, just like a CodE
, also replaces a piece of information (an element of the plaintext that may consist of a letter or word or string of symbols) with another object. The difference is that the replacement is made according to a rule defined by a secret key known only to the transmitter and legitimate receiver(s) in the expectation that an outsider, ignorant of the key, will not be able to invert the replacement to decrypt the cipher. In the past, the blurring of the distinction between codes and ciphers was relatively unimportant. In contemporary communications, however, information is frequently both encoded and encrypted so that it is important to understand the difference. A satellite communications link, for example, may encode information in ASCII characters if it is textual, or pulse-code modulate and digitize it in binary-coded decimal form if it is an analog signal such as speech. It then encrypts the resulting coded data into ciphers by using the DataEncryptionStandard
(DES). Finally, the CipheR
stream itself is encoded again, using error-correcting codes for transmission from the ground station to the orbiting satellite. These operations are undone, in reverse order, by the intended receiver to recover the original information.