A fast secure userspace pseudorandom number generator
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libottery -- drop-in secure replacement for your RNG.

This project is meant to provide a drop-in replacement for the horrible random number generator you are using today.

The RNG you are using today is probably horrible because:

  • If it's fast, it's probably very insecure. If you have ever gotten bytes from random() or rand() or mt_rand() or whatever and exposed them to the world, you have typically just let the world know what bytes you're going to be using from now to the end of time.

  • If it's secure, it's probably slow. So you probably have an insecure one that you use when you need random numbers fast.

  • If it's arc4random from an older OpenBSD (before October 2013), or any of the dozens of copies of the old arc4random() that have found their ways into other codebases, then you probably think it's secure, but it isn't, really. Even after you do the "discard the first N bytes" trick, RC4 still has detectable statistical biases. See also "What's wrong with RC4-based arcreandom() below.

    (The arc4random() from newer OpenBSDs is better, and doesn't use RC4 any more. The architecture there seems basically sound.)

This tool aims to be a speed-competitive replacement for arc4random, for your libc's random() or rand() function. It aims to be so fast that you will never need to think "do I need to use a secure random number generator here, or can I use a fast one?"

It currently uses Dan Bernstein's ChaCha stream cipher as its base. When it goes fast, it goes fast because of Ted Krovetz's implementation.


YOU WOULD HAVE TO BE SOME KIND OF LUNATIC TO USE THIS IN PRODUCTION CODE RIGHT NOW. It is so alpha that it begins the Greek alphabet. It is so alpha that Jean-Luc Godard is filming there. It is so alpha that it's 64-bit RISC from the 1990s. It's so alpha that it'll try to tell you that you belong to everyone else. It's so alpha that when you turn it sideways, it looks like an ox. It's so alpha that the Planck constant wobbles a little bit whenever I run the unit tests.

I will break backward compatibility more than once. (Probably.)

I will change something you were depending on. (Or at least, I won't promise not to.)

There might be horrible security bugs left in it. If there are, I won't be very embarrassed: I told you not to use it yet!

If it breaks, you don't get to keep both pieces! I will come over and break the pieces into even smaller pieces, then break something else that you actually liked, then point at them and laugh and laugh and laugh.


(To people without my particular sense of humor: the purpose of this section is to make you not use libottery in production code yet, because it isn't ready. If it makes you nervous about using this version of the software in production: good! That's the point.)

How to build it

make check

If that doesn't work, debug the program.

Yes, I know autotools is a pain, but I've outgrown what I'm happy doing in gmake alone. I welcome ports to other build tools, but only if they get the full functionality of the current build system.

I've tested with clang and gcc, on Linux and OSX. I've done most of the testing on Intel x86_64 chips, and a little bit on ARM. I will try other places eventually. I especially value interoperability with free platforms.

How to use it (the basics)

/* Include the main header. */
#include <ottery.h>

/* Get a random value between 0 and UINT_MAX, inclusive. */
unsigned u = ottery_rand_unsigned();

/* Get a random value between 0 and 1337, inclusive. */
unsigned u2 = ottery_rand_range(1337);

/* Get 128 random bytes. */
unsigned char buf[128];
ottery_rand_bytes(buf, sizeof(buf));

You can build your program with:

gcc -Wall -O2 -g myprog.c -o myprog -lottery

See the comments in ottery.h and ottery_st.h for more information.


A libottery PRNG seeds itself from your operating system's (hopefully) secure random number generator. This seed is used as the key and nonce for a (hopefully) strong, fast stream cipher. Every time it generates a "block" (about 1024 bytes), it uses a portion of its own output to replace its key and nonce, so that the key it used to make those bytes is unrecoverable. After it extracts bytes from the RNG, it clears them from its internal buffers. (This is based on a construction described in XXX, as summarized by DJB.)

