Marshal converts a jwk encodec ECDH public key into the form specified in section 4.3.6 of ANSI X9.62. (This is the format go-ipfs uses)
function marshalPublicKey (jwk) {
  const byteLen = curveLengths[jwk.crv]

  return Buffer.concat([
    Buffer.from([4]), // uncompressed point
    toBn(jwk.x).toArrayLike(Buffer, 'be', byteLen),
    toBn(jwk.y).toArrayLike(Buffer, 'be', byteLen)
  ], 1 + byteLen * 2)
}