[Narrator] A popular toy cipher is a monoalphabetic substitution cipher
where each letter in the alphabet is mapped to some substitution letter.
This could be the same letter, could be any other letter in the alphabet,
and decryption is done by the reverse mapping.
We provided python code to make this more specific,
what a monoalphabetic substitution cipher does.
One way to prove that a monoalphabetic substitution cipher is imperfect
is to use Shannon's key space theorem, which says that
the number of keys in a perfect cipher must exceed the number of messages or be equal to it.
Assuming a 26-letter alphabet, what length message do we need to show
that the key space for the monoalphbetic cipher is too small to be a perfect cipher?
For your answer give a number, which is the minimum length message needed for this proof.
