Why are we returning here a list of cleartexts, when message_kit always corresponds to a single message?
I know that Bob.retrieve() returns a list of cleartexts, but only because it's associated to WorkOrders, which may have more than one capsule (although currently they don't).
This may not be an issue to solve here, but perhaps we should consider a better API for retrieve/decrypt methods: it doesn't make any sense to have methods that accept only a single capsule, but that output a list of cleartexts (of length 1). Either we make the input also a list, or we return the only cleartext.

Originally posted by @cygnusv in #1031