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

Nico Williams <nico@cryptonector.com> Mon, 27 July 2015 19:40 UTC

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Date: Mon, 27 Jul 2015 14:40:21 -0500
From: Nico Williams <nico@cryptonector.com>
To: Kathleen Moriarty <kathleen.moriarty.ietf@gmail.com>
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Subject: Re: [saag] AD review of draft-iab-crypto-alg-agility-06
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On Sat, Jul 18, 2015 at 10:30:19AM +0200, Kathleen Moriarty wrote:
> > On Jul 17, 2015, at 7:18 PM, Stephen Farrell <stephen.farrell@cs.tcd.ie>; wrote:
> > 2.9: I'm not really a fan of blessing weaker algs for OS, but I lost
> > that argument before. I wonder if we would get consensus if this
> > said that weak algs are better than no encryption but still MUST be
> > deprecated as soon as feasible?
> 
> I don't think we've really debated this enough to get consensus.  I
> don't think weaker algs fit into our agreed definitions for OS.  I
> just recall your debate with Pete on another draft, but think a wider
> debate is needed to see what the consensus is.  I don't think weaker
> algorithms should fit into the definition.

If OS means "upgrade from cleartext when you can" (it does), then
failing to use weak crypto -> fallback on cleartext.

Even using 1DES is better than cleartext: because if everyone were using
1DES then the cost of massive eavesdropping (gather ciphertexts,
cryptanalyze) goes up significantly.

The key thing is that weak crypto must not lead to real-time exploitable
downgrade attacks.

Now, perhaps OS should mean "upgrade from cleartext when you can, but
fail if weak crypto is selected" (i.e., no fallback on cleartext).  But
then we have to have two OS definitions, one for SMTP, and one for other
protocols: because after all we do want [some, e.g., to postmaster]
e-mail to flow no matter what.  And even so, remember that the user
would gladly use no crypto if none is offered, so I don't think this
makes sense.

Think of OS as a migration strategy.  If we make the jump from cleartext
to encrypted too difficult/expensive, then we'll fail to complete the
migration.  Spurious failures resulting from attempting to upgrade could
mean failure to migrate: because users will simply turn off OS.  Yes, OS
could be required-to-be-enabled, thus preventing this social failure
mode, but we're still far from that.

Permitting weak crypto (with the above downgrade caveat) with OS is
rather necessary then.  We can only forbid weak crypto in OS
applications when the market share of such crypto is negligible.

Nico
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