A java 1.6 ccompatible class that provides RSA encryption using a base64 encoded public key string.

Crypto.java

/**
 * A java 1.6 ccompatible class that provides RSA encryption using a 
 * base64 encoded public key string.
 * 
 * To encrypt, do 
 * Crypto.encrypt("Text to encrypt"); // returns base64 encoded string for encrypted data
 * 
 * @author Ibrahim Lawal (ibrahim@lawal.me)
 * @version 2016/02/29
 */
import javax.xml.bind.DatatypeConverter;

import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import javax.crypto.Cipher;

// No inheriting this class - hence the final
public final class Crypto {
    // This class should not be instantiated. Only valid action is Crypto.encrypt(String)
    private Crypto() {
        throw new RuntimeException("This class should not be instantiated. Only valid action is Crypto.encrypt(String)");
    }

  private static String ALGORITHM = "RSA";
  private static String CIPHER = "RSA/ECB/PKCS1Padding";
  private static String PUBLIC_KEY = "ENTER//THE//BASE//64//ENCODED//KEY//String" +
      "HERE///////DO//NOT//INCLude//boundaries==";
  
 private static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;

    try {

      // get an RSA cipher object
      final Cipher cipher = Cipher.getInstance(CIPHER);

      //init cipher and encrypt the plain text using the public key
      cipher.init(Cipher.ENCRYPT_MODE, key);
      cipherText = cipher.doFinal(text.getBytes());

    } catch (Exception e) {

      e.printStackTrace();
    }
    return cipherText;
  }

  public static String encrypt(String text) throws Exception {
      
    return new String(DatatypeConverter.printBase64Binary(encrypt(text, getPublicKeyFromString())));
  }

  private static PublicKey getPublicKeyFromString() throws Exception {

    PublicKey key;

    try {
      //init keyfactory
      KeyFactory kf = KeyFactory.getInstance(ALGORITHM);

      //decode the key into a byte array
      byte[] keyBytes = DatatypeConverter.parseBase64Binary(PUBLIC_KEY);

      //create spec
      X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);

      //generate public key
      key = kf.generatePublic(spec);
    } catch (InvalidKeySpecException | NoSuchAlgorithmException e) {
      throw new Exception("Invalid publishable key: " + e.getMessage());
    }
    return key;

  }
}