Currently if a versioned object has a field that is an encrypted nested object, that object is also versioned. Examples:

• EncryptedKeyFrag (contains an encrypted AuthorizedKeyFrag)
• EncryptedTreasureMap (contains an encrypted AuthorizedTreasureMap)

This way the structure of the nested object can be changed independently later.

MessageKit, on the other hand, has its plaintext encrypted just as a bytestring - without any structure. If it is conceivable that we may add something to it later (signatures, keys, or whatever), it may be better to make it a versioned object too (even though it might seem trivial right now).