Replace complex ec operations with a hash in LNPBP-0004

[afilini]

The fifth step of LNPBP-0004 says that

For each of the slots that remain empty (the slot number is represented by j):

• tweak public key R with it's own hash H(R) j times: Rj = R + J * H(R) * G)
• compute a 256-bit bitcoin hash of Rj and serialize it into the slot j using bitcoin-style hash serialization format.

Where R is entropy * G.

I think we could replace that with a simple SHA256(entropy || j)

[dr-orlovsky]

My consideration: we need to force that the amount of entropy is sufficient. How we can enforce that the size of the hashed entropy would not be lees than some specific level?

[dr-orlovsky]

After the discussion on the today call it was decided to stick to simple deterministic hashes and do not use either EC math or verifiable delay functions