Substitution Ciphers

Keyword: "Generates" the mapping from 
plain text to cipher text.

Vigenere Cipher: 
  multiple alphabet substitution.
  Keyword: shared secret
  
  Plain text: hellothere
  Keyword: cpu
  
  hellothere
  cpucpucpuc
  
 h, c -> j
 
 
 jtfndnjtlg
 
 One time pad:
 Key word longer than the message
 Could use a book, page, paragraph as keyword.
 
 
 http://cs.gettysburg.edu/CompSci103/Coding.html
 
 
 Public key crypotography: RSA
    public key - shared with everyone
    private key - not shared at all
    
 Alice sends a message to Bob encrypted with Bob's public
    key. (public key alone is not enough to decrypt)
    
 Bob decrypts the message with his private key.