### Perfect Secrecy > The best the intruder can do is just guess no matter how much computational power he has One time pad is perfectly secret - Message Space - A collection of all possible X letter messages - Key Space - A collection of all possible outcomes of X randomly generate shifts between 1 and 26 - Cipher Text Space - All possible results of an encryption All people know is that every message is equally likely, no amount of computational power could improve a blind guess We have to share the long keys in advance - Relax our definition of secrecy - Develop a definition of Psuedo Randomness