// Copyright 2024 Patial Tech (Ankit Patial).
//
// This file is part of code.patial.tech/go/appcore, which is MIT licensed.
// See http://opensource.org/licenses/MIT

package crypto

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"io"
)

// NewEncryptionKey will generate new key as base64 encoded string
//
// should be of 16, 24 or 32 bytes in length
func NewEncryptionKey(size uint8) (string, error) {
	b, err := RandomBytes(size)
	if err != nil {
		return "", err
	}

	return base64.StdEncoding.EncodeToString(b), nil
}

func decodeEncryptionKey(str string) ([]byte, error) {
	return base64.StdEncoding.DecodeString(str)
}

// Encrypt text using the encyption key
func Encrypt(key, text string) (string, error) {
	b, err := decodeEncryptionKey(key)
	if err != nil {
		return "", fmt.Errorf("decrypted base64 string: %w", err)
	}

	gcm, err := getGCM(b)
	if err != nil {
		return "", err
	}

	// create a nonce. Nonce should be from GCM
	nonce := make([]byte, gcm.NonceSize())
	if _, err = io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}

	// encrypt the data using gcm.Seal
	ciphertext := gcm.Seal(nonce, nonce, []byte(text), nil) // prefix nonce to the ciphertext
	return fmt.Sprintf("%x", ciphertext), nil
}

// Decrypt text using the encryption key
func Decrypt(key, text string) (string, error) {
	b, err := decodeEncryptionKey(key)
	if err != nil {
		return "", fmt.Errorf("decrypted base64 string: %w", err)
	}

	// create a new Cipher Block from the key
	gcm, err := getGCM(b)
	if err != nil {
		return "", err
	}

	nonceSize := gcm.NonceSize()
	enc, err := hex.DecodeString(text)
	if err != nil {
		return "", err
	}
	nonce, ciphertext := enc[:nonceSize], enc[nonceSize:]

	// decrypt
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}

	return string(plaintext), nil
}

func getGCM(key []byte) (gcm cipher.AEAD, err error) {
	// cipher block from the key
	b, err := aes.NewCipher(key)
	if err != nil {
		return gcm, err
	}

	// GCM - https://en.wikipedia.org/wiki/Galois/Counter_Mode
	// https://golang.org/pkg/crypto/cipher/#NewGCM
	gcm, err = cipher.NewGCM(b)
	if err != nil {
		return gcm, err
	}

	return gcm, nil
}