"When the NSA and GCHQ compromised the security of potentially billions of phones (3g/4g encryption relies on the shared secret resident on the sim)," Snowden wrote in the AMA, "they not only screwed the manufacturer, they screwed all of us, because the only way to address the security compromise is to recall and replace every SIM sold by Gemalto."

Gemalto is one of the leading makers of SIM cards used in billions of mobile phones around the world to secure the communications of telecom customers of AT&T, T-Mobile, Verizon, Sprint and more than 400 other wireless carriers in 85 countries. Stealing the crypto keys essentially allows the spy agencies to wiretap and decipher encrypted phone communications at will without the assistance of telecom carriers or the oversight of a court or government. The keys also allow the agencies to decrypt previously intercepted messages they hadn't been able to crack.

But in stealing the keys with the aim of targeting the communications of specific customers, the spy agencies undermine the security of billions of other customers.