Caveat: I am not a lawyer or legal expert.
You should not have to encrypt the assets. I don't see anything in the licence that stipulates encryption be used.
Besides, that would only increase the difficulty and not out-right prevent stealing. Ultimately if someone wants to steal those assets they will find a way.
You may be referring to clause 4.2.c which states that the licence does not permit you to allow your users to extract the assets from your product. My interpretation of this is that you are fine as long as you do not facilitate extraction and do not say anywhere that it's ok if they take it. The licence is not asking you to actively prevent extraction, it is simply saying it does not allow you to allow your users to extract the assets.
If you're worried, an easy way of explicitly complying with that requirement is to leave a text file in there saying "Users of this product are not allowed to take these assets". I would usually include the license itself alongside the assets (many licenses actually require this anyway), which should completely cover you.