Multiple symetric cyphers

[Eric Hughes]

   From: cactus at seabsd.hks.net (L. Todd Masco)

   >Strength is not right aspect.  Global risk is reduced, simply because
   >the aggregate cost of a breach is reduced.

   Isn't it?  If an attacker does not know what cipher is used and breaking
   each is computationally expensive (though not prohibitively so) doesn't
   that add extra complexity?

Suppose that several symmetric ciphers are used and that one of them
is broken.  You then attempt to break all of the messages; the ones
that don't break are presumed to be one of the other ciphers.  So it
does nothing to improve strength.  Note, though, that the _rest_ of
the messages remain unbroken.

I am assuming that it's unlikely that all of the ciphers will be
broken simultaneously.

   Related: is there, in general or in any known specific cases, any
   loss of security in using sym. cipher A on ciphertext B (of another
   sym. cipher) with the same key?  With different keys (I would think
   not, but I vaguely remember mention of something here long ago)?

If you use the same key, the size of exhaustive search does not
increase.

Eric