On Friday, 16 September 2016 17:26:03 CET Hubert Kario wrote:
> I've been running tests on the openssl 1.1.0 release recently and I've
> noticed that if the client doesn't include the supported_groups extension,
> OpenSSL will pick curve with id 0x001d, that is ecdh_x25519, as the curve
> to do ECDHE over.
> While this is not incorrect behaviour according to the standard (it is quite
> explicit that if client doesn't provide this extension, server can pick any
> curve it wants), I'm afraid that this will cause interoperability problems.
> The majority of servers (71%) support *only* prime256v1 curve and of the
> ones that default to ECDHE key exchange nearly 83% will also default to
> this curve. OpenSSL 1.0.2h also defaults to this curve if there are no
> curves advertised by client.
> So it is very likely that any client that doesn't advertise curves will
> expect the server to select prime256v1. At the same time it is very
> unlikely that it will support x25519 (given how new it is).

I've filed a bug on github so that it doesn't fall off the radar...

Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purky┼łova 99/71, 612 45, Brno, Czech Republic

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