core-lightning/common/pseudorand.c

#include "config.h"
#include <assert.h>
#include <ccan/crypto/sha256/sha256.h>
#include <ccan/crypto/siphash24/siphash24.h>
#include <ccan/isaac/isaac64.h>
#include <ccan/likely/likely.h>
#include <ccan/tal/tal.h>
#include <common/pseudorand.h>
#include <sodium/randombytes.h>
#include <string.h>

static struct isaac64_ctx isaac64;
static struct siphash_seed siphashseed;
static bool pseudorand_initted = false;

static void init_if_needed(void)
{
	if (unlikely(!pseudorand_initted)) {
		unsigned char seedbuf[16];
		struct sha256 sha;

		randombytes_buf(seedbuf, sizeof(seedbuf));
		memcpy(&siphashseed, seedbuf, sizeof(siphashseed));

		/* In case isaac is reversible, don't leak seed. */
		sha256(&sha, seedbuf, sizeof(seedbuf));
		isaac64_init(&isaac64, sha.u.u8, sizeof(sha.u.u8));
		pseudorand_initted = true;
	}
}

uint64_t pseudorand(uint64_t max)
{
	init_if_needed();

	assert(max);
	return isaac64_next_uint(&isaac64, max);
}

uint64_t pseudorand_u64(void)
{
	init_if_needed();

	return isaac64_next_uint64(&isaac64);
}

double pseudorand_double(void)
{
	init_if_needed();

	return isaac64_next_double(&isaac64);
}

const struct siphash_seed *siphash_seed(void)
{
	init_if_needed();

	return &siphashseed;
}


void tal_arr_randomize_(void *arr, size_t elemsize)
{
	/* Easier arith. */
	char *carr = arr;
	size_t n = tal_bytelen(arr) / elemsize;

	assert(tal_bytelen(arr) % elemsize == 0);

	/* From Wikipedia's Fischer-Yates shuffle article:
	 *
	 * for i from 0 to n−2 do
	 *      j ← random integer such that i ≤ j < n
	 *      exchange a[i] and a[j]
	 */
	if (n < 2)
		return;

	for (size_t i = 0; i < n - 1; i++) {
		size_t j = i + pseudorand(n - i);
		char tmp[elemsize];

		memcpy(tmp, carr + i * elemsize, elemsize);
		memcpy(carr + i * elemsize, carr + j * elemsize, elemsize);
		memcpy(carr + j * elemsize, tmp, elemsize);
	}
}