Re: [TLS] First TLS cached information draft posted

[Simon Josefsson <simon at josefsson.org>]

Martin Rex <Martin.Rex at sap.com> writes:

> Simon Josefsson wrote:
>> 
>> Martin Rex <Martin.Rex at sap.com> writes:
>>
>> >Simon Josefsson wrote: 
>> >> 
>> >> Good point, but how would a client know whether a server replaced the
>> >> cached information with a hash or not if there is no signaling whether
>> >> this happened or not?
>> >
>> > The client knows which Hash it would find, if the server would sent
>> > the hash, so it could check for that.  Even a collision of the
>> > hash and the original handshake data would not cause a problem
>> > with this "heuristical" determination.
>> >
>> > I'm wondering about the following: we should give a strong recommendation
>> > about the minimum size of an object in order for caching to make any
>> > sense at all, like >= 2x the size of the hash.
>> >
>> > Maybe the minimum size should be rather 3x hash size, considering
>> > the rest of the protocol overhead.
>> 
>> This seems like a good idea -- and it solves the problem in the client
>> to identify whether the server replaced the data or not.  If the minimum
>> size is 3x hash size, the client can look at the length of the data.  If
>> it is longer than 3x hash size, it knows the server opted out from
>> sending the hash.  If it is shorter than 3x hash size, it knows the
>> server actually did replace the data with a hash value.
>
> Not quite.
>
> The caching extension is supposed to be generic, and the to-be-cached
> real data may in some situations be quite short or even the same size
> than the hash value (probably not for the server certificate caching,
> but maybe for the list of certification authorities in the certificate
> request message, which consists of distinguished names only, and
> there could be just one very short DName (or none at all with TLS v1.1+)

Then why use the extension?  The point of the extension, as I understood
it, is to replace large data with a small hash of the same data.  If the
data is small, the extension doesn't serve any purpose and should not be
used.  That is why I liked your suggestion to limit the use of the
extension when the data is larger than 3x hash size.

/Simon