[Orgmode] alter all subtrees containing specific tag

[news]

Hi,
   I use pgg to encrypt some org subtrees (e.g. the ones containing
   passwords). At the moment I have to encrypt/decrypt them one at a
   time by selecting the appropriate region and running
   pgg-encrypt-region/pgg-decrypt-region. 
   I want to make a function that will automatically encrypt/decrypt all
   subtrees (in the current buffer) whose headings contain an :ENCRYPT: tag.
   I tried using org-map-entries, but it seems to restore the buffer to
   its original state after returning.
   Is there a quick way to get the start and end points of a subtree, or
   place region around it? and a quick way to jump to the next heading
   with a given tag?

-- 
aleblanc