Encryption

encryption

  <algorithm, cryptography> Any procedure used in {cryptography}
  to convert {plaintext} into {ciphertext} in order to prevent
  any but the intended recipient from reading that data.

  There are many types of data encryption, and they are the
  basis of network security.  Common types include {Data
  Encryption Standard} and {public-key encryption}.

  The Unix command crypt performs encryption.

--> PHP GPG

How to create encrypted file systems on Linux:

http://weigand.home.texas.net/efs.html

http://un.bewaff.net/

http://koeln.ccc.de/archiv/drt//crypto/linux-disk.html

General cryptography:

http://www.cryptographyworld.com