shitty_streamcipher.c

/* License: CC0-1.0 */
/* Usage: shitty_streamcipher <key> < plaintext > ciphertext */
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>



const uint64_t RAND_A = 6364136223846793005;
const uint64_t RAND_C = 1442695040888963407;

// Linear congruential generator
uint64_t rand_next(uint64_t old)
{
    return old * RAND_A + RAND_C;
}

int main(int argc, char **argv)
{
    if (argc < 2) {
        fprintf(stderr, "Missing key argument\n");
        return 1;
    }

    // Seed random generator from first argument
    uint64_t rand_state = strtol(argv[1], NULL, 0);
    if (errno == ERANGE) {
        fprintf(stderr, "Key value is out of range\n");
        return 1;
    }
    if (rand_state == 0) {
        fprintf(stderr, "Failed to convert key\n");
        return 1;
    }

    char *rand_bytes = (char*)&rand_state;
    size_t rand_pos = 0;
    rand_state = rand_next(rand_state);

    int c_in = 0;
    while ((c_in = getchar()) != EOF) {
        int key = rand_bytes[rand_pos++];
        int c_out = c_in ^ key;
        putchar(c_out);
        if (rand_state >= sizeof(rand_state)) {
            rand_state = rand_next(rand_state);
            rand_pos = 0;
        }
    }

    return 0;
}