Encryption algorithms can be divided into block ciphers and stream ciphers. Stream ciphers take plain text input one bit (or one byte) at a time, and output a corresponding cipher text bit (byte) right away. The manner in which a bit (byte) is encrypted will depend both upon the key used and upon the encryption of the plain text stream leading up to this bit (byte).

In contrast to stream ciphers, block ciphers require an entire block of plain text input before they will perform any encryption (typically blocks are 64-bits or more). In addition, given an identical plain text input block, and an identical key, a block cipher will produce the same cipher text no matter where in an input stream it is encountered.

Although stream ciphers have some advantages where immediate responses are required -- for example on a socket -- the majority of widely-used modern encryption algorithms are block ciphers. (In this tutorial, whenever symmetric encryption algorithms are discussed generically, the user should assume the tutorial is referring to block ciphers.)