Post-recorded podcast of the lecture. (I forgot to press "record" at the start of the live lecture, so this is a recording of a repeat run given without any audience. )
Handwritten notes from the lecture. (The handwritten notes from the podcast are available here.)
Waiting room video: interview with Olga Paris-Romaskevich.

Explained the RSA public key cryptosystem.

For more background on the RSA cryptosystem take a look at the wikipedia page here.

It is worth considering, with the benefit of hindesight, the following quote

"both Gauss and lesser mathematicians may be justified in rejoicing that there is one science at any rate [number theory], and that their own, whose very remoteness from ordinary human activities should keep it gentle and clean."

from G.H. Hardy's A Mathematician's Apology. This short book is well worth a read and is available online here.