----- Forwarded message from Jerry Leichter <leich...@lrw.com> -----
Date: Tue, 10 Sep 2013 07:42:55 -0400
From: Jerry Leichter <leich...@lrw.com>
To: Phillip Hallam-Baker <hal...@gmail.com>
Cc: "cryptogra...@metzdowd.com" <cryptogra...@metzdowd.com>, ianG
<i...@iang.org>
Subject: Re: [Cryptography] The One True Cipher Suite
X-Mailer: Apple Mail (2.1283)
On Sep 9, 2013, at 12:00 PM, Phillip Hallam-Baker wrote:
> Steve Bellovin has made the same argument and I agree with it. Proliferation
> of cipher suites is not helpful.
>
> The point I make is that adding a strong cipher does not make you more
> secure. Only removing the option of using weak ciphers makes you more secure.
I'm not so sure I agree. You have to consider the monoculture problem,
combined with the threat you are defending against.
The large burst of discussion on this list was set off by Perry's request for
ways to protect against the kinds of broad-scale, gather-everything attacks
that Snowden has told us the NSA is doing. So consider things from the side of
someone attempting to mount this kind of attack:
1. If everyone uses the same cipher, the attacker need only attack that one
cipher.
2. If there are thousands of ciphers in use, the attacker needs to attack some
large fraction of them.
As a defender, if I go route 1, I'd better be really, really, really sure that
my cipher won't fall to any attacks over its operational lifetime - which, if
it's really universal, will extend many years *even beyond a point where a
weakness is found*.
On the other hand, even if most of the ciphers in my suite are only moderately
strong, the chance of any particular one of them having been compromised is
lower.
This is an *ensemble* argument, not an *individual* argument. If I'm facing an
attacker who is concentrating on my messages in particular, then I want the
strongest cipher I can find.
Another way of looking at this is that Many Ciphers trades higher partial
failure probabilities for lower total/catastrophic failure probabilities.
Two things are definitely true, however:
1. If you don't remove ciphers that are found to be bad, you will eventually
pollute your ensemble to the point of uselessness. If you want to go the "many
different ciphers" approach, you *must* have an effective way to do this.
2. There must be a large set of potentially good ciphers out there to choose
from. I contend that we're actually in a position to create reasonably good
block ciphers fairly easily. Look at the AES process. Of the 15 round 1
candidates, a full third made it to the final round - which means that no
significant attacks against them were known. Some of the rejected ones failed
due to minor "certificational" weaknesses - weaknesses that should certainly
lead you not to want to choose them as "the One True Cipher", but which would
in and of themselves not render breaking them trivial. And, frankly, for most
purposes, any of the five finalists would have been fine - much of the final
choice was made for performance reasons. (And, considering Dan Bernstein's
work on timing attacks based on table lookups, the performance choices made bad
assumptions about actual hardware!)
I see no reason not to double-encrypt, using different keys and any two
algorithms from the ensemble. Yes, meet-in-the-middle attacks mean this isn't
nearly as strong as you might naively think, but it ups the resource demands on
the attacker much more than on the defender.
Now, you can argue that AES - the only cipher really in the running for the One
True Symmetric Cipher position at the moment - is so strong that worrying about
attacks on it is pointless - the weaknesses are elsewhere. I'm on the fence
about that; it's hard to know. But if you're going to argue for One True
Cipher, you must be explicit about this (inherently unprovable) assumption;
without it your argument fails.
The situation is much more worse for the asymmetric case: We only have a few
alternatives available and there are many correlations among their potential
weaknesses, so the Many Ciphers approach isn't currently practical, so there's
really nothing to debate at this point.
Finally, I'll point out that what we know publicly about NSA practices says
that (a) they believe in multiple ciphers for different purposes; (b) they
believe in the strength of AES, but only up to a certain point. At this point,
I'd be very leery of taking anything NSA says or reveals about it practices at
face value, but there it is.
-- Jerry
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