(Android™) Get RSA Key Modulus from .cer or .key

Demonstrates how to get the RSA key modulus from either the certificate (.cer) or RSA key (.key). OpenSSL commands to do the same would be:

openssl x509 -inform DER -in "test.cer"  -modulus -noout 

openssl pkcs8 -inform DER -in​ "test.key"​ -outform PEM -passin pass:"12345​678a​"
   | openssl rsa -inform PEM -modulus -noout 

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);

    CkPrivateKey privKey = new CkPrivateKey();

    String password = "12345678a";
    boolean success = privKey.LoadPkcs8EncryptedFile("qa_data/certs/test_12345678a.key",password);
    if (success != true) {
        Log.i(TAG, privKey.lastErrorText());
        return;
        }

    CkXml xml = new CkXml();
    xml.LoadXml(privKey.getXml());

    // The XML contains the parts of the key in base64.
    Log.i(TAG, "Private Key XML:");
    Log.i(TAG, xml.getXml());

    // We can get the base64 modulus like this:
    String modulus = xml.getChildContent("Modulus");
    Log.i(TAG, "base64 modulus = " + modulus);

    // To convert to hex:
    CkBinData binDat = new CkBinData();
    binDat.AppendEncoded(modulus,"base64");
    String hexModulus = binDat.getEncoded("hex");
    Log.i(TAG, "hex modulus = " + hexModulus);

    // Now get the modulus from the cert:
    CkCert cert = new CkCert();

    success = cert.LoadFromFile("qa_data/certs/test_12345678a.cer");
    if (success != true) {
        Log.i(TAG, cert.lastErrorText());
        return;
        }

    // The cert contains the public key, which is composed of the
    // modulus + exponent (for RSA keys).
    CkPublicKey pubKey = cert.ExportPublicKey();

    xml.LoadXml(pubKey.getXml());
    Log.i(TAG, "Public Key XML:");
    Log.i(TAG, xml.getXml());

    // Proceed in the same way as before....
    modulus = xml.getChildContent("Modulus");
    Log.i(TAG, "base64 modulus = " + modulus);

    // To convert to hex:
    binDat.Clear();
    binDat.AppendEncoded(modulus,"base64");
    hexModulus = binDat.getEncoded("hex");
    Log.i(TAG, "hex modulus = " + hexModulus);


  }

  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."
  }
}