Re: [TLS] Wrapping up cached info

["Joseph Salowey (jsalowey)" <[e-mail]>]

This approach seems equivalent to the server assigns name approach that
has been brought up several times previously and not received enough
support to replace the current mechanism.  In addition, it doesn't seem
to solve the issue of how to hash the objects that the working group is
currently struggling with.  

Joe

> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]] On Behalf Of
> Michael D'Errico
> Sent: Wednesday, May 19, 2010 8:03 AM
> To: Stefan Santesson
> Cc: [e-mail]
> Subject: Re: [TLS] Wrapping up cached info
> 
> Stefan Santesson wrote:
> > Not sure how I should interpret the silence?
> >
> > I guess it means that everyone agrees with me :)
> 
> Then by that logic everyone also agrees with me to use http caching
> as the model!
> 
> Seriously, I think that the current draft might be too simple.  For
> example, consider caching the list of certificate_authorities in a
> CertificateRequest message.  A web application might have several
> different lists of certificate_authorities based on what the resource
> is and who is trying to access it.  If cached-info can only cache one
> object of any particular type, then it will not be very helpful for
> such an application.
> 
> Also, exactly how would you hash the Diffie-Hellman parameters in a
> ServerKeyExchange message?  You can't just hash the message since it
> also contains a unique Ys which changes frequently if not every
> handshake.
> 
> Allowing the server to instead name the objects, and keep track of
> timestamps for each object, allows you to handle these two cases.  In
> the multiple-CA-list example above, the server might have USER and
> ADMIN lists.  If it can't be sure when exactly those lists were
> created, the timestamp can simply be when the server was started or
> when it last read its configuration.
> 
> When the server forces the client to renegotiate when accessing a
> protected resource as an ADMIN for example, the server's cached info
> extension would contain:
> 
>      type=cert_req_ca_list,name=ADMIN,timestamp=20100519123456789
> 
> When requesting a less-sensitive, but not public resource, the server
> would send:
> 
>      type=cert_req_ca_list,name=USER,timestamp=20100519123456789
> 
> If the client's cached-info extension contained an exact match, then
> the server would omit that data from the handshake.
> 
> This is sort of like cipher suite selection.  The client sends a list
> of possible matches and the server selects one from the list.  It
> differs in that there might be no match, in which case the server just
> sends the info for the object it decided to send in the handshake.
> 
> I'm sure there are more details to be worked out, but I think it would
> be a good idea to entertain this approach.
> 
> Mike
> 
> 
> 
> > To repeat in short. Here is what I suggest based on the latest
traffic:
> >
> > 1) A client, caching information also caches what hash algorithm was
> used to
> > calculate the finished message at the time of caching.
> >
> > 2) On repeated connections, the client indicate cached info by
sending a
> > hash of the cached object using the cached hash algorithm. (No use
of
> FNV
> > hash)
> >
> > 3) The server accepts by returning the received hash instead of the
> cached
> > data.
> >
> > 4) The only hash agility provided is that the client will send a
hash
> > algorithm identifier with the hash.
> >
> > 5) The client MUST NOT send more than one hash per cached object,
and
> MUST
> > used the cached hash algorithm.
> >
> >
> > This solves all issues raised (securely binds the cached data to the
> > finished calculation) and removes the need for hash agility
> > Syntactically it just requires adding a hash identifier and
adjusting
> the
> > vector length for hash data.
> >
> > So I basically wander:
> > - Would this be acceptable?
> > - Who could NOT live with this solution?
> > - Who think it is worth the effort to agree on a better solution,
and
> why?
> >
> > Regards
> >
> >
> >
> >
> > On 10-05-18 12:24 AM, "Stefan Santesson" <[e-mail]> wrote:
> >
> >> As Martin pointed out to me privately, the hash used in the
finished
> >> calculation becomes known to the client only after receiving the
serer
> >> hello.
> >>
> >> It would therefore be natural for the client to use the hash
function
> used
> >> to calculate the finished message at the time when the data was
cached.
> >>
> >> The client would then indicate which hash algorithm it used and
upon
> >> acceptance, the server will honor the request.
> >>
> >> /Stefan
> >>
> >>
> >> On 10-05-17 10:34 PM, "Stefan Santesson" <[e-mail]>
wrote:
> >>
> >>> Guys,
> >>>
> >>> Where I come from we have a say "don't cross the river to get to
the
> water".
> >>> And to me this proposal to change the finished calculation is just
> that.
> >>>
> >>> Look at it.
> >>>
> >>> The proposal is to bind the cached data by adding a hash of the
cached
> data
> >>> to the finished calculation.
> >>>
> >>> The proposal is further to avoid hash agility by picking the hash
> algorithm
> >>> used by TLS's Finished message computation.
> >>>
> >>>
> >>> Now there are two ways to achieve this goal.
> >>>
> >>> 1) The crossing the river to get water approach:  Exchange FNV
hashes
> of the
> >>> cached data in the handshake protocol exchange and then inject
hashes
> of the
> >>> same data into the finished calculation through an alteration of
the
> TLS
> >>> protocol.
> >>>
> >>>
> >>> 2) The simple approach: Use the hash algorithm of the finished
> calculation
> >>> to hash the cached data (according to the current draft).
> >>>
> >>>
> >>> Alternative 2 securely bind the hashed data into the finished
message
> >>> calculation without altering the algorithm.
> >>>
> >>> Alternative 2 requires at most a hash algorithm identifier added
to
> the
> >>> protocol, if at all. We don't need to add negotiation since we
always
> use
> >>> the hash of the finished message calculation. Adding this
identifier
> would
> >>> be the only change made to the current draft.
> >>>
> >>> Alternative 2 don't require additional security analysis. If the
hash
> used
> >>> to calculate the finished message is broken, then we are screwed
> anyway.
> >>>
> >>> /Stefan
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> On 10-05-17 9:16 PM, "Martin Rex" <[e-mail]> wrote:
> >>>
> >>>> Joseph Salowey wrote:
> >>>>> I agree with Uri, that if you determine you need SHA-256 then
you
> should
> >>>>> plan for hash agility.  TLS 1.2 plans for hash agility.
> >>>>>
> >>>>> What about Nico's proposal where a checksum is used to identify
the
> >>>>> cached data and the actual handshake contains the actual data
hashed
> >>>>> with the algorithm used in the PRF negotiated with the cipher
suite?
> >>>>>
> >>>>> This way we don't have to introduce hash agility into the
extension,
> but
> >>>>> we have cryptographic hash agility where it matters in the
Finished
> >>>>> computation.  Does it solve the problem?
> >>>> Yes, I think so.
> >>>> This approach should solve the issue at the technical level.
> >>>>
> >>>> Going more into detail, one would hash/mac only the data that got
> >>>> actually replaced in the handshake, each prefixed by a (locally
> computed)
> >>>> length field.
> >>>>
> >>>> -Martin
> >>>> _______________________________________________
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> >>>
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> >>
> >
> >
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