package encryption

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"errors"
	"io"
)

//16 24 32

// Encrypt uses AES-GCM to encrypt the given plaintext with the given key. The
// plaintext is sealed with a 12-byte nonce, which is prepended to the ciphertext.
func Encrypt(key, plaintext []byte) ([]byte, error) {
	switch len(key) {
	case 16, 24, 32: // AES-128, AES-192, AES-256
	default:
		return nil, errors.New("invalid key length")
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}

	nonce := make([]byte, gcm.NonceSize())
	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, err
	}

	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)

	return ciphertext, nil
}

// Decrypt uses AES-GCM to decrypt the given ciphertext with the given key. This
// function expects that the first 12 bytes of the ciphertext are the nonce that
// was used to encrypt the plaintext.
func Decrypt(key, ciphertext []byte) ([]byte, error) {
	switch len(key) {
	case 16, 24, 32: // AES-128, AES-192, AES-256
	default:
		return nil, errors.New("invalid key length")
	}

	if len(ciphertext) < 12 {
		return nil, errors.New("ciphertext too short")
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}

	nonceSize := gcm.NonceSize()
	if len(ciphertext) < nonceSize {
		return nil, errors.New("ciphertext too short")
	}

	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]

	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, err
	}

	return plaintext, nil
}