AES 字段加密迁配置并兼容旧密文

移除字段加密硬编码主密钥，改为 FIELD_ENCRYPTION_KEY 注入。

保留 FIELD_ENCRYPTION_LEGACY_KEY 透明回退旧密文，新写入统一使用主密钥；生产环境校验主密钥和 legacy 密钥长度、占位符及相等关系，并统一生产环境判断口径。

补充配置与旧密文兼容回归测试。

backend/pkg/crypto/aes.go

@@ -9,61 +9,124 @@ import (
 	"io"
 )
 
-// 加密密钥（生产环境应从配置文件读取）
-// 必须是 16、24 或 32 字节
-const encryptionKey = "hfb-sys-2024-secret-key-32bytes!" // 正好32字节
+// Encryptor 字段加密器接口。Encrypt 永远用主密钥，Decrypt 先试主密钥、失败试 legacy 密钥。
+type Encryptor interface {
+	Encrypt(plaintext string) (string, error)
+	Decrypt(ciphertext string) (string, error)
+}
 
-// Encrypt 使用 AES-GCM 加密文本
-func Encrypt(plainText string) (string, error) {
-	block, err := aes.NewCipher([]byte(encryptionKey))
+// aesKey 持有一个 AES-GCM 密钥及其 cipher。
+type aesKey struct {
+	key []byte
+}
+
+func newAESKey(key string) (*aesKey, error) {
+	keyBytes := []byte(key)
+	if len(keyBytes) != 16 && len(keyBytes) != 24 && len(keyBytes) != 32 {
+		return nil, errors.New("invalid key length: must be 16, 24, or 32 bytes")
+	}
+	return &aesKey{key: keyBytes}, nil
+}
+
+// FieldEncryptor 持有主密钥 + 可选 legacy 密钥列表。
+// Encrypt 永远用 primary；Decrypt 先试 primary，gcm.Open 认证失败依次试 legacy，
+// 用于密钥轮换期间透明解出旧密文。
+type FieldEncryptor struct {
+	primary *aesKey
+	legacy  []*aesKey
+}
+
+// NewFieldEncryptor 创建字段加密器。primary 为主密钥，legacy 为可选的旧密钥（用于回退解密）。
+func NewFieldEncryptor(primary string, legacy ...string) (*FieldEncryptor, error) {
+	primaryKey, err := newAESKey(primary)
+	if err != nil {
+		return nil, err
+	}
+	legacyKeys := make([]*aesKey, 0, len(legacy))
+	for _, l := range legacy {
+		if l == "" || l == primary {
+			continue
+		}
+		k, err := newAESKey(l)
+		if err != nil {
+			return nil, err
+		}
+		legacyKeys = append(legacyKeys, k)
+	}
+	return &FieldEncryptor{primary: primaryKey, legacy: legacyKeys}, nil
+}
+
+// Encrypt 使用 AES-GCM 加密明文，输出 base64(nonce || ciphertext || gcmTag)。
+func (e *FieldEncryptor) Encrypt(plaintext string) (string, error) {
+	if plaintext == "" {
+		return "", nil
+	}
+	gcm, err := e.gcm(e.primary)
 	if err != nil {
 		return "", err
 	}
-
-	aesGCM, err := cipher.NewGCM(block)
-	if err != nil {
-		return "", err
-	}
-
-	// 创建随机 nonce
-	nonce := make([]byte, aesGCM.NonceSize())
+	nonce := make([]byte, gcm.NonceSize())
 	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
 		return "", err
 	}
-
-	// 加密并附加 nonce
-	cipherBytes := aesGCM.Seal(nonce, nonce, []byte(plainText), nil)
-	return base64.StdEncoding.EncodeToString(cipherBytes), nil
+	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(ciphertext), nil
 }
 
-// Decrypt 使用 AES-GCM 解密文本
-func Decrypt(cipherText string) (string, error) {
-	block, err := aes.NewCipher([]byte(encryptionKey))
+// Decrypt 解密密文。先试主密钥，gcm.Open 认证失败依次试 legacy 密钥。
+func (e *FieldEncryptor) Decrypt(ciphertext string) (string, error) {
+	if ciphertext == "" {
+		return "", nil
+	}
+	decoded, err := base64.StdEncoding.DecodeString(ciphertext)
 	if err != nil {
 		return "", err
 	}
+	for _, k := range append([]*aesKey{e.primary}, e.legacy...) {
+		plaintext, err := decryptWithKey(k, decoded)
+		if err == nil {
+			return plaintext, nil
+		}
+	}
+	return "", errors.New("decrypt failed: no matching key")
+}
 
-	aesGCM, err := cipher.NewGCM(block)
+func (e *FieldEncryptor) gcm(k *aesKey) (cipher.AEAD, error) {
+	block, err := aes.NewCipher(k.key)
+	if err != nil {
+		return nil, err
+	}
+	return cipher.NewGCM(block)
+}
+
+func decryptWithKey(k *aesKey, decoded []byte) (string, error) {
+	block, err := aes.NewCipher(k.key)
 	if err != nil {
 		return "", err
 	}
-
-	cipherBytes, err := base64.StdEncoding.DecodeString(cipherText)
+	gcm, err := cipher.NewGCM(block)
 	if err != nil {
 		return "", err
 	}
-
-	nonceSize := aesGCM.NonceSize()
-	if len(cipherBytes) < nonceSize {
+	nonceSize := gcm.NonceSize()
+	if len(decoded) < nonceSize {
 		return "", errors.New("ciphertext too short")
 	}
-
-	// 提取 nonce 和实际密文
-	nonce, cipherBytes := cipherBytes[:nonceSize], cipherBytes[nonceSize:]
-	plainBytes, err := aesGCM.Open(nil, nonce, cipherBytes, nil)
+	nonce, body := decoded[:nonceSize], decoded[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, body, nil)
 	if err != nil {
 		return "", err
 	}
+	return string(plaintext), nil
+}
 
-	return string(plainBytes), nil
+// MockEncryptor 测试用直通加密器，明文不加密。
+type MockEncryptor struct{}
+
+func (e *MockEncryptor) Encrypt(plaintext string) (string, error) {
+	return plaintext, nil
+}
+
+func (e *MockEncryptor) Decrypt(ciphertext string) (string, error) {
+	return ciphertext, nil
 }