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Re: [TLS] Resumption ticket/PSK

Eric Rescorla <ekr@rtfm.com> Thu, 19 May 2016 18:35 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 19 May 2016 11:31:53 -0700
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To: Kyle Rose <krose@krose.org>
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Subject: Re: [TLS] Resumption ticket/PSK
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Yes, I think this would be good text. PR wanted :)

-Ekr

On Thu, May 19, 2016 at 11:19 AM, Kyle Rose <krose@krose.org> wrote:

> Regarding the ability for passive observers' tracking of clients
> across connections (and potentially across IPs) via a session ticket
> used more than once, should there be any language around recommended
> practice here, especially for clients?
>
> An appropriately-configured server can help the client avoid this
> problem without performance penalty by issuing a new session ticket on
> every connection (for non-overlapping handshakes) and/or multiple on
> one (to cover that gap), and a client can help by keeping only the
> most recent ticket for a particular session and/or using a given
> ticket only once.
>
> Thoughts on adding language under "Implementation Notes" such as:
>
> "Clients concerned with privacy against tracking by passive observers
> SHOULD use a PSK/session ticket at most once. Servers SHOULD issue
> more than one session ticket per handshake, or issue a new session
> ticket on every resumption handshake, to assist in the privacy of the
> client while maintaining the performance advantage of session
> resumption."
>
> For pure PSK I assume tracking is less of an issue, but I'm happy to
> entertain thoughts there, as well.
>
> Kyle
>
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