package envelope

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

// aesGCMSeal encrypts plaintext with AES-256-GCM under key, returning
// nonce || ciphertext. key must be 32 bytes.
func aesGCMSeal(key, plaintext []byte) ([]byte, error) {

	gcm, err := newGCM(key)
	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
}

// aesGCMOpen reverses aesGCMSeal.
func aesGCMOpen(key, blob []byte) ([]byte, error) {

	gcm, err := newGCM(key)
	if err != nil {
		return nil, err
	}
	if len(blob) < gcm.NonceSize() {
		return nil, fmt.Errorf("envelope: ciphertext too short")
	}
	nonce, ciphertext := blob[:gcm.NonceSize()], blob[gcm.NonceSize():]
	return gcm.Open(nil, nonce, ciphertext, nil)
}

func newGCM(key []byte) (cipher.AEAD, error) {

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