package aes

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"errors"
	"fmt"
	"io"
	mrand "math/rand"
	"time"
)

const (
	keySize      = 32
	allowedRunes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
)

// RandomKey returns a randomly generated 32 bytes long key.
func RandomKey() (key []byte) {
	mrand.Seed(time.Now().UnixNano())

	key = make([]byte, keySize)

	for i := range key {
		key[i] = allowedRunes[mrand.Intn(len(allowedRunes))]
	}

	return key
}

// Encrypt encrypts plaintext with key and returns resulting bytes.
func Encrypt(plaintext, key []byte) ([]byte, error) {
	if len(key) != keySize {
		return nil, fmt.Errorf("key size invalid: %d, must be %d\n", len(key), keySize)
	}

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

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

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

	return gcm.Seal(nonce, nonce, plaintext, nil), nil
}

// Decrypt decrypts ciphertext with key and returns resulting bytes.
func Decrypt(ciphertext, key []byte) ([]byte, error) {
	c, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}

	gcm, err := cipher.NewGCM(c)
	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:]

	return gcm.Open(nil, nonce, ciphertext, nil)
}