package client

import (
	"encoding/binary"
	"github.com/cbeuw/Cloak/internal/ecdh"
	"github.com/cbeuw/Cloak/internal/util"
	"io"
	"sync/atomic"
)

const (
	UNORDERED_FLAG = 0x01 // 0000 0001
)

type authenticationPayload struct {
	randPubKey        [32]byte
	ciphertextWithTag [64]byte
}

// makeAuthenticationPayload generates the ephemeral key pair, calculates the shared secret, and then compose and
// encrypt the authenticationPayload
func makeAuthenticationPayload(sta *State, randReader io.Reader) (ret authenticationPayload, sharedSecret []byte) {
	/*
		Authentication data:
		+----------+----------------+---------------------+-------------+--------------+--------+------------+
		|  _UID_   | _Proxy Method_ | _Encryption Method_ | _Timestamp_ | _Session Id_ | _Flag_ | _reserved_ |
		+----------+----------------+---------------------+-------------+--------------+--------+------------+
		| 16 bytes | 12 bytes       | 1 byte              | 8 bytes     | 4 bytes      | 1 byte | 6 bytes    |
		+----------+----------------+---------------------+-------------+--------------+--------+------------+
	*/
	ephPv, ephPub, _ := ecdh.GenerateKey(randReader)
	copy(ret.randPubKey[:], ecdh.Marshal(ephPub))

	plaintext := make([]byte, 48)
	copy(plaintext, sta.UID)
	copy(plaintext[16:28], sta.ProxyMethod)
	plaintext[28] = sta.EncryptionMethod
	binary.BigEndian.PutUint64(plaintext[29:37], uint64(sta.Now().Unix()))
	binary.BigEndian.PutUint32(plaintext[37:41], atomic.LoadUint32(&sta.SessionID))

	if sta.Unordered {
		plaintext[41] |= UNORDERED_FLAG
	}

	sharedSecret = ecdh.GenerateSharedSecret(ephPv, sta.staticPub)
	ciphertextWithTag, _ := util.AESGCMEncrypt(ret.randPubKey[:12], sharedSecret, plaintext)
	copy(ret.ciphertextWithTag[:], ciphertextWithTag[:])
	return
}