package storage

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

// ErrCiphertextTooShort is returned when ciphertext is shorter than the GCM nonce size.
var ErrCiphertextTooShort = errors.New("ciphertext too short")

// Encrypt encrypts plaintext using AES-256-GCM with a random nonce.
// The nonce is prepended to the returned ciphertext.
// key must be exactly 32 bytes (AES-256).
func Encrypt(plaintext []byte, key []byte) ([]byte, error) {
	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
	}
	// Seal appends encrypted data to nonce, so nonce is prepended
	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)
	return ciphertext, nil
}

// Decrypt decrypts ciphertext produced by Encrypt.
// Expects the nonce to be prepended to the ciphertext.
func Decrypt(ciphertext []byte, key []byte) ([]byte, error) {
	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, ErrCiphertextTooShort
	}
	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
	return gcm.Open(nil, nonce, ciphertext, nil)
}
