(Android™) RSA Decrypt Binary

When something is RSA encrypted, the output is always equal in size to the RSA key. For example, if "Hello World" is encrypted with a 1024-bit RSA key, the output is 128 bytes (because 1024 bits / 8 bits per byte = 128 bytes). You can see that the size of the RSA key imposes a limit on how many bytes can be encrypted. Thus RSA encryption is for small amounts of data. A typical use for RSA encryption is to encrypt a bulk (symmetric) encryption key.

In this example, we have a binary file containing the result of RSA encryption. The size of the file tells us the RSA key size. For example, if the file is exactly 256 bytes, we know that the RSA key required to decrypt must be a 2048-bit key.

Chilkat Android™ Downloads Android™ Java Libraries Android C/C++ Libraries

// Important: Don't forget to include the call to System.loadLibrary
// as shown at the bottom of this code sample.
package com.test;

import android.app.Activity;
import com.chilkatsoft.*;

import android.widget.TextView;
import android.os.Bundle;

public class SimpleActivity extends Activity {

  private static final String TAG = "Chilkat";

  // Called when the activity is first created.
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkRsa rsa = new CkRsa();

    CkPrivateKey privKey = new CkPrivateKey();

    boolean success = privKey.LoadPemFile("myPrivateKey.pem");
    if (success != true) {
        Log.i(TAG, privKey.lastErrorText());
        return;
        }

    success = rsa.ImportPrivateKeyObj(privKey);
    if (success != true) {
        Log.i(TAG, rsa.lastErrorText());
        return;
        }

    // Load the encrypted bytes.
    // This will typically be a file that is 128, 256, etc. bytes in length.
    // For example, maybe it is a file containing an encrypted passphrase...
    CkBinData bdEncrypted = new CkBinData();
    success = bdEncrypted.LoadFile("qa_data/passphrase.enc");
    if (success != true) {
        Log.i(TAG, "Failed to load file.");
        return;
        }

    // In this case, we know that it was a string that was encrypted,
    // so decryption should result in a string.
    // To make things easy, we'll pass the RSA encrypted data as a Base64 string 
    // to the decryptor.
    rsa.put_EncodingMode("base64");
    String passphrase = rsa.decryptStringENC(bdEncrypted.getEncoded("base64"),true);
    if (rsa.get_LastMethodSuccess() != true) {
        Log.i(TAG, rsa.lastErrorText());
        return;
        }

    Log.i(TAG, "Decrypted passphrase: " + passphrase);

  }

  static {
      System.loadLibrary("chilkat");

      // Note: If the incorrect library name is passed to System.loadLibrary,
      // then you will see the following error message at application startup:
      //"The application <your-application-name> has stopped unexpectedly. Please try again."
  }
}