package pnyx

import (
	"crypto/ed25519"
	"crypto/rand"
	"crypto/sha512"
	"fmt"

	"filippo.io/edwards25519"
)

func NewSessionOpen(key ed25519.PrivateKey) ([]byte, ed25519.PrivateKey, error) {
  if key == nil {
    return nil, nil, fmt.Errorf("Cannot create a SESSION_OPEN packet without a key")
  }

  public, private, err := ed25519.GenerateKey(rand.Reader)
  if err != nil {
    return nil, nil, fmt.Errorf("Failed to generate ecdh key: %w", err)
  }

  packet := make([]byte, SESSION_OPEN_LENGTH)
  cur := 0

  copy(packet[cur:], []byte(key.Public().(ed25519.PublicKey)))
  cur += PUBKEY_LENGTH

  copy(packet[cur:], []byte(public)) 
  cur += PUBKEY_LENGTH

  signature := ed25519.Sign(key, packet[:cur])
  copy(packet[cur:], signature)
  cur += SIGNATURE_LENGTH

  return packet, private, nil
}

func ParseSessionOpen(session_open []byte) (ed25519.PublicKey, ed25519.PublicKey, error) {
  if len(session_open) != SESSION_OPEN_LENGTH {
    return nil, nil, fmt.Errorf("Bad SESSION_OPEN length: %d/%d", len(session_open), SESSION_OPEN_LENGTH)
  }

  cur := 0

  client_pubkey := (ed25519.PublicKey)(session_open[cur:cur+PUBKEY_LENGTH])
  cur += PUBKEY_LENGTH

  client_ecdh := (ed25519.PublicKey)(session_open[cur:cur+PUBKEY_LENGTH])
  cur += PUBKEY_LENGTH
  
  digest := session_open[:cur]
  signature := session_open[cur:cur+SIGNATURE_LENGTH]
  cur += SIGNATURE_LENGTH

  if ed25519.Verify(client_pubkey, digest, signature) == false {
    return nil, nil, fmt.Errorf("SESSION_OPEN signature verification failed")
  }

  return client_pubkey, client_ecdh, nil
}

func ECDH(public ed25519.PublicKey, private ed25519.PrivateKey) ([]byte, error) {
  public_point, err := (&edwards25519.Point{}).SetBytes(public)
  if err != nil {
    return nil, err
  } 


  h := sha512.Sum512(private.Seed())
  private_scalar, err := (&edwards25519.Scalar{}).SetBytesWithClamping(h[:32])
  if err != nil {
    return nil, err
  }

  shared_point := public_point.ScalarMult(private_scalar, public_point)

  return shared_point.BytesMontgomery(), nil
}