从AndroidKeystore加载私钥时出现问题

Android提供了一个API，用于生成用于加密操作的密钥，并将其存储在Android系统密钥存储中或从中存储。

我遵循KeyGenParameterSpec类的JavaDocs中的Example。密钥的生成，存储和加载工作。但是，如果我尝试使用密钥，则密码对象的init（）调用将失败。我调试了一下，可以看到加载的Key类型为“ android.security.keystore.AndroidKeyStoreSecretKey”。该实现防止密钥的byte []被暴露。我出于安全原因了解这一点，但是如果我想使用密钥，则必须获取密钥内容。所以，我必须做错什么。也许，还有另一种在Android中使用加密操作的方法吗？还是密钥的加载代码错误？

这是代码：

KeyGenerator keyGenerator = KeyGenerator.getInstance(
                KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore");

KeyGenParameterSpec.Builder builder = new KeyGenParameterSpec.Builder("demo-alias", KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT);
builder.setKeySize(256);
builder.setBlockModes(KeyProperties.BLOCK_MODE_CBC);
builder.setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7);
keyGenerator.init(builder.build());

// this key will work with a CipherObject ...
SecretKey key = keyGenerator.generateKey();

// Load the key from the Keystore
KeyStore keyStore = KeyStore.getInstance("AndroidKeyStore");
keyStore.load(null);

// This key will not work with the Cipher Object
SecretKey notWorkingKey = (SecretKey) keyStore.getKey("demo-alias", null);

Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
// That call fails
cipher.init(Cipher.ENCRYPT_MODE, key, new IvParameterSpec(new byte[]{87, 99, -94, 23, -17, 26, 84, -117, 59, -59, 25, -88, -66, 86, -42, 78}));

byte[] crypted = cipher.doFinal("testdata".getBytes());

密码的init（...）失败，但有以下异常：

java.lang.NullPointerException: Attempt to get length of null array
    at com.android.org.bouncycastle.crypto.params.KeyParameter.<init>(KeyParameter.java:13)
    at com.android.org.bouncycastle.jcajce.provider.symmetric.util.BaseBlockCipher.engineInit(BaseBlockCipher.java:557)
    at javax.crypto.Cipher.tryTransformWithProvider(Cipher.java:608)
    at javax.crypto.Cipher.tryCombinations(Cipher.java:532)
    at javax.crypto.Cipher.getSpi(Cipher.java:437)
    at javax.crypto.Cipher.init(Cipher.java:909)
    at javax.crypto.Cipher.init(Cipher.java:859)
    at de.demo.crypt.LoginActivity.executeLogin(LoginActivity.java:95)
    at de.demo.crypt.LoginActivity.access$000(LoginActivity.java:37)
    at de.demo.crypt.LoginActivity$1.onClick(LoginActivity.java:58)
    at de.demo.crypt.ActionButton.buttonClicked(ActionButton.java:104)
    at de.demo.crypt.ActionButton.access$000(ActionButton.java:17)
    at de.demo.crypt.ActionButton$1.onClick(ActionButton.java:60)
    at android.view.View.performClick(View.java:5198)
    at android.view.View$PerformClick.run(View.java:21147)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)