It's called steganography, defined as "the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message." Over the past decade or more, it's been a trick commonly used by terrorists and criminals as well, other CIA veterans say.
But CIA headquarters managers were wary of supplying it to the field. Possession of specialized steganography software would be every bit as incriminating as a radio set during World War Two and the Cold War.
"The agency fought back against the field ops officers on this, because if one asset got compromised, then that particular version of the software would have been assumed to be compromised and have to be removed" from circulation, the former operative continued.
So the field operatives did a work-around, this person said, "by asking their assets to give them their passwords for their work or personal e-mail accounts. Then the asset would write up a phony e-mail and leave it in ‘drafts,' perhaps with a document attached. The ops officer would log on remotely, collect the document, and then debrief the asset about it when they met in person."
But why not use at least some commonly available commercial encryption software, such as PGP (Pretty Good Privacy) to further obscure the messages and documents?
Because a spy -- and above all a spy chief -- would have a hard time explaining why he was using encrypted email out of approved channels.
"Using PGP was also an indication that you were using spy gear," the former operative said. Foreign security services constantly scan email traffic looking for encrypted messages, he said. So does the FBI and NSA, who have the tools to break it.
As for Petraeus, his agency-approved and encrypted message channels are monitored -- he couldn't risk tapping mash notes via those. But he would be trusted not to use his personal email to discuss business. And according to news accounts, no one would have paid attention to his personal accounts had his girlfriend not started sending harassing emails to a perceived rival, who then called a male friend in the FBI.
The shared Gmail account Petraeus and Broadwell used on their home and office computers was good enough to hide evidence of their liaison from coworkers and unsuspecting spouses, another operative points out, but not much else.
"Once law enforcement zeroes in on you, if you're using your home computer, it doesn't matter what technique you use," he said. The origin of the Gmail is the smoking gun.