Jon Callas, a cryptographer and security expert who co-founded the widely-used email encryption software PGP, is now working at Apple -- again.

It will be Callas' third time at the company after he worked there in the 1990s, and later between 2009 and 2011, when he developed full-disk encryption for Macs and paved the way for government certifications for iPhones.

Reuters was first to report the story.

Apple did not return an email when we asked about his new role. A separate email to Callas was not immediately returned.

According to his LinkedIn profile, Callas was said to be in "personal stealth mode" since last month. He most recently worked at Silent Circle, a security software maker, where he spent much of his time working on the company's Blackphone project.

What Callas will be doing at Apple remains a mystery -- except it's almost certainly something to do with crypto given the hostilities between the US government and the iPhone maker in recent months over the use of unbreakable encryption.

A fervent advocate of cryptography and strong encryption, Callas has spent most of his adult life protecting users from hackers, governments, but also more often than not, themselves.

When we spoke to him earlier this year -- albeit prior to the legal brouhaha between Apple and the FBI and his appointment, Callas attributed some of the privacy "push back" by Silicon Valley giants to the brightest minds who now work there.

He said in late January on the phone that as advertising networks stand as the "biggest privacy problem today" and not governments, encryption is more important than ever to keep personal data safe, secure, and private.

He said that everyone deserves strong encryption, regardless of whether they have something to hide or not -- a reference to a long-standing argument by those who are in favor of government surveillance.

"The 'nothing to hide argument' is -- there's no other word than bulls**t," he said. "Everyone has something to hide. There are a lot of people who don't necessarily have something to hide but they don't want their church to know that they play poker, or they drink wine, or don't necessarily want the people they work with to know who they vote for."

"There is an appropriateness of conversation we all have," he said.

Callas' other comments may give some speculative insight as to what he's working on at Apple.

He said that Apple's proprietary messaging service iMessage, used by iPhones and iPads, which allows users to send end-to-end encrypted messages -- much to the chagrin of the government, is "extraordinarily good but there are some limitations." That echoes similar sentiments by security experts, who have long said that iMessage is one of the best services going for the mass market, despite its occasional flaws.

But it's those smaller flaws that still punch a giant hole in the security of the system.

A flaw found in 2013 could have let Apple (and an adversary) read messages if subject to a government order. A separate flaw this year showed how researchers could decrypt some data sent by iMessage but stored in Apple's cloud. Both flaws were swiftly fixed.

Callas' skill set and years of experience shows that encryption is not only timeless, but now going mainstream.

"Crypto isn't magic pixie dust," he said. "You can't just sprinkle crypto on something and make it secure and private."

Whatever Callas is doing at Apple, you can bet millions of people will benefit from it down the line.