Re: [TLS] add challenge in TLS v1.3 to prevent DDOS attack?

Peter Gutmann <> Mon, 08 June 2015 09:20 UTC

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From: Peter Gutmann <>
To: Bingzheng Wu <>, tls <>
Thread-Topic: [TLS] add challenge in TLS v1.3 to prevent DDOS attack?
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Date: Mon, 8 Jun 2015 09:19:56 +0000
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Subject: Re: [TLS] add challenge in TLS v1.3 to prevent DDOS attack?
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Bingzheng Wu <>; writes:

>So, could we add a challenge-response mode in TLS v1.3 to increase the attacker's cost ?

You don't need anything that complex, OpenVPN has for a number of years
supported a very effective way of dealing with this, all you need to do is
formalise that.  It doesn't really need TLS 1.3 either, since you can do it in
a standard extension.  OpenVPN gives it the very misleading name tls-auth,
it's just a use of a PSK to MAC incoming packets, so the client-hello is
authenticated and dropped unless it has a valid MAC signature.  This has
protected OpenVPN against a number of OpenSSL vulnerabilities (as well as
generic port-scanning and similar), most notably Heartbleed, where any attempt
to exploit the vuln just bounced off, because unless you have the MAC key you
can't get past even the first hello message.

I started work on an RFC draft for this a while back, but it got shelved
because of conflicts over making it OpenVPN-compatible or not, it shouldn't be
too much work to dig it out and finish it (albeit in a non-OpenVPN-compatible