I'm looking to use Drupal to pull in data through forms etc, probably in the form of content types, and then expose these, probably via REST, to other things. I've had a look at the services module, but I'm concerned about securing this, and am going to be using certificates to do so. Is there anything out there that currently supports this sort of thing, or am I getting into custom made module territory here?
Services has a pluggable architecture for authentication, so if there's nothing that supports certificate authentication, all you need to write yourself is such a plugin.
That is basically responsible for mapping that certificate to a Drupal user and then everything after that will check that this user has permissions to access whatever entity/content it requested.
Drupal Module as web service with SSL looks like a related question but doesn't have any answers.