ENCRYPTION
- cryptography - making and breaking codes
- private key encryption
        - substitution cipher
        - encoding vs decoding
        - aka single key encryption or symmetric encryption - when
                same key is used for encoding and decoding
        - locked box example (you lock box, friend opens it with same key)
        - breaking a substitution cipher
        - square box code
        - problem of key security
- public key encryption - double key or asymmetric
        - needs two keys, a public and a private
        - public is visible to the world, private only to the owner
        - the private key cannot be deduced from the public key (not easily)
        - to send an encrypted message, you encode the msg with your friend's
                public key, he decrypts with his private key
        - since i used your public key, only you can read it because only
                you have your private key
        - locked box example
- encryption strength
        - STRONG VS WEAK (weak = can be cracked in a practical amount of time)
        - 10-bit keys - 1024 possible keys
        - 20-bit, 40 bit, 56 bit, 64 bit, 128 bit, 256 bit
        - can crack 56 bit code in less than a week (might be a few days now)
        - 64 bit were reasonably secure
- digital signatures
        - "reverse" of encrypting - with encrypting, that makes sure
                only the recipient can read it (provides ID of recipient)
        - with a dig sig, it makes sure only the sender can send it 
                (proves ID of sender)
        - you "sign" a document by using special software with your private
                key and the document
        - then anyone who wants to verify the document simply uses your private
                key on the document.
        - differs from a hand written signature because each digital signature
                is unique to the document - can't copy between docs
- public key encryption works because some problems are hard to solve
        but easy to verify the solution