By default, I'm trying to make libottery as safe to use as possible. Therefore, I am making it threadsafe by default, and forksafe by default. (That is, you don't need to tell it to lock itself to tolerate concurrent access, and you don't need to explicitly reinitialize it after each fork().) It also tries to make it impossible to accidentally use it unseeded. Later versions will try to resist even more common mistakes.

Getting good performance for large-sized requests is a simple matter of choosing a fast secure stream generator. Fortunately, we have a bunch of those. Right now, libottery uses Dan Bernstein's ChaCha cipher, where even a naive implementation is pretty fast, and an optimized one can be downright scary.

For small request performance (say, somebody wants to generate a bunch of random 4-byte unsigned ints), the overhead dominates. Right now, for a 4-byte request, the majority of our time goes into locking, checking whether we've forked, and making sure that the RNG is really initialized.

[1] http://csrc.nist.gov/groups/ST/toolkit//documents/rng/HashBlockCipherDRBG.pdf

Speed comparison

On my Core i7 laptop running OSX: If you're only generating a 4-byte unsigned, all the ChaCha implementations are now faster than OSX's RC4-based arc4random(), even when you're only generating 4 bytes at a time. We're about as fast than libc random() once you're generating at least 16 bytes per go, depending on how many ChaCha rounds you're using.

In single-threaded code with locks turned off (NOT RECOMMENDED!), libottery with ChaCha8 is faster than libc random() even for 8- byte requests.

At this point, I'm not sweating performance too badly. Eventually, I'll add a section about how to get better performance with compiler choices and different options, but for now, don't sweat it.

One area where I do want to do better in the future is performance in high-contention scenarios where the PRNG is getting used by many threads at once.

Digression: What's wrong with RC4-based arc4random()?

RC4 is a broken old cipher: Go read the wikipedia page. Discarding the first N bytes of its output mitigates the very worst attacks against it, but it still has detectable statistical biases.

Also, nearly any implementation of it does secret-dependent data lookups. That's a big no-no in modern cryptographic design. It can enable timing side-channels under some circumstances.

Furthermore, the way that it used RC4 provided only limited backtracking-resistance. Any attacker who managed to read the PRNG state -- via something like the Heartbleed attack, or whatever -- could reconstruct all previous updates of the PRNG back to the last time that it was reseeded. While arc4random() did try to re-seed every N bytes, N was in practice so large that compromising the arc4random() state would compromise a huge amount of secret outputs.

Least of all, RC4 not really performance-competitive any more. But if we can go faster and more secure, why wouldn't we?

The RC4 cipher might have been a good choice back in the mid-90s, when OpenBSD first added arc4random. But nowadays, we can do better, and should. (OpenBSD, for example, has switched its arc4random implementation to a construction more or less identical to the one Libottery uses.)

Digression: What's wrong with libc's random()/rand() calls?

It's probably a linear congruential generator, depending on what your operating system provides. That's completely insecure: anybody who sees one or two outputs from it can predict all past and future outputs. Or they could search the seed-space, which is not very big. Worse, it's probably slower than a well-designed modern stream cipher, so it doesn't even have an excuse for being insecure.

Digression: What's wrong with your favorite cryptography library's PRNG?

Probably nothing in terms of its security, assuming it does a reasonable job of seeding itself. But odds are good that it's pretty slow, which can make trouble for programmers: in performance critical code that eats a ton of random numbers, they're going to find themselves tempted to just use random(), because really, what could it matter?

Intellectual Property Notices

There are no known present or future claims by a copyright holder that the distribution of this software infringes the copyright. In particular, the author of the software is not making such claims and does not intend to make such claims. Additionally, Nick Mathewson has dedicated his code here to the public domain using the CC0 public domain dedication; see doc/cc0.txt or http://creativecommons.org/publicdomain/zero/1.0/ for full details.

There are no known present or future claims by a patent holder that the use of this software infringes the patent. In particular, the author of the software is not making such claims and does not intend to make such claims.

This code is in the public domain.