[TLS] Resumption ticket/PSK

Kyle Rose <krose@krose.org> Thu, 19 May 2016 18:24 UTC

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From: Kyle Rose <krose@krose.org>
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Subject: [TLS] Resumption ticket/PSK
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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