Re: [saag] Section 2.9: was Re: AD review of draft-iab-crypto-alg-agility-06

[Viktor Dukhovni <[e-mail]>]

On Tue, Sep 01, 2015 at 07:02:01PM +0200, Eliot Lear wrote:

> > Rapid deprecation of weaker crypto is net loss for OS when the
> > result is loss of interoperability and/or fallback to cleartext.
> 
> Of course, this holds true when there are no alternatives from which to
> select.  And it ties back to the second advantage you cited:
>
> > Makes it possible to phase out old deprecated algorithms.

Eventually, when "essentially everyone" supports better options,
the old can be dropped even by OS clients and servers.  So the
second benefit is "delayed" for OS, we quickly lift the "ceiling"
and as time goes by and this becomes practical lift the "floor".

> But what is it one has to do?  Why is the answer simply to specify MTI
> two actively used suites and then vary which two over some (hopefully
> long) period over time?

Two might not in practice be enough for OS, if deployed software
lasts 10+ years without upgrades.  Sure, only two suites might be
MTI at a given time, but implementations will often need to continue
to support "past" MTI suites, if those are the best available for
a sufficient number of peers.

Thus a suite that used to be "MTI", but no longer is, becomes
de-facto required for OS.  Actually this applies not just OS, in
practice even mandatory security protocols need to worry about
interoperability.  

The difference is mostly in how quickly deprecated crypto is
abandoned, because cleartext is rarely better than weak crypto
(modulo facilitation of downgrades against even non-OS peers ala
logjam and EXPORT ciphers).

While browsers and webservers are moving quick to abandon RC4 in
2015, SMTP servers and clients are going to continue to support it
for at least another year or two.  Some SMTP serevrs already don't
tolerate RC4 and create pressure on the legacy systems to upgrade.
For most, it makes more sense to continue to receive mail than to
enforce crypto-correctness.  The overall process takes longer.

--
	Viktor.