Re: Clearing cache digests

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
>
>

Received on Monday, 18 July 2016 08:30:10 UTC