Cryptosystem

A cryptosystem consists of sets , ,
and functions:

Such that for all and
Breaking Cryptosystems
Key Equivocation
Message Equivocation
Perfect Secrecy
Unicity Distance

Lemma

Message EquivocationKey Equivocation

Proof

Since , we know
Now calculate:

Exmples

Substitution cipher
Vigenere Cipher
Stream Cipher
One Time Pad
Rabin Cryptosystem
RSA Cryptosystem
Diffie Hellman Key Exchange