Expose primeOrder

[jeffallen]

From dedis/onet#256: Would it be possible to add a function to get the cardinality of the elliptic curve that is used ?

[jeffallen]

David says: I would just need to be able to access primeOrder and primeOrderScalar or just (primeOrderScalar) that are in group/edwards25519/const.go. so maybe just an uppercase ? 🙂

[nikkolasg]

I would maybe suggest that david makes a copy of that variable inside his own code if "Only wants that value" and not the cardinality for all possible groups. Only a suggestion...

[ineiti]

In Javascript and Java I had to access that value for different reasons, so it is definitively something that will be used if it's exposed. Can we put that into some generic parameter-readonly field or a method?

[bford]

It might be worth adding an (optional) interface somewhere by which a caller can query multiple curve parameters that may be of interest in different circumstances, such as (a) the curve order, (b) the order of the underlying field used in the curve's representation, (c) the large prime and cofactor (possibly 1) comprising the curve's order.

[jeffallen]

It is not clear enough what we expect here, and there are easy workarounds if David needs them.
So not going in kyber v1.

[pierluca]

It seems that the use cases discussed in this issue have found workarounds.

@si-co do you think we should leave this open, or shall we close it as part of the ongoing clean-up ?

[si-co]

The optional interface proposed by @bford would be useful and not difficult to implement, but since there are workarounds and given the multiple urgent issues in the repo I'd close (for now).