/*
Copyright © 2021 Sniptt <support@sniptt.com>

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/
package encrypt

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

type EncryptedBytes struct {
	Ciphertext []byte
	Key        []byte
	Nonce      []byte
}

// Bytes encrypts the provided data using AES-256-GCM with a randomly generated
// key and nonce. The nonce is prepended to the returned ciphertext.
func Bytes(bytes []byte) (EncryptedBytes, error) {
	if len(bytes) == 0 {
		return EncryptedBytes{}, fmt.Errorf("input bytes cannot be empty")
	}

	// Key should be 16 bytes (AES-128), 24 bytes (AES-192) or 32 bytes (AES-256)
	key := make([]byte, 32)
	_, err := rand.Read(key)
	if err != nil {
		return EncryptedBytes{}, err
	}

	// Generate a new aes cipher using the key above
	block, err := aes.NewCipher(key)
	if err != nil {
		return EncryptedBytes{}, err
	}

	// gcm or Galois/Counter Mode, is a mode of operation
	// for symmetric key cryptographic block ciphers
	// - https://en.wikipedia.org/wiki/Galois/Counter_Mode
	aesGCM, err := cipher.NewGCM(block)
	if err != nil {
		return EncryptedBytes{}, err
	}

	// Create a new byte array the size of the nonce,
	// populate with a cryptographically secure random sequence
	nonce := make([]byte, aesGCM.NonceSize())
	_, err = rand.Read(nonce)
	if err != nil {
		return EncryptedBytes{}, err
	}

	// Encrypt and authenticate plaintext
	// Note: Seal() prepends the nonce to the ciphertext
	ciphertext := aesGCM.Seal(nonce, nonce, bytes, nil)

	return EncryptedBytes{ciphertext, key, nonce}, nil
}

// ExtractNonce returns the nonce from the ciphertext where it was prepended
func (e EncryptedBytes) ExtractNonce() []byte {
	return e.Ciphertext[:len(e.Nonce)]
}

// CiphertextWithoutNonce returns the ciphertext without the prepended nonce
func (e EncryptedBytes) CiphertextWithoutNonce() []byte {
	return e.Ciphertext[len(e.Nonce):]
}