文档介绍：The symmetric key algorithms fall into two categories: ? Block ciphers: ? Operate on 64-bit message blocks. Even though most of the block ciphers operate on a 64-bit block, it is not an absolute requirement. ? Stream ciphers: ? Operate on a stream of data, which basically means they operate on a byte ata time. Chaining Mechanics in Block Cipher In the block ciphers, it is necessary to break up larger messages into 64-bit blocks and somehow chain them together. mon chaining mechanisms called modes exist, and each mode defines a method bining the plain text (the message that is not encrypted), the secret key, and the cipher text (the encrypted text) to generate the stream of cipher text that is actually transmitted to the recipient. These four modes are as follows: 1. Electronic codebook (ECB) The ECB chaining mechanism encodes each 64-bit block independently but uses the same key. The result is that the same plain text will always result in the same cipher text. This weakness can be exploited in multiple ways. For example, ifa snooper knows the plain text and the corresponding cipher text, that person can at least understand some parts ofa message. Another Vulnerability is the opportunity for an eavesdropper to analyze and perform pattern matching. A much simpler vulnerability is that an eavesdropper can recognize a change of information (when the cipher text changes) and make i