b ^ key[i % len(key)] for i, b in enumerate(enc))
print(flag.decode())

Output: ESPILON{st4rt_th3_w1r3}

📸 [screenshot: Python decryption script running and printing the flag]

Key concepts

• ESP32 flashing: esptool.py writes bootloader, partition table, and application binary at their respective flash offsets (0x1000, 0x8000, 0x10000).
• UART monitoring: ESP32 default baud rate is 115200 baud, 8N1, no parity.
• XOR cipher: The key LAIN is broadcast in plaintext — trivial for an intro challenge; the real-world lesson is that hardcoded keys in .rodata are easily extracted with strings.

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Flag

ESPILON{st4rt_th3_w1r3}