This lecture explores public key cryptosystems (PKC). It contrasts the asymmetric ciphers of PKC with the symmetric ciphers discussed in the previous lecture. It explores how RSA encryption works and considers some of the ways in which it can be attacked. It highlights the relative slowness of PKC and how this limits its application to the encryption of small things such as symmetric cipher keys and hashes (the latter providing a digital signature for message authentication). The lecture concludes with a look at the issue of public key trust, and the role of certificates and certificate authorities in achieving this.