Add support for inverting and negating private keys

[kripod]

No description provided.

[jprichardson]

FYI, @fanatid is AFK for 6 more days.

[fanatid]

Looks good on first view, thank you @kripod
The main question, are we need this? Or this can be useful for (from issue #124)? Originally this library is bindings for bitcoin-core/secp256k1 and that library is not export new functions.

Will review when return back, sorry for delay @kripod

[kripod]

Actually, I would need this functionality to implement a variant of mental poker.

[fanatid]

Why we need while loop here? Loop body will be executed 1 time at max..

exports.privateKeyNegate = function (privateKey) {
  var bn = BN.fromBuffer(privateKey)
  if (bn.isZero()) return Buffer.alloc(32)

  if (bn.ucmp(BN.n) > 0) bn.isub(BN.n)
  return BN.n.sub(bn).toBuffer()
}

[kripod]

You are right, the while statement can be changed to an if statement, and the condition can be concatenated with the previous if.

[fanatid]

Changed in b963603

[kripod]

@fanatid Thank you for merging, I would highly appreciate if you could publish a new minor release soon.

[fanatid]

@kripod thank you for PR, will publish new version after CI tests for latest commit :)