I'm a little surprised not to hear any mention of JSON Pointer (RFC 6901). It deals with exactly this:

http://tools.ietf.org/html/rfc6901

With JSON Pointer syntax, it would be something like:

    JWalk.getStringPtr(user, "/them/public_keys/primary/bundle");

It's concise, complete/unambiguous, and has implementations in a growing number of environments, so I think it could be worth mentioning as an approach. It also defines a useful URL fragment syntax for referencing nodes within documents, which would be a good thing for the JSON world.