This strategy is exactly what I was thinking, too. It saves having to deal with two different hashes (which several of the other responders recommended and certainly would work as they offer).

As ikegami notes, however, either this strategy (where only a single hash is used) or the two-hash strategy, only works if it is a true one-to-one mapping of keys to values. If that requirement is met, then it seems to me that neither strategy will work. In that case, I think the op has a whole different challenge.