SK to PK function in API

[CarlBeek]

A function that would be very useful to have specified as a part of the API is SK2PK, basically a function that takes in a secret key and returns a public key.

Such a function would be highly useful for the following purposes amongst others:

• Persons need only store the SK and can derive the PK when needed
• Deterministic Wallets (HD Wallets) which use alternate SK derivation schemes
• Secret sharing schemes that operate over the SK

SK2PK is already implicitly defined in 2.3 KeyGen this is just a presentation of the case for making it an explicit API function by breaking out the following steps into their own function:

   1.  xP = x * P
   2.  PK = point_to_pubkey(xP)
   3.  return PK

[kwantam]

Thanks for the suggestion! This looks like it could be a useful helper function; for example, the definition of PopProve might benefit from it.

I have to admit, I'm not totally convinced that something this low-level needs to be part of the API, but that's more like a nebulous first impression than a reasoned judgment. So: I'll think about it some more!