DNS provides a solution to this problem. DNS is already used as a ubiquitous lookup service to look up mail exchangers and their IP addresses, something all email application need to do. Thus DNS is already an integral part of email appli- cations today. The costs of adding support in the application to be able to look up other data are small. And unlike LDAP, additional configuration by the user will not be required. We have implemented S/MIME functionality in a email client, with certificate fetching from DNS, as a proof of concept. The implementation is based on the ex- tensible mail client Gnus [90]. The following figures.
