package crypto

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

// SecretBox AES-256-GCM 加密器，用 sha256(key) 派生 32 字节密钥
type SecretBox struct {
	key []byte
}

// NewSecretBox 创建加密器
func NewSecretBox(masterKey string) *SecretBox {
	h := sha256.Sum256([]byte(masterKey))
	return &SecretBox{key: h[:]}
}

// Encrypt 加密明文，返回 base64 编码的密文（含 12 字节 nonce 前缀）
func (s *SecretBox) Encrypt(plaintext string) (string, error) {
	block, err := aes.NewCipher(s.key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return "", err
	}
	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
	return base64.StdEncoding.EncodeToString(ciphertext), nil
}

// Decrypt 解密密文，plaintext 应为 Encrypt 的返回值
func (s *SecretBox) Decrypt(cipherB64 string) (string, error) {
	ciphertext, err := base64.StdEncoding.DecodeString(cipherB64)
	if err != nil {
		return "", err
	}
	block, err := aes.NewCipher(s.key)
	if err != nil {
		return "", err
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return "", err
	}
	nonceSize := gcm.NonceSize()
	if len(ciphertext) < nonceSize {
		return "", errors.New("ciphertext too short")
	}
	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return "", err
	}
	return string(plaintext), nil
}