History

 Cryptographical methods used to come in these kinds:
   - substitution ciphers
   - transposition ciphers
   - codebooks

 Breaking them typically relied on letter frequency tables,
 word frequency tables, and guessing.  

These days

 * symmetric key algorithms (DES, Solitaire)
   Encrypt and decrypt using the same key.  
   Either in blocks or stream.  Usually pretty fast.

 * asymmetric key alrogithms (RSA, PGP)
   Two different keys on the encrypt/decrypt ends.
   Public/private keys, based on big prime numbers.

 * hashing (crypt)
   One way summary (encryption)

We discussed these in class on Nov 7.

Think of the message as a number M (a *big* number) and the encoded
message as another number C=f(M,k1) for some function f,
where k1 is some kind of secret key.  Then decoding is 
some other function M=fInverse(C,k2) where k2 is some other
function 

The issues are
    - How much shared information does the sender/receiver need
      to have in common?  (Hard copy code books?  The algorithm?
      A "key"?)
    - How hard is to encrypt a message?
    - How hard is it to decrypt a message?
    - How hard is it for a 3rd party to break?

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Links

* Encryption tutorial (PGP, hashes)
  http://hotwired.lycos.com/webmonkey/programming/php/tutorials/tutorial1.html

* Cryptonomicon http://www.cryptonomicon.com
  - http://www.well.com/user/neal/cypherFAQ.html

* Solitaire - Bruce Schneier http://www.counterpane.com/solitaire.html

* 636 PDF slides (!) http://www.cs.auckland.ac.nz/~pgut001/tutorial/



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Other methods

 * Hiding text inside images and multimedia
   * search "digital watermarks" on google
   * "Data Hiding in Digital Binary Images"
      http://www.ee.princeton.edu/~minwu/research/icme00_binaryDH.html
   * Hide and recover encrypted in GIF files with Stego.
      http://www.stego.com