Cryptography at Khan Academy

http://www.youtube.com/watch?v=Kf9KjCKmDcU&feature=share&list=EC7AEDF86AABA1AA9A

Khan Academy - Cryptography playlist 

Fundamental theorem of arithmetic - Composite numbers have a unique prime factorization.  

Substitution  Cipher (example Caesar Cipher - actually used by Julius Caesar) Fixed shift of every letter - Broken based on the frequency of the Letters in the English language. 

Polyalphabetic Ciphers - Shift based on a password. 

One Time Pad - random shifts for every letter based on a key - The encrypted message has equal letter frequencies. So code cannot be broken just based on the Cipher Text.