package secret

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"strings"
)

var ErrEmpty = errors.New("secret is empty")

type Box struct {
	gcm cipher.AEAD
}

func NewBox(keyMaterial string) (*Box, error) {
	if keyMaterial == "" {
		return nil, fmt.Errorf("encryption key material is required")
	}
	sum := sha256.Sum256([]byte(keyMaterial))
	block, err := aes.NewCipher(sum[:])
	if err != nil {
		return nil, err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &Box{gcm: gcm}, nil
}

func (b *Box) Encrypt(plain string) (string, error) {
	if plain == "" {
		return "", ErrEmpty
	}
	nonce := make([]byte, b.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ciphertext := b.gcm.Seal(nonce, nonce, []byte(plain), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

func (b *Box) Decrypt(encoded string) (string, error) {
	if encoded == "" {
		return "", ErrEmpty
	}
	raw, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		return "", err
	}
	nonceSize := b.gcm.NonceSize()
	if len(raw) < nonceSize {
		return "", fmt.Errorf("invalid ciphertext")
	}
	nonce, ciphertext := raw[:nonceSize], raw[nonceSize:]
	plain, err := b.gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}

func IsDecryptFailure(err error) bool {
	if err == nil {
		return false
	}
	return strings.Contains(err.Error(), "message authentication failed")
}

func DecryptHint(err error) error {
	if err == nil {
		return nil
	}
	if IsDecryptFailure(err) {
		return fmt.Errorf("DNS API 凭据无法解密，请在 DDNS 页面重新保存 API Token（服务加密密钥可能已变更）")
	}
	return err
}