BitTorrent Sync has received an end -of-2013 update — adding an often requested feature and several important fixes.
The latest update adds a 4-digit passcode lock protection (currently available for English language users only), to help protect your data from falling into the wrong hands.
It also fixes several issues relating to the the disappearance of some folders and reappearance of deleted files, as well as eliminating a few small bugs and generally upping stability.
Source: iTunes