interview

home / developersection / interviews / what is a file signature?

Ask Question
ICSM Computer

IT-Hardware & Networking

Ravi Vishwakarma is a dedicated Software Developer with a passion for crafting efficient and innovative solutions. With a keen eye for detail and years of experience, he excels in developing robust software systems that meet client needs. His expertise spans across multiple programming languages and technologies, making him a valuable asset in any software development project.

Follow
Can you answer this question?
Answer

1 Answers

ICSM Computer

ICSM Computer

14-May-2025

A file signature (also called a magic number) is a unique sequence of bytes at the beginning of a file that identifies its file type — regardless of the file extension.

These signatures help operating systems and applications understand the true format of a file, even if the extension is incorrect or missing.

Examples of Common File Signatures

File Type Hex Signature (first few bytes) ASCII Equivalent
PDF 25 50 44 46 %PDF
PNG 89 50 4E 47 0D 0A 1A 0A PNG (non-ASCII)
JPG/JPEG FF D8 FF (binary)
GIF 47 49 46 38 GIF8
ZIP 50 4B 03 04 PK..
DOCX/XLSX 50 4B 03 04 (because they are ZIP) PK..

Note: Some file types share the same signature (e.g., .zip, .docx, .xlsx) because they are all ZIP-based and need deeper inspection.

Why Use File Signature?

  1. More secure than trusting file extension (which can be renamed).
  2. Helps avoid spoofed file uploads (e.g., virus.exe.jpg).

How to Detect File Type from Signature in C#

Step 1: Build a signature map

var knownSignatures = new Dictionary<string, byte[]>
{
    { ".pdf", new byte[] { 0x25, 0x50, 0x44, 0x46 } },          // %PDF
    { ".png", new byte[] { 0x89, 0x50, 0x4E, 0x47 } },          // PNG
    { ".jpg", new byte[] { 0xFF, 0xD8, 0xFF } },                // JPG
    { ".gif", new byte[] { 0x47, 0x49, 0x46, 0x38 } },          // GIF8
    { ".zip", new byte[] { 0x50, 0x4B, 0x03, 0x04 } },          // PK..
};

Step 2: Detect file extension by matching signature

string DetectFileExtension(string filePath)
{
    byte[] buffer = new byte[8];

    using (var fs = File.OpenRead(filePath))
    {
        fs.Read(buffer, 0, buffer.Length);
    }

    foreach (var pair in knownSignatures)
    {
        var sig = pair.Value;
        if (buffer.Take(sig.Length).SequenceEqual(sig))
            return pair.Key;
    }

    return "unknown";
}

Usage:

string ext = DetectFileExtension("upload.dat");
Console.WriteLine($"Detected file type: {ext}");

Limitations

  1. Some formats share the same signature (e.g., .zip, .docx, .xlsx).
  2. To distinguish them, you'll need to unzip and inspect internal content types.
advertising
advertising

Liked By

We use cookies to ensure you have the best browsing experience on our website. By using our site, you acknowledge that you have read and understood our Cookie Policy & Privacy Policy