RSA encrypt with base64 encoded public key in Android

10
votes

How to do RSA encryption of byte array with base-64 encoded public key?

After reading the couple of articles( of google search ) on how to do RSA encryption in Java, found the following snippet 

public byte[] rsaEncrypt(byte[] data) {
  PublicKey pubKey = readKeyFromFile("/public.key");
  Cipher cipher = Cipher.getInstance("RSA");
  cipher.init(Cipher.ENCRYPT_MODE, pubKey);
  byte[] cipherData = cipher.doFinal(src);
  return cipherData;
}

I have public key as a base64 encoded string (126 characters), How do i create 'PublicKey' with the encoded string and use it with Cipher.

2
javaandroidrsa
How can we know what is contained in your base64 encoded string? It is probably a modulus and a public exponent, but what form? Probably the DER encoding of an X509EncodedKeySpec. – President James K. Polk

2 Answers

8
votes

Your base64 string is possibly an X509EncodedKeySpec. I can only guess. If so, you should base64 decode the string to obtain a byte []. Then construct an X509EncodedKeySpec from this byte []. Then create an instance of an RSA KeyFactory, and use the generatePublic() method of this KeyFactory to obtain a PublicKey. This public key can then be passed to Cipher.init().

Note: to perform base64 decoding use either the apache commons codec, or the Harder base64 decoder.

UPDATE March 8, 2017: In better-late-than-never news, Java 8 now includes a Base64 encoding/decoding class, java.util.Base64

4
votes

this is how you can generate Public and Private key pair below is the function to store them on hard dist

enter code here
public static void GenerateKeyPair()
{       
    try{
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(2048);
        KeyPair kp = kpg.genKeyPair();

        KeyFactory fact = KeyFactory.getInstance("RSA");
        RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(),
          RSAPublicKeySpec.class);
        RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(),
          RSAPrivateKeySpec.class);

        saveToFile("public.key", pub.getModulus(),
                  pub.getPublicExponent());
        saveToFile("private.key", priv.getModulus(),
                  priv.getPrivateExponent());
    }catch(Exception e){
        System.out.println(e.getMessage());
    }
}

public static void saveToFile(String fileName,
  BigInteger mod, BigInteger exp) throws Exception {
  ObjectOutputStream oout = new ObjectOutputStream(
    new BufferedOutputStream(new FileOutputStream(fileName)));
  try {
    oout.writeObject(mod);
    oout.writeObject(exp);
  } catch (Exception e) {
    throw new Exception("error", e);
  } finally {
    oout.close();
  }
}