🔐 LSB Steganography Tool

Hide secret files inside BMP images using Least Significant Bit (LSB) manipulation — written in pure C.

---

📌 What is LSB Steganography?

Steganography is the art of hiding secret information inside ordinary files without anyone noticing.

This tool embeds any secret file (.txt, .c, .csv, etc.) into a .bmp image by replacing the Least Significant Bit of each pixel byte with bits from the secret data. The resulting image looks visually identical to the original — but carries hidden information inside it.

Original BMP  +  Secret File  →  Stego BMP  (looks identical, carries secret)

---

🗂️ Project Structure

steganography-tool/
├── main.c          # Entry point — argument parsing, operation routing
├── encode.c        # Encoding logic — hides secret file into BMP
├── encode.h        # Encoding function declarations & EncodeInfo struct
├── decode.c        # Decoding logic — extracts secret file from BMP
├── decode.h        # Decoding function declarations & DecodeInfo struct
├── common.h        # Shared constants (MAGIC_STRING)
├── types.h         # Custom types: Status, OperationType, uint
├── Makefile        # Build automation
└── beautiful.bmp   # Sample source image for testing

---

⚙️ How It Works

Encoding Flow

Source BMP  →  Copy 54-byte BMP Header
            →  Encode Magic String  (#*)
            →  Encode Extension Size  (32 bits)
            →  Encode File Extension  (.txt etc.)
            →  Encode File Size  (32 bits)
            →  Encode File Data  (LSB of each pixel byte)
            →  Copy Remaining Image Data
            →  Output: Stego BMP

Decoding Flow

Stego BMP  →  Verify Magic String  (confirm stegged image)
           →  Decode Extension Size
           →  Decode File Extension
           →  Decode File Size
           →  Decode File Data
           →  Output: Recovered Secret File

LSB Technique

Each 1 byte of secret data is spread across 8 bytes of image data — 1 bit per pixel byte (LSB only). This causes a maximum pixel value change of ±1, making it visually undetectable.

// Encoding: set LSB of image byte to bit i of secret byte
image_buffer[i] = (image_buffer[i] & 0xFE) | ((data >> i) & 1);

// Decoding: extract LSB from each image byte, reconstruct secret byte
ch |= (image_buffer[i] & 1) << i;

---

🚀 Getting Started

Prerequisites

• GCC compiler
• Linux / Unix environment
• A .bmp source image

Build

git clone https://github.com/santhoshpandiansg/steganography-tool.git
cd steganography-tool
make

Or manually:

gcc main.c encode.c decode.c -Wall -o stego

---

💻 Usage

Encode — Hide a secret file in an image

./stego -e <source.bmp> <secret_file> [output.bmp]

Example:

./stego -e beautiful.bmp secret.txt stego.bmp

Output:

[*] Operation  : ENCODE
[*] Source BMP : beautiful.bmp
[*] Secret File: secret.txt
[*] Output BMP : stego.bmp

  [1/7] Files opened successfully
  [2/7] Capacity check passed
  [3/7] BMP header copied
  [4/7] Magic string encoded
  [5/7] Extension size encoded
  [6/7] Extension encoded: .txt
  [7/7] Secret data encoded (22 bytes)

[+] Encoding Successful!
[+] Stego image saved as: stego.bmp

---

Decode — Extract hidden file from stego image

./stego -d <stego.bmp> [output_filename]

Example:

./stego -d stego.bmp recovered

Output:

[*] Operation   : DECODE
[*] Stego Image : stego.bmp

  [1/5] Stego image opened
  [2/5] Magic string verified
  [3/5] File extension decoded: .txt
  [4/5] File size decoded: 22 bytes
  [5/5] Secret data extracted successfully

[+] Decoding Successful!
[+] Secret file saved as: recovered.txt

---

🔍 Key Concepts Demonstrated

Concept	Where Used
Bitwise operations (&, |, >>)	LSB encode/decode
Binary file I/O (fread, fwrite, rb, wb)	All file operations
Struct-based design	EncodeInfo, DecodeInfo
Modular C programming	Separate encode/decode modules
Memory management (malloc, free)	Secret data buffer
BMP file format parsing	Header copy, pixel data access
Magic string validation	Tamper detection
Error handling with enums	Status, OperationType

---

📐 Capacity Formula

Max secret size (bytes) = (Image pixel bytes - overhead) / 8

Overhead = (MAGIC_STRING + extension_size(4) + extension + file_size(4)) * 8 bits

For a 800×600 BMP: 800 × 600 × 3 = 1,440,000 bytes → can hide up to ~179,000 bytes (~175 KB) of secret data.

---

⚠️ Limitations

• Only supports 24-bit uncompressed BMP images as cover files
• Secret file must fit within image capacity (tool checks this automatically)
• Not intended for security-critical use — LSB is a basic steganography technique

---

👨‍💻 Author

Santhosh Pandian SG Embedded Systems Engineer | Emertxe ECEP Trainee

---

📄 License

This project is licensed under the MIT License.