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#ifdef NDEBUG
#error Asserts must work here
#endif
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <stdint.h>
#include <assert.h>
// This gives log2(64)*24 = 6*24 = 144 bits of entropy.
#define ALPHABET_SIZE 64
#define KEY_LENGTH 24
__attribute__((noreturn))
static void die(const char *msg) {
fprintf(stderr, "%s\n", msg);
exit(1);
}
//////////////////// RANDOM GENERATOR ////////////////////
struct randgen {
FILE *urandom;
uint8_t buffer[256];
size_t cursor;
};
static void randgen_refresh(struct randgen *gen) {
size_t nr = fread(gen->buffer, 1, sizeof gen->buffer, gen->urandom);
if (nr < sizeof gen->buffer) die("Cannot read from /dev/urandom");
gen->cursor = 0;
}
static struct randgen randgen_init(void) {
FILE *f = fopen("/dev/urandom", "r");
if (!f) die("Cannot open /dev/urandom");
struct randgen gen;
gen.urandom = f;
randgen_refresh(&gen);
return gen;
}
static uint8_t randgen_gen_byte(struct randgen *gen) {
if (gen->cursor == sizeof gen->buffer) randgen_refresh(gen);
return gen->buffer[gen->cursor++];
}
static uint64_t randgen_gen(struct randgen *gen, uint64_t below) {
if (below == 0) return 0;
int nbytes = 0;
while (nbytes < 8 && (1ULL << (8 * nbytes)) < below) nbytes++;
// This check allows us to do fearless modulo in the main loop
if (nbytes == 8) assert(false && "'below' too large");
const uint64_t gen_limit = 1ULL << (8 * nbytes);
while (true) {
uint64_t sample = 0;
for (int i = 0; i < nbytes; i++) {
sample |= (uint64_t)randgen_gen_byte(gen) << (8 * i);
}
if (sample < gen_limit - gen_limit % below) {
return sample % below;
}
}
}
//////////////////// MAIN FUNCTIONALITY ////////////////////
static char gen_key_char(struct randgen *gen) {
// 64 characters, so 6 bits
static const char alphabet[] =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_";
assert(strlen(alphabet) == ALPHABET_SIZE);
return alphabet[randgen_gen(gen, strlen(alphabet))];
}
int main() {
struct randgen gen = randgen_init();
char key[KEY_LENGTH + 1];
for (size_t i = 0; i < KEY_LENGTH; i++) {
key[i] = gen_key_char(&gen);
}
key[KEY_LENGTH] = '\0';
printf("%s\n", key);
}
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