[tor-dev] Proposal 222: Stop sending client timestamps

[George Kadianakis]

Nick Mathewson <nickm at torproject.org> writes:

> Filename: 222-remove-client-timestamps.txt
> Title: Stop sending client timestamps
> Authors: Nick Mathewson
> Created: 22 August 2013
> Target: 0.2.5.x
> Status: Open
>
> 0. Summary
>
>    There are a few places in Tor where clients and servers send
>    timestamps.  I list them and discuss how to eliminate them.
>
> 1. Introduction
>
>    Despite this late date, many hosts aren't running NTP and
>
> <snipage>
>
>    field, so this change should be safe.
>
> 2.2. AUTHENTICATE (server)
>
>    The AUTHENTICATE cell is not ordinarily sent by clients. It
>    contains an 8-byte timestamp and a 16-byte random value.
>    Instead, let's replace both with a 24-byte (truncated) HMAC of
>    the current time, using a random key.
>
>    This will achieve the goal of including a timestamp in the
>    cell (preventing replays even in the presence of bad entropy),
>    while at the same time not including the time here.
>

Hey Nick,

how does the client verify the contents of the AUTHENTICATE cell
(including the timestamp), if the timestamp is encrypted with a random
key?