Callas has worked for Apple twice before, first from 1995 to 1997 and again from 2009 to 2011. During the second period, he oversaw the upgrades to FileVault in OS X Lion, the first full-disk encryption system to cover the computer’s entire home directory.
It is still unclear as to what Callas will be working on at Apple, but the move underscores Apple’s commitment to disk encryption on its devices, a feature that was at the center of the San Bernardino case earlier this year.
Apple is presumably looking to strengthen the encryption used in OS X, iOS, and/or iCloud. Although those products are already thoroughly encrypted, the company has become increasingly concerned about hacking attempts by criminals, spy agencies, and other parties, whether foreign or domestic.
Callas is best known for his work on the pioneering PGP encryption system, including authoring the openPGP standard for IETF and developing PGP’s universal server software. He co-founded the secure-messaging system Silent Circle in 2012.