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Re: [Qemu-devel] error: qcrypto_random_bytes() tried to read from /dev/[

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] error: qcrypto_random_bytes() tried to read from /dev/[u]random, even on windows
Date: Mon, 24 Apr 2017 14:36:24 +0100
User-agent: Mutt/1.7.1 (2016-10-04)

On Mon, Apr 24, 2017 at 02:30:25PM +0100, Peter Maydell wrote:
> On 24 April 2017 at 13:50, Daniel P. Berrange <address@hidden> wrote:
> > For subject line, better to describe the change made, rather than
> > the problem.
> >
> > On Mon, Apr 24, 2017 at 02:17:56PM +0200, address@hidden wrote:
> >> Now it calls CryptGenRandom() if is it compiled for windows.
> >>
> >> It might be possible to save the cryptographic provider in between
> >> invocations, e.g. by making it static -- I have no idea how computationally
> >> intensive that operation actually is.
> >
> > I'd think most people should really just enable gnutls during build. This 
> > just
> > has to provide a fallback that's good enough to be functional. If someone 
> > really
> > cares about performance of this fallback, they can send patches later....
> What I think is more worrying is the suggestion in the CryptAcquireContext
> docs that this might for instance prompt the user for a password to
> decrypt keys. We definitely don't want to do that every time. In
> fact we'd rather not do it at all.
> Googling suggests that the approach used by web browsers is
> to use the not-very-documented RtlGenRandom function:
> https://dxr.mozilla.org/mozilla-central/source/security/nss/lib/freebl/win_rand.c#141
> https://chromium.googlesource.com/chromium/src.git/+/master/base/rand_util_win.cc
> That looks much simpler than trying to use the official crypto
> APIs. (It's also what the MS C runtime library does to implement
> rand_s().)

FYI, both gnutls and openssl use these CryptAcquireContext/CryptGenRandom
methods, so I'd prefer to stick with that.

It seems we merely need to set CRYPT_SILENT in the flags to prevent any
chance of interactive prompts.


"CRYPT_SILENT is intended for use with applications for which the UI
 cannot be displayed by the CSP."

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