Re: [RFC][PATCH 0/6] /dev/random - a new approach

[Stephan Mueller <smueller@chronox.de>]

Am Donnerstag, 21. April 2016, 15:03:37 schrieb Nikos Mavrogiannopoulos:

Hi Nikos,

> On Thu, Apr 21, 2016 at 11:11 AM, Stephan Mueller <smueller@chronox.de> 
wrote:
> > Hi Herbert, Ted,
> > 
> > The venerable Linux /dev/random served users of cryptographic mechanisms
> > well for a long time. Its behavior is well understood to deliver entropic
> > data. In the last years, however, the Linux /dev/random showed signs of
> > age where it has challenges to cope with modern computing environments
> > ranging from tiny embedded systems, over new hardware resources such as
> > SSDs, up to massive parallel systems as well as virtualized environments.
> > 
> > With the experience gained during numerous studies of /dev/random, entropy
> > assessments of different noise source designs and assessing entropy
> > behavior in virtual machines and other special environments, I felt to do
> > something about it.
> > I developed a different approach, which I call Linux Random Number
> > Generator (LRNG) to collect entropy within the Linux kernel. The main
> > improvements compared to the legacy /dev/random is to provide sufficient
> > entropy during boot time as well as in virtual environments and when
> > using SSDs. A secondary design goal is to limit the impact of the entropy
> > collection on massive parallel systems and also allow the use accelerated
> > cryptographic primitives. Also, all steps of the entropic data processing
> > are testable. Finally massive performance improvements are visible at
> > /dev/urandom / get_random_bytes.
> 
> [quote from pdf]
> 
> > ... DRBG is “minimally” seeded with 112^6 bits of entropy.
> > This is commonly achieved even before user space is initiated.
> 
> Unfortunately one of the issues of the /dev/urandom interface is the
> fact that it may start providing random numbers even before the
> seeding is complete. From the above quote, I understand that this
> issue is not addressed by the new interface. That's a serious
> limitation (of the current and inherited by the new implementation),
> since most/all newly deployed systems from "cloud" images generate
> keys using /dev/urandom (for sshd for example) on boot, and it is
> unknown to these applications whether they operate with uninitialized
> seed.

That limitation is addressed with the getrandom system call. This call will 
block until the initial seeding is provided. After the initial seeding, 
getrandom behaves like /dev/urandom. This behavior is implemented alredy with 
the legacy /dev/random and is preserved with the LRNG.
> 
> While one could argue for using /dev/random, the unpredictability of
> the delay it incurs is prohibitive for any practical use. Thus I'd
> expect any new interface to provide a better /dev/urandom, by ensuring
> that the kernel seed buffer is fully seeded prior to switching to
> userspace.
> 
> About the rest of the design, I think it is quite clean. I think the
> DRBG choice is quite natural given the NIST recommendations, but have
> you considered using a stream cipher instead like chacha20 which in
> most of cases it would outperform the DRBG based on AES?

This can easily be covered by changing the DRBG implementation -- the current 
DRBG implementation in the kernel crypto API is implemented to operate like a 
"block chaining mode" on top of the raw cipher. Thus, such change can be 
easily rolled in.

Ciao
Stephan