解密回调

说明

为了保证安全性，支付在回调通知接口中，对关键信息进行了 AES-256-GCM 加密，商户收到报文后，要解密出明文，解密时使用 APP 的密钥。本章节详细介绍了加密报文的格式，以及如何进行解密。

加密报文格式

首先，商户先从应答中获取以下信息。

• AES-GCM 是一种 NIST 标准的 认证加密算法， 是一种能够同时保证数据的保密性、 完整性和真实性的一种加密模式。它最广泛的应用是在 TLS 中

• 回调报文使用的加密密钥为 APP 的密钥。。

• 对于加密的数据，我们使用了一个独立的 JSON 对象来表示。为了方便阅读，示例做了 Pretty 格式化，并加入了注释。

{
    "original_type": "transaction", // 加密前的对象类型
    "algorithm": "AEAD_AES_256_GCM", // 加密算法

    // Base64编码后的密文
    "ciphertext": "...",

    // 加密使用的随机串初始化向量）
    "nonce": "...",
}

注意：加密的随机串，跟签名时使用的随机串没有任何关系，是不一样的。

解密

• 算法接口的细节，可以参考 rfc5116。

• 大部分编程语言（较新版本）都支持了 AEAD_AES_256_GCM。开发者可以参考下列的示例，了解如何使用您的编程语言实现解密。

代码示例

Java

import java.io.IOException;
import java.security.GeneralSecurityException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;
import javax.crypto.Cipher;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AesUtil {
  static final int KEY_LENGTH_BYTE = 32;
  static final int TAG_LENGTH_BIT = 128;
  private final byte[] aesKey;

    public AesUtil(byte[] key) {
      if (key.length != KEY_LENGTH_BYTE) {
        throw new IllegalArgumentException("无效的AppSecret，长度必须为32个字节");
        }
      this.aesKey = key;
    }

  public String decryptToString( byte[] nonce, String ciphertext)
  throws GeneralSecurityException, IOException {
    try {
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        SecretKeySpec key = new SecretKeySpec(aesKey, "AES");
        GCMParameterSpec spec = new GCMParameterSpec(TAG_LENGTH_BIT, nonce);

        cipher.init(Cipher.DECRYPT_MODE, key, spec);

        return new String(cipher.doFinal(Base64.getDecoder().decode(ciphertext)), "utf-8");
       } catch (NoSuchAlgorithmException | NoSuchPaddingException e) {
         throw new IllegalStateException(e);
       } catch (InvalidKeyException | InvalidAlgorithmParameterException e) {
         throw new IllegalArgumentException(e);
        }
    }
}

PHP

class AesUtil {
  /**
  * AES key
  *
  * @var string
  */
  private $aesKey;

  const KEY_LENGTH_BYTE = 32;
  const AUTH_TAG_LENGTH_BYTE = 16;

 /**
  * Constructor
  */
  public
  function __construct($aesKey) {
    if (strlen($aesKey) != self::KEY_LENGTH_BYTE) {
      throw new InvalidArgumentException('无效的AppSecret，长度应为32个字节');
    }
     $this - > aesKey = $aesKey;
    }

   /**
    * Decrypt AEAD_AES_256_GCM ciphertext
    *
    * @param string    $associatedData     AES GCM additional authentication data
    * @param string    $nonceStr           AES GCM nonce
    * @param string    $ciphertext         AES GCM cipher text
    *
    * @return string|bool      Decrypted string on success or FALSE on failure
    */
  public
  function decryptToString($associatedData, $nonceStr, $ciphertext) {
    $ciphertext = \base64_decode($ciphertext);
    if (strlen($ciphertext) <= self::AUTH_TAG_LENGTH_BYTE) {
      return false;
    }

    // ext-sodium (default installed on >= PHP 7.2)
    if (function_exists('\sodium_crypto_aead_aes256gcm_is_available') && \sodium_crypto_aead_aes256gcm_is_available()) {
        return \sodium_crypto_aead_aes256gcm_decrypt($ciphertext, $associatedData, $nonceStr, $this - > aesKey);
      }

    // ext-libsodium (need install libsodium-php 1.x via pecl)
    if (function_exists('\Sodium\crypto_aead_aes256gcm_is_available') && \Sodium\crypto_aead_aes256gcm_is_available()) {
      return \Sodium\crypto_aead_aes256gcm_decrypt($ciphertext, $associatedData, $nonceStr, $this - > aesKey);
    }

    // openssl (PHP >= 7.1 support AEAD)
    if (PHP_VERSION_ID >= 70100 && in_array('aes-256-gcm', \openssl_get_cipher_methods())) {
    $ctext = substr($ciphertext, 0, -self::AUTH_TAG_LENGTH_BYTE);
    $authTag = substr($ciphertext, -self::AUTH_TAG_LENGTH_BYTE);

    return \openssl_decrypt($ctext, 'aes-256-gcm', $this - > aesKey, \OPENSSL_RAW_DATA, $nonceStr,$authTag, $associatedData);

    }
    throw new \RuntimeException('AEAD_AES_256_GCM需要PHP 7.1以上或者安装libsodium-php');
  }
}

  </CodeGroupItem>

  <CodeGroupItem title="Java">

```java:no-line-numbers
OkHttpClient client = new OkHttpClient().newBuilder()
   .build();
MediaType mediaType = MediaType.parse("application/json");
RequestBody body = RequestBody.create(mediaType, "<body data here>");
Request request = new Request.Builder()
   .url("https://api.ttpay.io/v1/wallet/update")
   .method("POST", body)
   .addHeader("Authorization", "<Authorization>")
   .addHeader("User-Agent", "TTPay API (https://ttpay.io)")
   .addHeader("Content-Type", "application/json")
   .build();
Response response = client.newCall(request).execute();

Aspnet

public class AesGcm {
  private static string ALGORITHM = "AES/GCM/NoPadding";
  private static int TAG_LENGTH_BIT = 128;
  private static int NONCE_LENGTH_BYTE = 12;
  private static string AES_KEY = "yourkeyhere";

  public static string AesGcmDecrypt(string associatedData, string nonce, string ciphertext)
  {
    GcmBlockCipher gcmBlockCipher = new GcmBlockCipher(new AesEngine());
    AeadParameters aeadParameters = new AeadParameters(
      new KeyParameter(Encoding.UTF8.GetBytes(AES_KEY)),
      128,
      Encoding.UTF8.GetBytes(nonce),
      Encoding.UTF8.GetBytes(associatedData));
    gcmBlockCipher.Init(false, aeadParameters);

    byte[] data = Convert.FromBase64String(ciphertext);
    byte[] plaintext = new byte[gcmBlockCipher.GetOutputSize(data.Length)];
    int length = gcmBlockCipher.ProcessBytes(data, 0, data.Length, plaintext, 0);
    gcmBlockCipher.DoFinal(plaintext, length);
    return Encoding.UTF8.GetString(plaintext);
  }
}

Python

  from cryptography.hazmat.primitives.ciphers.aead import AESGCM
  import base64
  def decrypt(nonce, ciphertext, associated_data):

    key = "Your32Apiv3Key"

    key_bytes = str.encode(key)
    nonce_bytes = str.encode(nonce)
    ad_bytes = str.encode(associated_data)
    data = base64.b64decode(ciphertext)

    aesgcm = AESGCM(key_bytes)
    return aesgcm.decrypt(nonce_bytes, data, ad_bytes)