The discipline relating to the use and development of techniques to encrypt and decrypt messages is called cryptography. The process of encryption entails taking a message and changing it to hide the original meaning from everybody but the intended recipient(s). Substitution Ciphers are based on the principle of replacing each character with another character in order to hide the actual meaning of the message. A common simple cipher, which can be found on some UNIX systems, is ROT13. International Data Encryption Algorithm (IDEA), like Data Encryption Standard, is a block cipher operating on 64-bit blocks of plaintext which uses the same algorithm for both encryption and decryption. Public Key or asymmetric cryptosystems differ from symmetric cryptosystems in that they use different keys for decryption and encryption. A digital signature is completely analogous to a handwritten signature used for centuries to authenticate documents. Another digital signature algorithm is ESIGN.