如何在不同活动的android studio中加密/解密文本

Question

I tried developing an text encryption/decryption app in android studio. So here on the MainActivity.java i ran a sample code of encryption & decryption.

MainActivity.java

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class MainActivity extends AppCompatActivity {

Button btn,btn2;
static final String TAG = "SymmetricAlgorithmAES";
String secr="k";
String secr2="d";

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

    //code to use my specified defined key
    byte[] key = new byte[0];
    try {
        key = (secr+secr2).getBytes("UTF-8");
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
    MessageDigest sha = null;
    try {
        sha = MessageDigest.getInstance("SHA-1");
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    key = sha.digest(key);
    key = Arrays.copyOf(key, 16); // use only first 128 bit

    SecretKeySpec sks = new SecretKeySpec(key, "AES");

    // Original text
    String theTestText = "This is just a simple test";
    TextView tvorig = (TextView)findViewById(R.id.tvorig);
    tvorig.setText("\n[ORIGINAL]:\n" + theTestText + "\n");

    // Encode the original data with AES
    byte[] encodedBytes = null;
    try {
        Cipher c = Cipher.getInstance("AES");
        c.init(Cipher.ENCRYPT_MODE, sks);
        encodedBytes = c.doFinal(theTestText.getBytes());
    } catch (Exception e) {
        Log.e(TAG, "AES encryption error");
    }
    TextView tvencoded = (TextView)findViewById(R.id.tvencoded);
    tvencoded.setText("" +
            Base64.encodeToString(encodedBytes, Base64.DEFAULT) + "\n");

    // Decode the encoded data with AES
    byte[] decodedBytes = null;
    try {
        Cipher c = Cipher.getInstance("AES");
        c.init(Cipher.DECRYPT_MODE, sks);
        decodedBytes = c.doFinal(encodedBytes);
    } catch (Exception e) {
        Log.e(TAG, "AES decryption error");
    }
    TextView tvdecoded = (TextView)findViewById(R.id.tvdecoded);
    tvdecoded.setText("[DECODED]:\n" + new String(decodedBytes) + "\n");

   }

The above code works properly with correct output. But when i try to modify the code and try to write encryption and decryption in different activities, but the decryption part does not work properly.

Here is the code for encryption part which works properly without any error.

Encryption.java

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class Encryption extends AppCompatActivity {

static final String TAG = "SymmetricAlgorithmAES";
String secr="k";
String secr2="d";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.encryption);
    enc_text_edt=(EditText)findViewById(R.id.enc_text_edt);
    enc_text_btn=(Button)findViewById(R.id.enc_text_btn);
    enctv=(TextView)findViewById(R.id.enctv);

    //code to use my specified defined key
    byte[] key = new byte[0];
    try {
        key = (secr+secr2).getBytes("UTF-8");
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
    MessageDigest sha = null;
    try {
        sha = MessageDigest.getInstance("SHA-1");
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    key = sha.digest(key);
    key = Arrays.copyOf(key, 16); // use only first 128 bit

    SecretKeySpec sks = new SecretKeySpec(key, "AES");

    final SecretKeySpec finalSks = sks;
    enc_text_btn.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            try {

                // Encode the original data with AES
                byte[] encodedBytes = null;
                try {
                    Cipher c = Cipher.getInstance("AES");
                    c.init(Cipher.ENCRYPT_MODE, finalSks);
                    encodedBytes = c.doFinal(enc_text_edt.getText().toString().getBytes());
                } catch (Exception e) {
                    Log.e(TAG, "AES encryption error");
                }


                enctv.setText("[ENCRYPTED]:\n" +
                        Base64.encodeToString(encodedBytes, Base64.DEFAULT) + "\n");


                enc_text_edt.setText("");


