Encryption algorithms can be divided into block ciphers and stream ciphers. Stream ciphers are able to 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 the previous plain text stream encrypted leading up to that point.

In contrast to stream ciphers, block ciphers require an entire block of plain text input before they can 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 in cases 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.