crypto_prng_buf
crypto_prng_buf(&prng, output, 1);
crypto_prng_buf(&prng, output + 1, 30);
crypto_prng_buf(&prng, output + 31, 1);
crypto_prng_buf(&prng->arc4_prng, buf, sizeof buf);
crypto_prng_buf(&prng->arc4_prng, &v, sizeof v);
crypto_prng_buf(&prng->arc4_prng, buf, len);
crypto_prng_buf(&prng->arc4_prng, seed, sizeof seed);
do crypto_prng_buf(&prng->arc4_prng, &r, sizeof r);