[gnutls-help] too few bits from gnutls_dh_params_generate2() ?

Brian Hinz bphinz at users.sourceforge.net
Tue Nov 11 00:12:48 CET 2014

On Mon, Nov 10, 2014 at 5:59 PM, Nikos Mavrogiannopoulos <nmav at gnutls.org>

> On Mon, 2014-11-10 at 11:48 -1000, Daniel Kahn Gillmor wrote:
> > > After some debugging it turns out that the failing criteria is that
> > > multiple of 64 bits requirement[1]. For some reason I've gotten a 1023
> > > bit prime, even though I called gnutls_dh_params_generate2() with 1024
> > > as the argument.
> > ugh.  Java is at fault here -- there's no sense in this particular
> > severe limitation.  if they're willing to use 512-bit DHE parameters and
> > 1024-bit DHE parameters, they should be willing to use 1023-bit DHE
> > parameters.
> That's indeed quite some arbitrary limitation.

I think that the actual limitation in question is that Java is requiring
the prime length to be a multiple of 64.  Presumably this dates back to
FIPS-186-1 which did require prime lengths to be multiples of 64.  The
limitation on the prime length is supposedly being relaxed in Java 8.

> > That said, i suppose it's possible that gnutls could always ensure that
> > the high bit is set when generating a prime of a given size.
> That should be the case in gnutls 3.3.x. That version delegates to
> nettle the DH parameter generation and nettle seems to be more precise.

Thanks, I'll try that.

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