            } catch (Exception e) {

                e.printStackTrace();
            }

        }
    });
}
}

code of Decryption

Decryption.java

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;
import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

public class Decryption extends AppCompatActivity {

Button dec_text_btn;
TextView dec_edtext_view, dectv;

static final String TAG = "SymmetricAlgorithmAES";
String secr = "k";
String secr2 = "d";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.decryption);
    dec_text_btn = (Button) findViewById(R.id.dec_text_btn);
    dec_edtext_view = (EditText) findViewById(R.id.dec_edtext_view);
    dectv = (TextView) findViewById(R.id.dectv);


    //code to use my specified defined key
    byte[] key = new byte[0];
    try {
        key = (secr + secr2).getBytes("UTF-8");
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
    MessageDigest sha = null;
    try {
        sha = MessageDigest.getInstance("SHA-1");
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    key = sha.digest(key);
    key = Arrays.copyOf(key, 16); // use only first 128 bit

    SecretKeySpec sks = new SecretKeySpec(key, "AES");


    final SecretKeySpec finalSks = sks;
    dec_text_btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            try {

                // Decode the encoded data with AES

                byte[] decodedBytes = null;
                try {
                    Cipher c = Cipher.getInstance("AES");
                    c.init(Cipher.DECRYPT_MODE, finalSks);
                    decodedBytes= c.doFinal(dec_edtext_view.getText().toString().getBytes());
                    } catch (Exception e) {
                        Log.e(TAG, "AES encryption error");
                    }

                    dectv.setText("[DECRYPTED]:\n" + new String(decodedBytes) + "\n");


                } catch (Exception e) {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "creptography exception see log cat....", Toast.LENGTH_SHORT).show();
                }
        }
    });
}
}

Please help me with the error. While executing Decryption part it directly shows exception "creptography exception see log cat" .

Answer 1

After reading your code I think I have found the problem, you encode to Base64 but never decode. In the Encryption you do the following

enctv.setText("[ENCRYPTED]:\n" +
                        Base64.encodeToString(encodedBytes, Base64.DEFAULT) + "\n");

and I can guess the user copies it to the decryption field but after they click the button you do

decodedBytes= c.doFinal(dec_edtext_view.getText().toString().getBytes());

instead of decoding from Base64. I would also like to add a few notes: You're security is not safe, you barely achieved any layer of security when the keys are in plane site like this.

Note 1:

Keys should be generated randomly with a SecureRandom . You can easily do it by doing the following:

byte[] key = new byte[16];
        new SecureRandom().nextBytes(key);

Note 2:

Use an initialization vector aka IV this is useful in case the user has typed the same message. For example consider the following scenario you encrypt "Hello World" and it comes out as "ABCDEFGHIJK". Now you send it again and it is again "ABCDEFGHIJK". With an IV it will be different everytime as long as you generate a new IV per message, you should append this IV to the message so later in decryption you can extract it.

Note 3:

When declaring a Cipher use more than AES . There is a great article about how to increase your security and knowledge: article link

Note 4:

If an exception occurs don't continue on like nothing happend, you should handle it correctly and not continue on code that depends on what caused the exception.

Note 5:

Learn Java more in depth instead of jumping to cryptography, you're fields should be private and some final don't declare null if you might be planning to use it later, if you do check if its null. Don't declare "UTF-8" in get bytes, have a constant declaring a Charset such as "UTF-8" this is easily done with Charset.forName("UTF-8")

Answer 2

I agree with everything OughtToPrevail said.

Also, you should probably get all of that out of your activity and into a helper class. That way it will be reusable, and you can test the in and out of it (without copying and pasting) with something that would look like this:

    public void myEncryptionTest(){
    String message = "This is the message to encrypt and decrypt.";
    String pass = "pass";
    String encryption = Crypto.myEncrypt(message.getBytes(), pass);
    byte[] decryption = Crypto.myDecrypt(encryption, pass);
    String decrypted = new String(decryption);
    Log.d("****DECRYPTION:  ", decrypted);
}

Where the helper class is called "Crypto" and the two static functions you're testing are "myEncrypt" and "myDecrypt."