Going by [1] it seems like PGP.
"Snowden only wanted to communicate securely using PGP encryption, for which Greenwald didn’t have the proper software installed at the time."
In an interview with The Huffington Post, Greenwald acknowledged that he's no expert in using such technology and said that Snowden even provided a step-by-step email and video to help secure their communication.
I wish Greenwald would publish that step-by-step email. It's probably one of the most valuable HOWTOs ever written, because only Snowden (and his colleagues) know for a fact what steps are NSA-proof.
It could be minimum key size and such, or potentially a specific older version of GPG if NSA ever managed to get something into the codebase, but I suspect it's simple.
Glenn Greenwald is really not very technical at all, so I imagine it was really about getting GPG installed and an appropriate plug-in for his mail client. I actually bet most of the e-mail was spent explaining why he needed to do this, because even now people are still reluctant to use GPG. There's been no explosion in it's use. People are still lazy.
I wish Greenwald would publish that step-by-step email. It's probably one of the most valuable HOWTOs ever written, because only Snowden (and his colleagues) know for a fact what steps are NSA-proof.