Public-Key Cryptography
Private key crypto => Alice and Bob must secretly choose K, exposure of EK renders system insecure => requires prior communication of the key K using secure channel before any ciphertext is transmitted.
Idea: to find a cryptosystem where computationally infeasible to determine D given E => E could be made public by publishing it in a directory.
=> Alice can send encrypted message using public E
Bob is only person who can decrypt it, using secret decryption rule.