Re: A cipher operates with more possible plaintexts than possible keys. What does this mean?

rossum <rossum48@xxxxxxxxxxxx> wrote:
On Tue, 08 Mar 2011 17:07:23 GMT, unruh
<unruh@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

Any cypher except theOTP has a smaller key than plaintexts. It means
that it is theoretically breakable, if the plaintext has structure to
it.
Nitpick: Any cypher except the OTP has a smaller key than _the largest
possible_ plaintexts.

I may want to encipher "No" which is smaller than most keys. Even
with padding it will not be larger than most block cypher keys.

While we're picking nits: There are many ciphers with a key that's larger
than the largest possible plaintext.

Example: The alphabet is Zn, the key is a sequence of pairs (ai,bi),
such that ai is invertible in Zn, and we encrypt (x1,x2,...,xn) as
(a1*x1+b1, ..., an*xn+bn).

The key is much larger than the largest possible plaintext.

--
kg
.

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