Seismic data encryption - review (chapter 10, Graham Ganssle)

[mycarta]

@gganssle :

I've never done any encryption, but after reading Simon Singh's The Code Book I've developed quite the interest for it. This chapter was a fun read. I loved it. The level is perfect! And great that you enable readers by providing links to the tools.

One thing needs to be fixed: the link for Diskcryptor (third link) is old or broken. Is there a new one, or alternative?
GnuPG link was instead incorrect but I fixed it already .

At 552 words the length is right but has 2 figures so I will leave final say to the expert (@kwinkunks ).