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Re: [TLS] Closing on 0-RTT

Bill Cox <waywardgeek@google.com> Tue, 13 June 2017 19:06 UTC

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From: Bill Cox <waywardgeek@google.com>
Date: Tue, 13 Jun 2017 12:06:26 -0700
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
Cc: Eric Rescorla <ekr@rtfm.com>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Closing on 0-RTT
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On Tue, Jun 13, 2017 at 4:32 AM, Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:

> Also:
>
> - Note that 0-RTT exporters are not safe for authentication unless
>   the server does global anti-replay on 0-RTT.


I do not think this is the case.  Nick Harper has proposed an RFC for token
binding over 0-RTT:

    https://tools.ietf.org/html/draft-nharper-0-rtt-token-binding-02

In the same way servers can ensure tickets are single-use (by binding them
to a server/metro/orbit and having local ticket caches), we can ensure that
each retransmission carries a unique auth signature.  I would state the
situation like this:

  - Note that 0-RTT exporters are not safe for authentication on servers
that do not enforce single-use tickets, or for clients that do not
recompute authentication signatures on retransmission of early data.

Even this is only partially true.  Anti-replay can be built above the TLS
layer.  I'm considering doing token-binding replay defense in the
authentication backend, to help ensure the token-binding guarantee: that
auth tokens taken from one device cannot be used from another device
without continued access to the first device's signing oracle.
Unfortunately, 0-RTT master resumption secrets are a new kind of auth
bearer token, and the token binding spec does not cover them.

Bill

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