Heapless RSA

A performant single-file, heap-less implementation of RSA encryption and signatures in Zig with OAEP+ padding and PEM key storage.

Change compile time variables in rsa.zig to use different modulus sizes

Features

• Zero heap allocation: Uses only stack buffers and compile-time constants
• Montgomery multiplication for efficient modular exponentiation
• PEM format key storage and loading
• RSA encryption/decryption with OAEP+ padding
• Digital signatures with SHA-256 hashing
• Miller-Rabin primality testing for secure key generation
• Configurable modulus size via compile-time constants

API Reference:

Key Management

pub fn gen_keys() Keys
pub fn save_private_key(keys: Keys, filename: []const u8) !void
pub fn save_public_key(keys: Keys, filename: []const u8) !void
pub fn load_private_key(filename: []const u8) !Keys
pub fn load_public_key(filename: []const u8) !Keys

Encryption/Decryption

pub fn encrypt(message: []const u8, public_key: PublicKey) !BitsType
pub fn decrypt(blob: BitsType, keys: Keys) ![MAX_MESSAGE_LEN]u8

Signatures

pub fn sign(message: []const u8, keys: Keys) BitsType
pub fn verify(message: []const u8, signature: BitsType, public_key: PublicKey) bool

Usage Example

const std = @import("std");
const rsa = @import("rsa.zig");

pub fn main() !void {
    const keys = rsa.gen_keys();
    
    try rsa.save_private_key(keys, "private.pem");
    try rsa.save_public_key(keys, "public.pem");
    
    const message = "Hello, RSA!";
    const ciphertext = try rsa.encrypt(message, keys.public_key);
    
    const decrypted = try rsa.decrypt(ciphertext, keys);
    
    const signature = rsa.sign(message, keys);
    
    const is_valid = rsa.verify(message, signature, keys.public_key);
    
    std.debug.print("Message: {s}\n", .{message});
    std.debug.print("Decrypted: {s}\n", .{decrypted[0..message.len]});
    std.debug.print("Signature valid: {}\n", .{is_valid});
}

Testing:

Run the test script:

zig run test.zig

Warning

This implementation has not been professionally auditted. Use at your own risk