// Package crypto provides AES-256-GCM symmetric encryption for sensitive
// secret fields persisted to the database (e.g. git credentials, LLM API keys).
//
// The master key is supplied by the caller (see config.MasterKey) and never
// stored alongside the ciphertext.
package crypto

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

// Encryptor 用 AES-256-GCM 加解密 secret。
// 密文格式：nonce(12) || ciphertext || tag。
type Encryptor struct {
	gcm cipher.AEAD
}

// NewEncryptor constructs an Encryptor from a 32-byte (AES-256) key.
// Any other key length yields an error rather than silently downgrading.
func NewEncryptor(key []byte) (*Encryptor, error) {
	if len(key) != 32 {
		return nil, fmt.Errorf("key must be 32 bytes (got %d)", len(key))
	}
	block, err := aes.NewCipher(key)
	if err != nil {
		return nil, fmt.Errorf("aes new cipher: %w", err)
	}
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("new gcm: %w", err)
	}
	return &Encryptor{gcm: gcm}, nil
}

// Encrypt seals plaintext with a fresh random nonce and returns
// nonce || ciphertext || tag.
func (e *Encryptor) Encrypt(plaintext []byte) ([]byte, error) {
	nonce := make([]byte, e.gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("rand nonce: %w", err)
	}
	// Seal 把 nonce 作为 dst 前缀写入
	return e.gcm.Seal(nonce, nonce, plaintext, nil), nil
}

// Decrypt verifies and opens a ciphertext produced by Encrypt.
// It returns an error if the input is too short or authentication fails.
func (e *Encryptor) Decrypt(ciphertext []byte) ([]byte, error) {
	if len(ciphertext) < e.gcm.NonceSize() {
		return nil, errors.New("ciphertext too short")
	}
	nonce, ct := ciphertext[:e.gcm.NonceSize()], ciphertext[e.gcm.NonceSize():]
	pt, err := e.gcm.Open(nil, nonce, ct, nil)
	if err != nil {
		return nil, fmt.Errorf("gcm open: %w", err)
	}
	return pt, nil
}