Here is a brief summary of the talk I gave at ROUGOL, for the benefit of all those who attended, and also for anyone who missed it.
1. The first part was about the general concepts of cryptography, and in particular symmetric (traditional) cryptography, useful for protecting (encrypting) sensitive data in computers. You can follow a slideshow illustrating the main points that I discussed at Part1. At the bottom of the first page there (you may need to open the page fully to see this) you can click on "Next" to go to the next page. Subsequent pages have similar buttons to move forward or backward.
2. The sequence in Part1 ends with a page showing two of my favourite RISC OS applications for protecting sensitive data, !Q-Lock and !Stealth, which I then demonstrated. I can't demonstrate them here online, but interested readers can download them from http://www.queen.clara.net/pgp/acorn.html and try them for themselves.
3. The final part of my talk concerned public-key cryptography and its practical application in RISC OS computers using GnuPG, with the aid of either a front end or a mail client such as Messenger Pro or Pluto which supports GnuPG. The main ideas are summarised in a second slideshow: Part2.
To save time, I omitted a discussion of why it is not easy to "invert" a public key to obtain the corresponding secret key. Mathematically, this involves a "one-way function", and this concept is illustrated in three images, which would normally follow page 2 of the slide show:
4. One important topic which I did not cover in my talk, to save time, was digital signatures, though I answered a few questions about this briefly at the end. Some information about PGP-signing a message or other data can be found here: https://www.gnupg.org/gph/en/manual/x135.html.
Some information about signing public keys and about the web of trust can be found here: http://www.pgp.net/pgpnet/pgp-faq/pgp-faq-key-signatures.html. That page refers to the use of PGP rather than GnuPG, but the principles are the same.
And finally: If readers have any questions about these topics, or comments or suggestions about any of this material, I can be contacted at n.m.queen [at] bham.ac.uk.