Table of Contents

hexdump in CTF: How to Inspect Binary Files and Extract Hidden Data

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✅ What is hexdump?

hexdump is a command-line utility that displays the binary contents of a file in hexadecimal format, often alongside ASCII representations.

In CTF challenges, hexdump is commonly used to:

Inspect unknown binaries
Examine file headers and magic numbers
Detect hidden flags or embedded data
Analyze memory or disk dumps

It’s a fast and lightweight tool for low-level forensic analysis.

🛠️ Basic hexdump Commands

1. Display a file in hex

hexdump file.bin

Outputs the file content in hexadecimal format.

2. Canonical format (hex + ASCII)

hexdump -C file.bin

Shows both hexadecimal bytes and their ASCII characters, making it easier to spot text and flags.

3. Specify bytes per line

hexdump -v -e '16/1 "%02X " "\n"' file.bin

Customizes the output format (16 bytes per line in hex).

4. Inspect specific offsets

hexdump -s 1024 -C file.bin

Starts reading from offset 1024 bytes into the file. Useful for skipping headers or irrelevant sections.

5. Output to a file

hexdump -C file.bin > output.txt

Saves the hex view for later inspection or searching.

🎯 How hexdump is Used in CTF Challenges

1. Analyze file headers

Identify file types via magic numbers (e.g., 89 50 4E 47 for PNG, 50 4B 03 04 for ZIP).
Useful for corrupted or disguised files.

2. Extract hidden flags

Flags may be embedded in non-printable bytes or appended to a file.
hexdump allows you to view and extract such data manually.

3. Examine binaries

Inspect compiled programs to find strings, offsets, or instruction sequences.
Helps in reverse-engineering challenges.

4. Analyze memory or disk dumps

hexdump is excellent for scanning RAM or disk images for hidden data fragments or ASCII sequences.

5. Detect steganography or padding

Patterns like repeated bytes, unusual null padding, or LSB encoding can be detected visually in hex.

🔍 Common CTF Patterns with Hexdump

Pattern	Description	How Hexdump Helps
Hidden flags	Appended or embedded text in binary	Displays ASCII for quick spotting
Corrupted files	PNG, ZIP, or ELF with altered headers	Identify and fix magic numbers manually
Custom binaries	Reverse-engineering tasks	Examine offsets and instructions
Memory fragments	ASCII or hex data in RAM dumps	Locate secret data quickly
Steganography	Subtle LSB or null byte patterns	Visual inspection reveals anomalies
Encoded data	Base64 or XORed content	Spot hints in ASCII regions for decoding

📌 Tips for Using hexdump in CTFs

Combine hexdump -C with grep to search for flag patterns.
Start by inspecting magic numbers to identify file types.
Use offset options (-s) for large files to skip irrelevant sections.
Save output to a file for offline analysis or to try decoding later.
hexdump is complementary to tools like strings, binwalk, and hexedit.

🧩 Recommended Workflow in CTF

Run hexdump -C file.bin to visualize the file content.
Inspect the header for magic numbers or anomalies.
Scan through ASCII portions for potential flags.
Examine specific offsets if the flag is appended or embedded.
Combine with other tools (strings, binwalk, zsteg) for multi-layer challenges.

hexdump