encryption of SSRS subscription emails and attachments
Not out of the box as far as I am aware, and if you need to distribute reports to external parties this is a bad idea as they will all have to install third party encryption software (incurring licensing and support costs).  It will also put your help desk people into the job of babysitting all the users as they forget their keys.

You're much better off providing a secure portal (this can be done with Sharepoint if you have this) and emailing out links to the reports.