The symmetric ciphers

[Robert J. Hansen]

On 10/30/2013 7:20 PM, Johan Wevers wrote:
> That's because ROT(N) is a group.

Yes, but good luck answering the inevitable next two questions: "what's
a group?" and "how do we know if something's a group?"  You very quickly
run into some complicated higher-level maths, and that's something best
avoided.

> I don't know wether the other symmetric ciphers are a group though,
> but I'm sure someone has investigated that.

There is no single answer to this.  The "other symmetric ciphers" need
to be evaluated combinatorically: for instance, are AES128, 3DES and
Camellia a group?  That answer may be different from AES192, 3DES and
Camellia.

Given there are 11 different symmetric algorithms as of 2.0.22, figuring
out all known-safe 3-cipher selections would require us to investigate
165 different combinations.  Frankly, I don't feel like doing that much
work.

> Assuming that the same key is used for all that is.

No.  I'm quite happy with my blanket statement: cryptographic algorithms
are subtle and should be left alone.  If you're Don Coppersmith then I
think you should feel free to get down with your bad self, but otherwise
this entire line of inquiry is one that I think goes nowhere good.