package seal

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"errors"
)

const keySize = 32

var ErrKey = errors.New("seal: key must be 32 random bytes in base64")

type Box struct {
	aead cipher.AEAD
}

func New(encodedKey string) (*Box, error) {
	key, err := base64.StdEncoding.DecodeString(encodedKey)
	if err != nil || len(key) != keySize {
		return nil, ErrKey
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, err
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &Box{aead: aead}, nil
}

func (b *Box) Seal(plain, purpose string) []byte {
	nonce := make([]byte, b.aead.NonceSize())
	if _, err := rand.Read(nonce); err != nil {
		panic(err)
	}
	return b.aead.Seal(nonce, nonce, []byte(plain), []byte(purpose))
}

func (b *Box) Open(sealed []byte, purpose string) (string, error) {
	size := b.aead.NonceSize()
	if len(sealed) < size {
		return "", errors.New("seal: ciphertext too short")
	}
	plain, err := b.aead.Open(nil, sealed[:size], sealed[size:], []byte(purpose))
	if err != nil {
		return "", errors.New("seal: cannot decrypt")
	}
	return string(plain), nil
}