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+# file-encryption
+
+A simple file encryption tool using memory-mapped I/O and XOR operation.
+
+<img src = "encryption.png" />
+
+## 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
+
+```bash
+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
+
+```bash
+./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
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