file-encryption

A simple file encryption tool using memory-mapped I/O and XOR operation.

Description

file-encryption modifies a binary file in-place by applying a bitwise XOR operation using a user-provided numeric key. It efficiently processes large files in memory page-sized blocks using mmap, avoiding explicit read/write loops.

This method can be used for lightweight encryption and decryption, as XOR is symmetric: applying the same key twice restores the original file.

Features

Fast in-place encryption using memory mapping (mmap)
Page-aligned memory processing for better performance
Supports partial blocks at the end of the file (byte-accurate)
Symmetric encryption: same operation encrypts and decrypts
No third-party dependencies, written in pure C

Usage

git clone https://git.scratko.xyz/file-encryption
cd file-encryption
gcc -Wall encryption.c -o encryption
./encryption <filename> <key>

<filename> — path to the target file <key> — numeric key used for encryption (unsigned integer)

Example

./encryption secret.bin 12345

This will XOR every 4-byte word of secret.bin with 12345. Running the same command again will restore the original content.

How it works

The file is opened with O_RDWR and memory-mapped using mmap().
The numeric key is bitwise-inverted: ~key.
The file is processed block-by-block in chunks of 4096 bytes (aligned to system page size).
For each 4-byte block in the mapped memory, a XOR with the key is applied.
If the file ends with fewer than 4 bytes remaining, only those bytes are XORed.

Limitations

Works only on POSIX-compatible systems (Linux, macOS)
Assumes int is 4 bytes and little-endian architecture
The file is modified in-place — no backups or safety checks