package tunnel

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"

	"github.com/oxtoacart/bpool"
)

var (
	// the local buffer pool
	// gcmStandardNonceSize from crypto/cipher/gcm.go is 12 bytes
	// 100 - is max size of pool
	noncePool = bpool.NewBytePool(100, 12)
)

// hash hahes the data into 32 bytes key and returns it
// hash uses sha256 underneath to hash the supplied key
func hash(key []byte) []byte {
	sum := sha256.Sum256(key)
	return sum[:]
}

// Encrypt encrypts data and returns the encrypted data
func Encrypt(data []byte, key []byte) ([]byte, error) {
	// generate a new AES cipher using our 32 byte key
	c, err := aes.NewCipher(hash(key))
	if err != nil {
		return nil, err
	}

	// gcm or Galois/Counter Mode, is a mode of operation
	// for symmetric key cryptographic block ciphers
	// - https://en.wikipedia.org/wiki/Galois/Counter_Mode
	gcm, err := cipher.NewGCM(c)
	if err != nil {
		return nil, err
	}

	// get new byte array the size of the nonce from pool
	// NOTE: we might use smaller nonce size in the future
	nonce := noncePool.Get()
	if _, err = rand.Read(nonce); err != nil {
		return nil, err
	}
	defer noncePool.Put(nonce)

	// NOTE: we prepend the nonce to the payload
	// we need to do this as we need the same nonce
	// to decrypt the payload when receiving it
	return gcm.Seal(nonce, nonce, data, nil), nil
}

// Decrypt decrypts the payload and returns the decrypted data
func Decrypt(data []byte, key []byte) ([]byte, error) {
	// generate AES cipher for decrypting the message
	c, err := aes.NewCipher(hash(key))
	if err != nil {
		return nil, err
	}

	// we use GCM to encrypt the payload
	gcm, err := cipher.NewGCM(c)
	if err != nil {
		return nil, err
	}

	nonceSize := gcm.NonceSize()

	if len(data) < nonceSize {
		return nil, ErrDecryptingData
	}

	// NOTE: we need to parse out nonce from the payload
	// we prepend the nonce to every encrypted payload
	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
	ciphertext, err = gcm.Open(ciphertext[:0], nonce, ciphertext, nil)
	if err != nil {
		return nil, err
	}

	return ciphertext, nil
}