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 
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 

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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