A fascinating tidbit in the Washington Post’s latest story on NSA leaker Edward Snowden: Snowden wanted the Washington Post to publish online a cryptographic key that he could use to prove to a foreign embassy that he was the source of the documents: Code Name ‘Verax’: Snowden, in Exchanges With Post Reporter, Made Clear He Knew Risks.

He alluded to other options, aware that he had secrets of considerable financial value, but said, “I have no desire to provide raw source material to a foreign government.”

To effect his plan, Snowden asked for a guarantee that The Washington Post would publish — within 72 hours — the full text of a PowerPoint presentation describing PRISM, a top-secret surveillance program that gathered intelligence from Microsoft, Facebook, Google and other Silicon Valley giants. He also asked that The Post publish online a cryptographic key that he could use to prove to a foreign embassy that he was the document’s source.

I told him we would not make any guarantee about what we published or when. (The Post broke the story two weeks later, on Thursday. The Post sought the views of government officials about the potential harm to national security prior to publication and decided to reproduce only four of the 41 slides.)

Snowden replied succinctly, “I regret that we weren’t able to keep this project unilateral.” Shortly afterward he made contact with Glenn Greenwald of the British newspaper the Guardian.