[Cryptography] upgrade mechanisms and policies

Ben Laurie ben at links.org
Sat Apr 11 16:21:13 EDT 2015

On 11 April 2015 at 19:50, Bill Frantz <frantz at pwpconsult.com> wrote:

> On 4/11/15 at 4:52 AM, michael at kjorling.se (Michael Kjörling) wrote:
>  On 10 Apr 2015 23:28 -0700, from frantz at pwpconsult.com (Bill Frantz):
>>> The only issue with only one crypto suite per version is that you
>>> can't assume that version n+1 is better than version n.
>> On what basis however can we assume that a hypothetical future TLS 1.5
>> will be "better" (either in some objectively measurable sense, or in
>> every sense) than a likewise TLS 1.4?
> All of the protocols I know of that have version negotiation design the
> protocol to try to agree on the highest numbered version. What I think we
> need is to have each end have a preference order for versions numbers and
> pick the offered version which is highest in the preference list.
> With this kind of mechanism, each version could have only one algorithm
> for each function, eliminating algorithm negotiation, and a lot of
> complexity from implementations. The preference ordered list is the place
> for each end to specify policy, as John Denker suggests. Note that if an
> endpoint considers a version to be spyware, it can leave it out of the list
> entirely, causing the connection attempt to fail unless another version can
> be agreed on. As Michael Kjörling points out:

>  Newer does not necessarily mean better,
>> especially in the security field, and in fact something that has stood
>> the test of time may actually be _better_ than something entirely
>> newfangled.
Wat? This is crazy talk.

Clearly the only sane policy is to believe that the latest version of X is
the most secure. And if you know about X you ought to also know about the
problems with X-1, X-2,.... So, sure, each end indicates which versions it
is prepared to use, but of the intersection, _surely_ highest wins?

> Cheers - Bill
> -----------------------------------------------------------------------
> Bill Frantz        | Concurrency is hard. 12 out  | Periwinkle
> (408)356-8506      | 10 programmers get it wrong. | 16345 Englewood Ave
> www.pwpconsult.com |                - Jeff Frantz | Los Gatos, CA 95032
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