Chapter 4. Modes of Operation


We’ve covered how to encrypt a 64-bit block with DES or IDEA, or a 128-bit block with AES, but that doesn’t really tell how to use these algorithms. For instance, sometimes messages to be encrypted don’t happen to be exactly one block long. We also mentioned in §2.4.5 Integrity Check that it was possible, using a secret key encryption scheme, to generate a MAC (message authentication code). This chapter will attempt to tie up these various loose ends.

Encrypting a Large Message

How do you encrypt a message larger than 64 bits? There are several schemes defined in [DES81]. These schemes would be equally applicable to IDEA or any secret key scheme that encrypted fixed-length blocks, and no doubt one could ...

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