[Cfrg] tcpinc: endpoint authentication and session ID privacy

Kyle Rose <krose@krose.org> Sat, 01 October 2016 15:32 UTC

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From: Kyle Rose <krose@krose.org>
Date: Sat, 01 Oct 2016 11:32:07 -0400
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Cc: tcpinc-chairs@ietf.org
Subject: [Cfrg] tcpinc: endpoint authentication and session ID privacy
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In the tcpinc working group we're working hard on finishing up the tcpcrypt
draft. One of the remaining items on my plate is to solicit analysis of
certain aspects of the protocol and to get advice on how best to word some
of the security assertions in the draft. I'm hoping some folks in cfrg can
help out.

The first issue relates to bootstrapping authentication on top of the
opportunistically-encrypted channel that tcpcrypt creates. Endpoint
authentication bootstrapped via tcpcrypt relies on the uniqueness over all
time of a session ID that is computed from a function of inputs (e.g.,
session transcript, output of key agreement) known to both endpoints of a
connection: "[the session ID] will with overwhelming probability be unique
for each individual TCP connection" from https://tools.ietf.org/html/dr

The authors claim (and I am inclined to agree, though I am not a
cryptographer) that these session IDs do not in general need to be private
(e.g., if they are to be signed by private keys known only to the
legitimate endpoints, with that signature used as proof of channel
integrity), only that an attacker in control of one endpoint of a
connection must not be able to manipulate the inputs to the session ID for
a connection with a non-cooperating endpoint in order to produce a
collision in which the ID would be the same on any two different
connections with non-vanishing probability.

First of all: does this achieve the properties we want? Secondly: is there
a straightforward way of explaining why this works in the context of the
protocol as-designed? To this latter point, I'm thinking it has something
to do with the session ID being a collision-resistant function of input
from both endpoints, making it computationally intractable for one endpoint
to influence the outcome in a useful way... but I'm not sure how to word it.

Any thoughts would be greatly appreciated.