(Android™) RSA-OAEP with SHA256 hashing

RSA encrypt a SHA256 hash with OAEP padding.

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.

    CkPublicKey pubkey = new CkPublicKey();

    CkStringBuilder sbPem = new CkStringBuilder();
    boolean bCrlf = true;
    sbPem.AppendLine("-----BEGIN PUBLIC KEY-----",bCrlf);
    sbPem.AppendLine("MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA33TqqLR3eeUmDtHS89qF",bCrlf);
    sbPem.AppendLine("3p4MP7Wfqt2Zjj3lZjLjjCGDvwr9cJNlNDiuKboODgUiT4ZdPWbOiMAfDcDzlOxA",bCrlf);
    sbPem.AppendLine("04DDnEFGAf+kDQiNSe2ZtqC7bnIc8+KSG/qOGQIVaay4Ucr6ovDkykO5Hxn7OU7s",bCrlf);
    sbPem.AppendLine("Jp9TP9H0JH8zMQA6YzijYH9LsupTerrY3U6zyihVEDXXOv08vBHk50BMFJbE9iwF",bCrlf);
    sbPem.AppendLine("wnxCsU5+UZUZYw87Uu0n4LPFS9BT8tUIvAfnRXIEWCha3KbFWmdZQZlyrFw0buUE",bCrlf);
    sbPem.AppendLine("f0YN3/Q0auBkdbDR/ES2PbgKTJdkjc/rEeM0TxvOUf7HuUNOhrtAVEN1D5uuxE1W",bCrlf);
    sbPem.AppendLine("SwIDAQAB",bCrlf);
    sbPem.AppendLine("-----END PUBLIC KEY-----",bCrlf);

    // Load the public key object from the PEM. 
    boolean success = pubkey.LoadFromString(sbPem.getAsString());
    if (success != true) {
        Log.i(TAG, pubkey.lastErrorText());
        return;
        }

    // Build a small string to encrypt
    CkJsonObject json = new CkJsonObject();
    json.UpdateString("example","123");
    json.UpdateString("hello","world");
    Log.i(TAG, json.emit());

    // This is the JSON to be RSA encrypted:  {"example":"123","hello":"world"}

    // IMPORTANT: RSA encryption is only used to encrypt small amounts of data.
    // RSA is only able to encrypt data to a maximum amount of your key size (2048 bits = 256 bytes) 
    // minus padding / header data (11 bytes for PKCS#1 v1.5 padding, 42 bytes for OAEP).
    // As a result it is often not possible to encrypt files with RSA directly. 
    // RSA is also not meant for this purpose. 
    // 
    // If you want to encrypt more data, you can use something like:
    // 1) Generate a 256-bit random keystring K
    // 2) Encrypt your data with AES-CBC with K
    // 3) Encrypt K with RSA
    // 4) Send both to the other side 

    CkRsa rsa = new CkRsa();
    rsa.put_OaepPadding(true);
    rsa.put_OaepHash("SHA-256");
    rsa.ImportPublicKeyObj(pubkey);
    rsa.put_EncodingMode("base64");

    // Note: The OAEP padding uses random bytes in the padding, and therefore each time encryption happens,
    // even using the same data and key, the result will be different --  but still valid.  One should not expect
    // to get the same output.
    boolean bUsePrivateKey = false;
    String encryptedStr = rsa.encryptStringENC(json.emit(),bUsePrivateKey);
    if (rsa.get_LastMethodSuccess() != true) {
        Log.i(TAG, rsa.lastErrorText());
        return;
        }

    Log.i(TAG, "Result: " + encryptedStr);

  }

  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."
  }
}