文件解密失败：javax.crypto.BadPaddingException：填充块已损坏

Question

I've created a ZIP-File on my pc (I compressed it with the os x zipper in the finder) and after that I encrypted it with my java program:

public class ResourceEncrypter {

    static byte[] salt = { (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
                               (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99 };

    public static void main(String[] args) {
        new ResourceEncrypter().encryptAllFiles();
        System.out.println("Okay, done");
    }



    private byte[] getKey() {
        try {
            KeyGenerator kgen = KeyGenerator.getInstance("AES");
            SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
            sr.setSeed(salt);
            kgen.init(128, sr); 
            SecretKey skey = kgen.generateKey();
            byte[] key = skey.getEncoded();
            return key;
        } catch(Exception e) {
            e.printStackTrace();
        }
        return null;
    }


    private void encryptAllFiles() {
        try {
            byte[] key = getKey();

            //Take a list of files and encrypt each file...
            String srcFilesPath = System.getProperty("user.dir") + "/srcFiles";
            String encryptedSrcFilesPath = System.getProperty("user.dir") + "/encryptedSrcFiles";
            File[] listOfFiles = new File(srcFilesPath).listFiles();
            for(int i = 0; i < listOfFiles.length; ++i) {
                if(listOfFiles[i].getAbsolutePath().contains(".zip")) {
                    //Encrypt this file!
                    byte[] data = Files.readAllBytes(Paths.get(listOfFiles[i].getAbsolutePath()));
                    byte[] encryptedData = ResourceEncrypter.encrypt(key, data);

                    String filename = listOfFiles[i].getName();
                    System.out.println("Write result to " + encryptedSrcFilesPath + "/" + filename);
                    FileOutputStream output = new FileOutputStream(encryptedSrcFilesPath + "/" + filename);
                    output.write(encryptedData);
                    output.close();
                }
            }
        } catch(Exception e) {
            e.printStackTrace();
        }
    }



    private static byte[] encrypt(byte[] key, byte[] data) throws Exception {
        SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
        byte[] encryptedData = cipher.doFinal(data);
        return encryptedData;
    }

So, this takes any zip file encrypts it and saves the result to another folder.

Now, I got an android app and I put the encrypyted zip file into an asset folder main/assets/pic.zip.encrypted .

In my android app I do the following:

public class MainActivity extends AppCompatActivity {

static byte[] salt = {  (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,
                                (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99 };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        decryptZipFile();
    }


    private byte[] getKey() {
        try {
            //Create the key for the encryption/decryption
            KeyGenerator kgen = KeyGenerator.getInstance("AES");
            SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
            sr.setSeed(salt);
            kgen.init(128, sr); // 192 and 256 bits may not be available
            SecretKey skey = kgen.generateKey();
            byte[] key = skey.getEncoded();
            return key;
        } catch(Exception e) {
            e.printStackTrace();
        }
        return null;
    }


    private static byte[] decrypt(byte[] key, byte[] encryptedData) throws Exception {
        SecretKeySpec skeySpec = new SecretKeySpec(key,"AES");
        Cipher cipher = Cipher.getInstance("AES");
        cipher.init(Cipher.DECRYPT_MODE, skeySpec);
        byte[] decrypted = cipher.doFinal(encryptedData);
        return decrypted;
    }


    public void decryptZipFile() {
        // First decrypt the zip file
        try {
            InputStream is = getResources().getAssets().open("pics.zip.encrypted");
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int count;
            while ((count = is.read(buffer)) != -1)
                baos.write(buffer, 0, count);
            byte[] encryptedData = baos.toByteArray();

            byte[] decryptedData = decrypt(getKey(), encryptedData);
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

When I now try to decrypt my zip file with this code I get the following error:

09-23 18:41:21.117  30799-30799/demo.zip.app.zipapp W/System.err﹕ javax.crypto.BadPaddingException: pad block corrupted
09-23 18:41:21.117  30799-30799/demo.zip.app.zipapp W/System.err﹕ at com.android.org.bouncycastle.jcajce.provider.symmetric.util.BaseBlockCipher.engineDoFinal(BaseBlockCipher.java:854)
09-23 18:41:21.117  30799-30799/demo.zip.app.zipapp W/System.err﹕ at javax.crypto.Cipher.doFinal(Cipher.java:1340)
09-23 18:41:21.117  30799-30799/demo.zip.app.zipapp W/System.err﹕ at demo.zip.app.zipapp.MainActivity.decrypt(MainActivity.java:63)

However, when I apply the same methods for decrypting on my PC it works fine.

What is happening here?

Answer 1

Okay, I found the solution.

The problem is that SecureRandom and in particular the setSeed method works differently on Android than on normal Java.

So, instead you should not construct a key from a salt key as I do above. Instead you should go to the PC and get the key from private byte[] getKey() as a Base64 string object. Then the key looks something like this in my case: n9dReP+BPwHWGCLpDQe+MQ== and then copy an paste this into the android method which becomes this:

private byte[] getKey() {
            byte[] key = Base64.decode(new String("n9dReP+BPwHWGCLpDQe+MQ==").getBytes(), 0);
            return key;
    }

That's it.