Consider using keyed Blake instead of prepend the tag?

[AnomalRoil]

Currently we prepend a fixed tag to our mesages when we are hashing data:

kyber/encrypt/ibe/ibe.go

Lines 132 to 138 in a780ab2

	h3, err := blake2s.NewXOF(maxSize, nil)
	if err != nil {
	panic(err)
	}
	if _, err := h3.Write(H3Tag()); err != nil {
	return nil, fmt.Errorf("err hashing h3 tag: %v", err)
	}

I think we should consider using keyed Blake instead of prepend the tag. It would be as simply as replacing the nil in the Blake2s instantiation with the H3Tag.

@nikkolasg It would simplify a bit the code and also, keyed hash functions are there exactly for this usecase, no?

[nikkolasg]

I'd like to have the possibility to run any hash function and methods we want, extract this bit into a generic or something or config, so we can change it - it's already an issue here #29
Here's blake2s but if we want to use this in a solidity context, I think we might want to go back to like sha256 or keccak and keep the simple "tags" to differentiate the "hash functions" as in the paper.
Maybe something like

IbeHashing interface {
   H1() hash.Hash
   H2() hash.Hash
   H3() hash.Hash
}

[AnomalRoil]

Solved in #34 I'd say.