(Android™) Duplicate openssl rsautl -sign -in small.dat -inkey private.pem -out rsautl.sig

Demonstrates how to duplicate this OpenSSL command:

openssl rsautl -sign -in small.dat -inkey private.pem -out rsautl.sig

How much data can be signed with "openssl rsautl"? The maximum size (in bytes) is the key size minus the overhead for the padding. The overhead size depends on the padding. With OAEP padding, the overhead is 42 bytes. With the default PKCSv1.5 padding, the overhead is 11 bytes.

Given a 2048-bit RSA key (256 bytes) and using PKCSv1.5 padding, the max data size that can be signed is 256 - 11 = 245 bytes.

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

    CkPrivateKey pkey = new CkPrivateKey();

    // Load the private key from an PEM file:
    boolean success = pkey.LoadPemFile("private.pem");
    if (success != true) {
        Log.i(TAG, pkey.lastErrorText());
        return;
        }

    // Load the file to be signed.
    CkBinData bd = new CkBinData();
    success = bd.LoadFile("small.dat");

    CkRsa rsa = new CkRsa();

    // Import the private key into the RSA component:
    success = rsa.ImportPrivateKeyObj(pkey);
    if (success != true) {
        Log.i(TAG, rsa.lastErrorText());
        return;
        }

    // OpenSSL uses big-endian.
    rsa.put_LittleEndian(false);

    // Sign bd.  If successful, bd will contain the RSA signature that contains the original data.
    success = rsa.OpenSslSignBd(bd);
    if (success != true) {
        Log.i(TAG, rsa.lastErrorText());
        return;
        }

    // Save the binary signature to a file.
    success = bd.WriteFile("rsautl.sig");
    if (success != true) {
        Log.i(TAG, "Failed to save output signature file.");
        return;
        }

    Log.i(TAG, "Success.");

  }

  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."
  }
}