The Modelling and Analysis of Security Protocols: the CSP Approach

0.5. Encryption modes

Block ciphers can be used in a number of ways or modes. The simplest is the electronic code book mode in which each block is enciphered separately and in turn. This has the merit of being straightforward but does suffer from the drawback of not providing any linkage between successive blocks. As a result, it is comparatively easy to delete, modify or re-order blocks, insert fake blocks, and so on.

Of course for short messages that fit in a single block this is really not an issue. Such messages do occur, particularly if the data is, say, a key or nonce: this might well comprise 128 bits, so if the cipher acts on 128-bit blocks then it is fine. However, many messages will span blocks and so these dangers appear. To foil ...