Combining Block Ciphers
Bruce Schneier · 2015
This chapter discusses the different ways to combine block algorithms to get new algorithms. The impetus behind these schemes is to try to increase security without going through the trouble of designing a new algorithm. Multiple encryption is one combination technique: using an algorithm to encrypt the same plaintext block multiple times with multiple keys. Cascading is like multiple encryption, but uses different algorithms. There are other techniques. Encrypting a plaintext block twice with the same key, whether with the same algorithm or a different one, is not smart. For the same algorithm, it does not affect the complexity of a brute-force search. For different algorithms, it may or may not. If people are going to use any of the techniques in this chapter, they make sure that the multiple keys are different and independent.