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

I use the following code to convert a PubliKey to PEM Base64 format

String publicKeyString = javax.xml.bind.DatatypeConverter.printBase64Binary(publicKey.getEncoded());

I hope it helps.

Answer 2

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.

Answer 3

Asymmetric keys like those from RSA are usually stored in X509 format. Therefor you can use X509EncodedKeySpecinstead.

A simple example is already in the Java 7 JavaDoc (just replace DSA with RSA): http://docs.oracle.com/javase/7/docs/api/java/security/KeyFactory.html

X509EncodedKeySpec bobPubKeySpec = new X509EncodedKeySpec(bobEncodedPubKey);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PublicKey bobPubKey = keyFactory.generatePublic(bobPubKeySpec);

If you need to deserialize the private from byte[], I've found that you must use PKCS8EncodedKeySpec.