'use strict'

const { Ber } = require('asn1')
const { Buffer: Asn1Buffer } = require('safer-buffer')
const { Buffer: RuntimeBuffer } = require('buffer')

const X25519_OID = '1.3.101.110'

function getNacl() {
  return require('tweetnacl')
}

/** @param {import('safer-buffer').Buffer | Uint8Array} buf */
function u8(buf) {
  const b = Asn1Buffer.from(buf)
  return new Uint8Array(b.buffer, b.byteOffset, b.byteLength)
}

function berSpkiX25519(pub32) {
  const pub = Asn1Buffer.from(pub32)
  const w = new Ber.Writer()
  w.startSequence()
  w.startSequence()
  w.writeOID(X25519_OID)
  w.endSequence()
  w.startSequence(Ber.BitString)
  w.writeByte(0x00)
  w._ensure(pub.length)
  w._buf.set(pub, w._offset)
  w._offset += pub.length
  w.endSequence()
  w.endSequence()
  return w.buffer
}

function berPkcs8X25519(priv32) {
  const priv = Asn1Buffer.from(priv32)
  const w = new Ber.Writer()
  w.startSequence()
  w.writeInt(0x00, Ber.Integer)
  w.startSequence()
  w.writeOID(X25519_OID)
  w.endSequence()
  w.startSequence(Ber.OctetString)
  w.writeBuffer(priv, Ber.OctetString)
  w.endSequence()
  w.endSequence()
  return w.buffer
}

/** Match bare-ssh2 kex: SPKI DER ends with 32-byte X25519 public key. */
function parseSpkiX25519PubDer(der) {
  const b = Asn1Buffer.from(der)
  if (b.length < 32) throw new Error('Invalid X25519 SPKI DER')
  return u8(b.slice(-32))
}

function parsePkcs8X25519PrivDer(der) {
  const { Ber: BerR } = require('asn1')
  let reader = new BerR.Reader(Asn1Buffer.from(der))
  reader.readSequence()
  if (reader.readInt() !== 0) throw new Error('Invalid PKCS#8 version')
  reader.readSequence()
  if (reader.readOID() !== X25519_OID) throw new Error('Invalid X25519 PKCS#8 OID')
  reader = new BerR.Reader(reader.readString(BerR.OctetString, true))
  return u8(reader.readString(BerR.OctetString, true))
}

class X25519PublicKey {
  /** @param {Uint8Array | import('safer-buffer').Buffer} pub32 */
  constructor(pub32) {
    this.type = 'public'
    this.asymmetricKeyType = 'x25519'
    this._pub = Asn1Buffer.from(pub32)
    if (this._pub.length !== 32) throw new Error('X25519 public key must be 32 bytes')
  }

  /** @param {{ type: string, format: string }} opts */
  export(opts) {
    if (!opts || opts.format !== 'der') throw new Error('unsupported export format')
    if (opts.type === 'spki') return berSpkiX25519(this._pub)
    if (opts.type === 'raw') return Asn1Buffer.from(this._pub)
    throw new Error('unsupported export type')
  }
}

class X25519PrivateKey {
  /**
   * @param {Uint8Array | import('safer-buffer').Buffer} priv32
   * @param {Uint8Array | import('safer-buffer').Buffer | null} pub32
   */
  constructor(priv32, pub32) {
    this.type = 'private'
    this.asymmetricKeyType = 'x25519'
    this._priv = Asn1Buffer.from(priv32)
    if (this._priv.length !== 32) throw new Error('X25519 private key must be 32 bytes')
    this._pub = pub32 ? Asn1Buffer.from(pub32) : null
  }

  /** @param {{ type: string, format: string }} opts */
  export(opts) {
    if (!opts || opts.format !== 'der') throw new Error('unsupported export format')
    if (opts.type === 'pkcs8') return berPkcs8X25519(this._priv)
    throw new Error('unsupported export type')
  }
}

function generateKeyPairSync() {
  const nacl = getNacl()
  const pair = nacl.box.keyPair()
  return {
    publicKey: new X25519PublicKey(pair.publicKey),
    privateKey: new X25519PrivateKey(pair.secretKey, pair.publicKey)
  }
}

/** @param {import('safer-buffer').Buffer} der */
function createPublicKeyFromSpkiDer(der) {
  return new X25519PublicKey(parseSpkiX25519PubDer(der))
}

/**
 * @param {{ privateKey: unknown, publicKey: unknown }} opts
 * @returns {import('safer-buffer').Buffer}
 */
function diffieHellman(opts) {
  const nacl = getNacl()
  let privU8
  let pubU8
  const sk = opts.privateKey
  const pk = opts.publicKey
  if (sk instanceof X25519PrivateKey) privU8 = u8(sk._priv)
  else if (sk && typeof sk.export === 'function')
    privU8 = parsePkcs8X25519PrivDer(sk.export({ type: 'pkcs8', format: 'der' }))
  else throw new Error('diffieHellman: unsupported private key')

  if (pk instanceof X25519PublicKey) pubU8 = u8(pk._pub)
  else if (pk && typeof pk.export === 'function')
    pubU8 = parseSpkiX25519PubDer(pk.export({ type: 'spki', format: 'der' }))
  else throw new Error('diffieHellman: unsupported public key')

  const shared = nacl.scalarMult(privU8, pubU8)
  // Use runtime `buffer` (Bare node-buffer) so kex `convertToMpint` / `hash.update` agree on `Buffer.isBuffer`.
  return RuntimeBuffer.from(shared)
}

module.exports = {
  generateKeyPairSync,
  createPublicKeyFromSpkiDer,
  diffieHellman,
  X25519PublicKey,
  X25519PrivateKey
}