Re: Revised draft of random(7) man page for review

[Nikos Mavrogiannopoulos <nmav-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

----- Original Message -----
> >    Usage recommendations
> >        The kernel random-number generator relies on  entropy  gathered
> >        from  device  drivers and other sources of environmental noise.
> >        It is designed to produce a small amount of  high-quality  seed
> >        material to seed a cryptographically secure pseudorandom number
> >        generator (CSPRNG).  It is designed for  security,  not  speed,
> >        and  is  poorly  suited  to generating large amounts of crypto‐
> >        graphic random data.  Users should be economical in the  amount
> >        of seed material that they consume via getrandom(2), /dev/uran‐
> >        dom, and /dev/random.
> > 
> >        ┌─────────────────────────────────────────────────────┐
> >        │FIXME                                                │
> >        ├─────────────────────────────────────────────────────┤
> >        │Is it really  necessary  to  avoid  consuming  large │
> >        │amounts from /dev/urandom? Various sources linked to │
> >        │by https://bugzilla.kernel.org/show_bug.cgi?id=71211 │
> >        │suggest it is not.                                   │
> >        │                                                     │
> >        │And: has the answer to the previous question changed │
> >        │across kernel versions?                              │
> >        └─────────────────────────────────────────────────────┘
> >        Consuming unnecessarily large  quantities  of  data  via  these
> >        interfaces  will  have  a negative impact on other consumers of
> >        randomness.
> 
> So "poorly suited" is definitely true.  Also true is that urandom is
> not engineered for use for non-cryptographic uses.  It's always going
> to be faster to use random(3) for those purposes.
> 
> As far as whether or not it has a negative impact, it depends on how
> much you trust the underlying cryptographic algorithms.  If the CSPRNG
> is seeded correctly with at least 256 bits of entropy that can't be
> guessed by the attacker, and if the underlying cryptographic
> primitives are secure, then it won't matter.  But *if* there is an
> unknown vulnerability in the underlying primitive, and *if* large
> amounts of data generated by the CSPRNG would help exploit that
> vulnerability, and *if* that bulk amount of CSPRNG output is made
> available to an attacker with the capability to break the underlying
> cryptographic vulnerability, then there would be a problem.
> 
> Obviously, no one knows of such a vulnerability, and I'm fairly
> confident that there won't be such a vulnerability across the
> different ways we've used to generate the urandom source --- but some
> people are professional paranoids, and would argue that we shouldn't
> make bulk output of the CSPRNG available for no good reason, just in
> case.

The above is certainly accurate, however, I think that such a discussion or text, when reflected to a man-page is going to cause problems. The audience of a man-page are not crypto people, and seeing such text would create confusion rather than clarify how these devices/apis should be used. The *if* part is not put into a perspective, suggesting that such an *if* is possible. However, if one clarifies, i.e., in that case, your TLS or SSH connection is most likely broken as well, and not because of any attack on /dev/urandom, then one can see that we are heading towards a theoretical discussion. 

My suggestion, on that particular text would be to remove it, but make it explicit somewhere in the text that all the assurances for the devices depend on the crypto primitives, rather than describing risks that may arise on particular usage patterns *if* primitives are broken. 

regards,
Nikos
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