Re: [TLS] Universal PSKs

Nikos Mavrogiannopoulos <> Mon, 18 June 2018 13:18 UTC

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From: Nikos Mavrogiannopoulos <>
To: Jonathan Hoyland <>, Hubert Kario <>
Cc: "" <>
Date: Mon, 18 Jun 2018 15:18:19 +0200
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Subject: Re: [TLS] Universal PSKs
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On Mon, 2018-06-18 at 13:56 +0200, Jonathan Hoyland wrote:
> The issue with the current draft is that it leaves a single PSK with
> two potential interpretations. 
> If the draft also changed the PSK identity value then each PSK would
> have a single interpretation.
> If the draft changes the identity then it can also change the PSK key
> without having to change the manner in which the binder is computed.
> In that case universal PSKs and regular PSKs do not need to be
> distinguished, because they are both validated in the same way. 
> A server unaware of universal PSKs would simply see an unrecognised
> PSK identity.
> If both the unmodified and the modified PSKs are sent, then it can
> simply select the unmodified version, ignoring the other. 
> A server that recognises both values can choose which to use.
> If the modified PSK identity was a channel binding, then the modified
> version would have stronger security properties, and thus presumably
> would be preferable.
> In this case the hash function used for the binder remains
> selectable.  
> Would that resolve your issue?

That may not be sufficient. A server which sees an unrecognized PSK may
chose to pretend it recognized it to avoid user enumeration attacks
similarly to TLS1.2 (optional) behavior. Hence a modified username
would cause negotiation  failure with such a server.

A new extension seems to be necessary to eliminate any interoperability
issues with servers that will not follow the universal psk draft.