Re: Clearing cache digests

Yes, that seems like the obvious answer.

-Ekr

On Mon, Jul 18, 2016 at 10:29 AM, Martin Thomson <martin.thomson@gmail.com>
wrote:

> Why are we concerned about this at all?  Can't we put this stuff into
> the same bucket as cookies?
>
> On 18 July 2016 at 08:31, Eric Rescorla <ekr@rtfm.com> wrote:
> > The draft says:
> >
> >    As a result, clients MUST mitigate for this threat when the user
> >    attempts to remove identifiers (e.g., "clearing cookies").  This
> >    could be achieved in a number of ways; for example: by clearing the
> >    cache, by changing one or both of N and P, or by adding new,
> >    synthetic entries to the digest to change its contents.
> >
> >    TODO: discuss how effective the suggested mitigations actually would
> >    be.
> >
> > Except for "clearing the cache", my initial impression is that the
> answer to
> > "how effective" is "not very", except for very naive uses of the cache
> as an
> > identifier.
> >
> > Consider that the general structure of this mechanism is that the client
> > gives the server an oracle which answers the question "do you have
> document
> > X" with false positive rate 2^-P. This implies that the server can use
> the
> > cache as a cookie B of length N bits by creating N resources R_1, R_1 ...
> > R_N and then to store the cookie:
> >
> > - If B_i == 1 then store R_i
> > - Otherwise don't
> >
> > You then query for the cookie in the cache the same way. This has an
> epsilon
> > error probability but you can correct for that by storing N + delta bits
> and
> > using an error correcting code.
> >
> > So, my claim is that any mechanism that retains the information in the
> cache
> > digest will allow for tracking, even if you change the way it is encoded
> > (e.g., changing N, P).
> >
> > -Ekr
> >
> >
>