woody-woodpacker/rsa64/primes.c

#include "rsa.h"

uint16_t get_random_bytes(int fd) {
	uint16_t ret;
	if (read(fd, &ret, sizeof(uint16_t)) == -1) {
		exit(1);
	}
	return ret;
}

// n pow e mod m
uint64_t pow_mod(uint64_t n, uint64_t e, uint64_t m) {
	uint64_t y = 1;

	while (e > 1) {
		if (e & 1) {
			y = (y * n) % m;
		}
		n = (n * n) % m;
		e = e >> 1;
	}
	return (n * y) % m;
}

bool is_prime(uint16_t n, size_t k_max, int fd) {
	uint16_t a = get_random_bytes(fd);
	uint16_t d = n - 1;
	uint16_t s = 0;

	while ((d & 1) == 0) {
		s++;
		d = d >> 1;
	}

	for (size_t k = 0; k < k_max; k++) {
		a = 0;
		while (a < 2 || a > (n - 2)) {
			a = get_random_bytes(fd);
		}
		uint16_t x = pow_mod(a, d, n);
		uint16_t y;
		for (uint16_t i = 0; i < s; i++) {
			y = pow_mod(x, 2, n);
			if (y == 1 && x != 1 && x != n - 1)
				return false;
			x = y;
		}
		if (y != 1) {
			return false;
		}
	}
	return true;
}

uint16_t generate_prime_fd(int fd) {
	uint16_t n = get_random_bytes(fd);
	n |= 1 << 15;
	n |= 1;

	while (!is_prime(n, 16, fd)) {
		n = get_random_bytes(fd);
		n |= 1 << 15;
		n |= 1;
	}
	return n;
}

uint16_t generate_prime() {
	int fd = open("/dev/urandom", O_RDONLY);
	uint16_t n = generate_prime_fd(fd);
	close(fd);
	return n;
}