[Android] how to get pem format public key from modulus and exponent, pemexponent

[Android] how to get pem format public key from modulus and exponent, pemexponent

To do this, we need to download 2 jar, which list below:

Bouncycastle. openssl + commons-codec-1.10

If you want to compile the app from android source code, you will need to write your own"Android. mk"

package com.example.getpubkey;import java.io.StringWriter;import java.math.BigInteger;import java.security.KeyFactory;import java.security.PublicKey;import java.security.interfaces.RSAPublicKey;import java.security.spec.RSAPublicKeySpec;import java.security.spec.X509EncodedKeySpec;import android.app.Activity;import android.os.Bundle;import android.util.Log;import android.view.Menu;import android.view.MenuItem;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;import org.apache.commons.codec.binary.Hex;import org.bouncycastle.openssl.*;public class MainActivity extends Activity {    String key_N = "f22f26ef784475df476e15e3f6595329a7b4f8ad02ffb7c2c8f24baae2c6f5363c0a6f2288935c244d541bfcbfd0fd2382160672d10e8ed362a59661311094f8263810112e4a67d44a07527c514dbd2f4af7f23852645489f191802d96d12f65825a6029eedc8c284bcd2732355bf9848cabe82aef1bb4a60fa7b0e8eb8478fa1dddc36f4eeb6e7e952306e88ebddb593db1b538bde8230a8edc1237575370a8ebc8c3c64588b49397e2cad4b707b4b66515c3bf0f78b6b27dfdf0c28cd5fdc4fc91383681f6ab2f39d524df96c3277ca5d855c890ce331f655ddec5e37c52bf2ad30ec1026b117d5dae899b25b87c5ca23cd005bcc970a3f33605591aeb0755";    String key_E = "00010001";    Button btn1 = null;    static String pubkey = null;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        btn1 = (Button)this.findViewById(R.id.btn1);        btn1.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                try {                    pubkey = convertPublicKey(key_N,key_E);                } catch (Exception e) {                    // TODO Auto-generated catch block                    e.printStackTrace();                }                Log.d("GetPub",pubkey);            }        });    }    private String convertPublicKey(String modulus, String exponent) throws Exception {        PublicKey pub;        StringWriter writer = new StringWriter();        BigInteger data_N = new BigInteger(Hex.decodeHex(modulus.toCharArray()));        BigInteger data_E = new BigInteger(Hex.decodeHex(exponent.toCharArray()));        /* use the N,E to generate the OpensslRSAPublickey */        RSAPublicKeySpec spec = new RSAPublicKeySpec(data_N, data_E);        KeyFactory factory = KeyFactory.getInstance("RSA");        pub = factory.generatePublic(spec);        /* use the pemwriter to generate the pem format key */        PEMWriter pemWriter = new PEMWriter(writer);        pemWriter.writeObject(pub);        pemWriter.flush();        pemWriter.close();        return writer.toString();    }}