证书和回调报文解密

为了保证安全性，微信支付在回调通知和平台证书下载接口中，对关键信息进行了AES-256-GCM加密。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

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。

证书和回调报文使用的加密密钥为APIv3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": "...", 
        // 附加数据包（可能为空字符串）
        "associated_data": "" 
}

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RFC 5116。

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。

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ApiV3Key，长度必须为32个字节");
    }
    this.aesKey = key;
  }

  public String decryptToString(byte[] associatedData, 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);
      cipher.updateAAD(associatedData);

      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ApiV3Key，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');
    }
}

.NET

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)