Cryptography

• The "Caesar" code: shift (mod 26). Just 26 keys, one for each letter of the alphabet.
  
  
• The permutation code. There are 26!, or about 400 million billion billion keys ...

References:

1. Childs, Lindsay N.: A Concrete Introduction to Higher Algebra, 2nd edition (Springer, 1995).
   
   
2. Singh, Simon: The Code Book (Anchor Books, 1999).
   
   
3. Stinson, Douglas R.: Cryptography, Theory and Practice (CRC 1995).