# Nim-Libp2p
# Copyright (c) 2023 Status Research & Development GmbH
# Licensed under either of
#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
#  * MIT license ([LICENSE-MIT](LICENSE-MIT))
# at your option.
# This file may not be copied, modified, or distributed except according to
# those terms.

## This module integrates BearSSL Cyrve25519 mul and mulgen
##
## This module uses unmodified parts of code from
## BearSSL library <https://bearssl.org/>
## Copyright(C) 2018 Thomas Pornin <pornin@bolet.org>.

# RFC @ https://tools.ietf.org/html/rfc7748

{.push raises: [].}

import bearssl/[ec, rand]
import stew/results
from stew/assign2 import assign
export results

const Curve25519KeySize* = 32

type
  Curve25519* = object
  Curve25519Key* = array[Curve25519KeySize, byte]
  Curve25519Error* = enum
    Curver25519GenError

proc intoCurve25519Key*(s: openArray[byte]): Curve25519Key =
  assert s.len == Curve25519KeySize
  assign(result, s)

proc getBytes*(key: Curve25519Key): seq[byte] =
  @key

proc byteswap(buf: var Curve25519Key) {.inline.} =
  for i in 0 ..< 16:
    let x = buf[i]
    buf[i] = buf[31 - i]
    buf[31 - i] = x

proc mul*(_: type[Curve25519], point: var Curve25519Key, multiplier: Curve25519Key) =
  let defaultBrEc = ecGetDefault()

  # multiplier needs to be big-endian
  var multiplierBs = multiplier
  multiplierBs.byteswap()
  let res = defaultBrEc.mul(
    addr point[0],
    Curve25519KeySize,
    addr multiplierBs[0],
    Curve25519KeySize,
    EC_curve25519,
  )
  assert res == 1

proc mulgen(_: type[Curve25519], dst: var Curve25519Key, point: Curve25519Key) =
  let defaultBrEc = ecGetDefault()

  var rpoint = point
  rpoint.byteswap()

  let size =
    defaultBrEc.mulgen(addr dst[0], addr rpoint[0], Curve25519KeySize, EC_curve25519)

  assert size == Curve25519KeySize

proc public*(private: Curve25519Key): Curve25519Key =
  Curve25519.mulgen(result, private)

proc random*(_: type[Curve25519Key], rng: var HmacDrbgContext): Curve25519Key =
  var res: Curve25519Key
  let defaultBrEc = ecGetDefault()
  let len = ecKeygen(
    PrngClassPointerConst(addr rng.vtable), defaultBrEc, nil, addr res[0], EC_curve25519
  )
  # Per bearssl documentation, the keygen only fails if the curve is
  # unrecognised -
  doAssert len == Curve25519KeySize, "Could not generate curve"

  res