Serialize and Deserialize an RSA public key

Question

KeyPairGenerator kpg = KeyPairGenerator.getInstance(\"RSA\");
kpg.initialize(1024);
KeyPair kp = kpg.genKeyPair();
Key publicKey = kp.getPublic();
Key privateKey = kp.ge         


        

Answer 1

Depends on what do you want to do with the serialized representation. If the consumer is no one but your own program, feel free to roll your own implementation. A public RSA key consists of two integers - exponent and modulus. Modulus is large - around 1024 bits, exponent is typically on the order of 17 bits. Both are available as BigInteger objects if you cast your public key object to RSAPublicKey.

So, to recap:

RSAPublicKey publicKey = (RSAPublicKey)kp.getPublic();
return publicKey.getModulus().toString() + "|" +
    publicKey.getPublicExponent().toString();

That's sufficient to restore the key. To deserialize:

String []Parts = MyKeyString.split("\\|");
RSAPublicKeySpec Spec = new RSAPublicKeySpec(
        new BigInteger(Parts[0]),
        new BigInteger(Parts[1]));
return KeyFactory.getInstance("RSA").generatePublic(Spec);

If the key needs to be passed to third party software, you better serialize to a standard format - PEM or DER.