"Various hurdles are preventing wide-spread adoption of PGP encrypted
   communication, with easy searchability being one of them. I propose a
   standard allowing users to search through encrypted email safely. The
   search is tailored for text messages containing distinct words.
   Searching is achieved through a search index in plain-text attached to
   each email. The index contains each of the unique words contained in a
   message, salt hashed. There will be 10-50 salts per email account used
   randomly on a per word basis. Larger amount of salts mean more safety
   and more expensive search. Incoming messages will also get appended
   with a search index using IMAP APPEND command. ..."
   [1]https://ipfs.io/ipfs/QmVntBs8AxmeuLacGappM5LhB8ao9k2RPijvByESt4HmgC

References

   1. https://ipfs.io/ipfs/QmVntBs8AxmeuLacGappM5LhB8ao9k2RPijvByESt4HmgC