Related Clues:
- Difficult to understand
- Nothing to write home about?
- Known to few
- Cloak-and-dagger
- Like some documents
- Like some handshakes
- Secret, mysterious
- Mysterious, secret
- It's not to be told
- Confidential, not to be revealed
- Mysterious and secret
- Mysterious or secret
- Abstruse or mysterious
- Private, hidden
- Requiring secret or mysterious knowledge