[Cryptography] OpenSSL and random

[Theodore Ts'o]

On Thu, Dec 01, 2016 at 09:47:02PM -0600, Nico Williams wrote:
> On Thu, Dec 01, 2016 at 09:42:59PM -0500, Theodore Ts'o wrote:
> > On Thu, Dec 01, 2016 at 06:39:46PM -0600, Nico Williams wrote:
> > > It might be nice to be get an indication of entropy quality from the OS.
> > > At minimum a boolean (true -> real entropy, false -> meh entropy).
> > > _Perhaps_ also an indication of when was the last time new entropy was
> > > stirred in.  (Anything more would be overkill and hard to use well.)
> > 
> > We do give such a an indication.   For example:
> > 
> > random: systemd: uninitialized urandom read (16 bytes read, 3 bits of entropy available)
> 
> I meant: in the API.  A dmesg does the app no good.
> 
> Python could use lame rng seeds for hash table randomization, note the
> lameness, and reseed later when cryptographically-secure an rng is
> needed.

The application can do this already using the existing API's.

int get_pseudorandom_hash(char *buf, int buflen)
{
	int fd, ret;

	ret = getrandom(buf, buflen, GRND_NONBLOCK);
	if (!ret)
		return 0;
	if (errno != EAGAIN)
		return -1;
	fd = open("/dev/urandom", O_RDONLY);
	if (fd < 0)
		return -1;
	while (buflen > 0) {
		ret = read(fd, buf, buflen);
		if (ret < 0)
			return -1;
		buf += ret;
		buflen -= ret;
	}
	close(fd);
	return 0;
}

Cheers,

					- Ted