Re: [OAUTH-WG] New Version Notification for draft-fett-oauth-dpop-03.txt

Neil Madden <neil.madden@forgerock.com> Wed, 27 November 2019 20:30 UTC

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From: Neil Madden <neil.madden@forgerock.com>
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Date: Wed, 27 Nov 2019 20:30:16 +0000
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To: Brian Campbell <bcampbell@pingidentity.com>
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Subject: Re: [OAUTH-WG] New Version Notification for draft-fett-oauth-dpop-03.txt
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> On 27 Nov 2019, at 19:19, Brian Campbell <bcampbell@pingidentity.com> wrote:
> 
>> On Wed, Nov 27, 2019 at 3:31 AM Neil Madden <neil.madden@forgerock.com> wrote:
>> 
>> That is true, but is IMO more of a hindrance than an advantage for a PoP scheme. The very fact that the signature is valid at every RS is why you need additional measures to prevent cross-RS token reuse. This downside of signatures for authentication was pointed out by djb 18 years ago (https://groups.google.com/forum/m/#!msg/sci.crypt/73yb5a9pz2Y/LNgRO7IYXOwJ), which is why most modern crypto protocols either use Diffie-Hellman for authN (https://noiseprotocol.org) or sign a hash of an interactive handshake transcript (TLS 1.3 - https://tools.ietf.org/html/rfc8446#section-4.4.3) so that the signature is tightly bound to a specific interactive protocol run.
>> 
> 
> Mostly for my own edification -  using Diffie-Hellman for authN (that a key was held) was effectively at the heart of the "tentative suggestion for an alternative design" that you had much early in this thread?

Yes, exactly.

— Neil