By using SSSS we distribute the key to "N" people where any "k" (N>=k) are required to participate to unlock the code. But what if I wish to have the people "x" and "y" always be part of "k". Any provisions for that??

The classic definition of a PRP includes efficient invertibility. Given that many modern cipher modes (CTR-based e.g. GCM) use only the forward direction of the block cipher, it seems that the efficient invertibility part of the definition is not actually necessary for practical purposes. Would s...