Writeup: Hello-ESP - EspilonCTF (ESP)

Challenge Overview

Flag

ESPILON{st4rt_th3_w1r3}

Files Provided

Solution Walkthrough

Step 1: Initial Recon

Extracted HELLO-ESP.zip to obtain three .bin files and a README with flashing instructions. Running strings on hello-espilon.bin revealed key output messages:

Welcome to ESPILON ctf new category ESP have fun!
=== Hello ESP ===
System ready.
Encrypted flag:
XOR Key:

This indicated the firmware prints an XOR-encrypted flag at runtime.

Step 2: Binary Structure Analysis

Parsed the ESP32 application image header (magic byte 0xE9):

Step 3: Locating the Encrypted Data

Found the format strings in DROM (Segment 0) at file offset 0x30E0. Searched for 32-bit cross-references to these string addresses in the code literal pool:

Step 4: Extracting Key and Ciphertext

XOR Key at file offset 0x756C:

4C 41 49 4E  ->  "LAIN"

Encrypted flag at file offset 0x7570 (23 bytes):

09 12 19 07 00 0E 07 35 3F 35 7D 3C 38 1E 3D 26 7F 1E 3E 7F 3E 72 34

Step 5: Decryption

Applied repeating-key XOR with key "LAIN":

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([enc[i] ^ key[i % 4] for i in range(len(enc))])
# Result: ESPILON{st4rt_th3_w1r3}

Tools Used

• Python 3 (struct, byte manipulation)
• strings, xxd (initial recon)
• Manual ESP32 image header parsing

Key Takeaways

• ESP32 firmware images use magic byte 0xE9 and contain multiple segments (DROM, DRAM, IRAM, IROM)
• Xtensa architecture uses L32R literal pools for loading constants, making cross-reference tracing straightforward
• No physical hardware or flashing was needed; static analysis of the binary was sufficient to recover the flag