It usually goes something like "mom, what are the two words (short authentication strings) you see on your phone?" Sometimes she even asks me--we make jokes about the combinations.

And yeah, it's true that some of the Snowden docs reference ZRTP attacks & spoofing voice so it's not necessarily foolproof (or her phone could be owned etc.), but another one of the things my mother told me when I was a kid was "the key is to begin." And that's what moxie's done...