Chapter 4.1,4.2,4.4 for lecture 21 September

1. The way to encrypt using DES was difficult to understand. It seems like something that I will need to work out a few times before I will really know how to use it and see how it works. Section 4.1 was helpful for understanding 4.4.

2. This has definitely been the most complicated chapter of the course so far. However a secure cryptosystem must not be easy to break, so it seems likely that further cryptosystems will be more complicated than the ones discussed in chapter 2.