如何在Bouncy Castle中从AES_CBC切换到AES_CFB解密私钥？

问题背景 #

在Android项目中使用基于Bouncy Castle的依赖库，加密模式从AES_CBC切换到AES_CFB后，原解密代码抛出异常，无法读取加密数据。

原AES_CBC模式下可用的Kotlin解密代码 #

private fun getPrivateKey(certObject: PKCS8EncryptedPrivateKeyInfo, keyPassPhrase: String): PrivateKey? {
    val bouncyCastleProvider = BouncyCastleProvider()
    val decryptionProvider: InputDecryptorProvider =
        JceOpenSSLPKCS8DecryptorProviderBuilder().setProvider(bouncyCastleProvider)
            .build(keyPassPhrase.toCharArray())
    val info = certObject.decryptPrivateKeyInfo(decryptionProvider)
    val converter = JcaPEMKeyConverter()
    return converter.getPrivateKey(info)
}

AES_CFB模式下抛出的异常 #

Method threw 'org.bouncycastle.pkcs.PKCSException' exception.

unable to read encrypted data: no key size for algorithm:2.16.840.1.101.3.4.1.44

注：OID 2.16.840.1.101.3.4.1.44 对应AES-256-CFB算法

附Java示例代码 #

public class Application {   

private final static String passphrase = "password";

  public static void main(String[] args) {
    try {
      BouncyCastleProvider securityProvider = new BouncyCastleProvider();
     
      Security.addProvider(securityProvider);
      PKCS8EncryptedPrivateKeyInfo encryptedPrivateKeyInfo = getPrivateKeyInfo();
      System.out.println("Private key algorithm encrypted: " + encryptedPrivateKeyInfo.getEncryptionAlgorithm().getAlgorithm());
      InputDecryptorProvider decryptionProvider = new JceOpenSSLPKCS8DecryptorProviderBuilder()
              .setProvider(securityProvider)
              .build(passphrase.toCharArray());
      PrivateKeyInfo privateKeyInfo = encryptedPrivateKeyInfo.decryptPrivateKeyInfo(decryptionProvider);
      System.out.println("Private key algorithm decrypted: " + privateKeyInfo.getPrivateKeyAlgorithm().getAlgorithm());
    }catch (Exception e){
      System.out.println(e);
    }   
  }

  private static PKCS8EncryptedPrivateKeyInfo getPrivateKeyInfo() throws IOException {     
    InputStream privateKeyInputStream = new FileInputStream("src/main/resources/key.k8");
    PEMParser pemParser = new PEMParser(new InputStreamReader(privateKeyInputStream, StandardCharsets.UTF_8));
    Object pemObject = pemParser.readObject();
    return (PKCS8EncryptedPrivateKeyInfo) pemObject;  
  } 
}

加密私钥文件内容（key.k8） #

-----BEGIN ENCRYPTED PRIVATE KEY----- 
<private key encrypted with AES_CFB_256 encryption using bouncy castle>
-----END ENCRYPTED PRIVATE KEY-----

解决方案 #

1. 确保Bouncy Castle Provider正确注册到Security框架 #

原代码仅创建了Provider实例，但未添加到Security全局配置中，这会导致部分算法无法被正确识别。修改代码时先检查并添加Provider：

private fun getPrivateKey(certObject: PKCS8EncryptedPrivateKeyInfo, keyPassPhrase: String): PrivateKey? {
    val bcProvider = BouncyCastleProvider()
    // 检查Provider是否已注册，避免重复添加
    if (Security.getProvider(bcProvider.name) == null) {
        Security.addProvider(bcProvider)
    }
    // 构建解密器时指定Provider
    val decryptorProvider = JceOpenSSLPKCS8DecryptorProviderBuilder()
        .setProvider(bcProvider)
        .build(keyPassPhrase.toCharArray())
    val privateKeyInfo = certObject.decryptPrivateKeyInfo(decryptorProvider)
    // 密钥转换时也需指定Provider
    val converter = JcaPEMKeyConverter().setProvider(bcProvider)
    return converter.getPrivateKey(privateKeyInfo)
}

2. 确认Bouncy Castle版本兼容性 #

确保使用的Bouncy Castle依赖版本支持AES_CFB的PKCS8解密，推荐使用适配Android的最新稳定版本：

// 在build.gradle中添加依赖
implementation 'org.bouncycastle:bcprov-android:1.77'

3. 处理Android加密强度限制（针对旧版本系统） #

如果运行在Android 6.0及以下版本，需要启用无限制加密政策文件。可以通过以下方式检查并启用：

// 检查是否支持强加密
val isUnlimited = Cipher.getMaxAllowedKeyLength("AES") >= 256
if (!isUnlimited) {
    // 尝试启用无限制政策（需要对应权限或系统支持）
    try {
        val field = Security::class.java.getDeclaredField("isRestricted")
        field.isAccessible = true
        field.set(null, false)
    } catch (e: Exception) {
        // 处理异常或提示用户
    }
}

内容的提问来源于stack exchange，提问作者user_8275