How to get a RSA PublicKey by giving a PrivateKey?

Question

I am looking for a Java function that will get an RSA PrivateKey and will return the correct RSA PublicKey?

Alternatively, is there a function that will tell us if the RSA PrivateKey/PublicKey is valid?

Answer 1

If you have your private key as an RSAPrivateCrtKey object, you can get the public exponent as well as modulous.

Then you could create the public key like so:

RSAPublicKeySpec publicKeySpec = new java.security.spec.RSAPublicKeySpec(modulus, exponent);   
try {   
     KeyFactory keyFactory = KeyFactory.getInstance("RSA");   

     PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);   
} catch (Exception e) {   
     e.printStackTrace();   
} 

Answer 2

I can't think of any good reason you'd need this. But here it is:

static boolean isValidRSAPair(KeyPair pair)
{
  Key key = pair.getPrivate();
  if (key instanceof RSAPrivateCrtKey) {
    RSAPrivateCrtKey pvt = (RSAPrivateCrtKey) key;
    BigInteger e = pvt.getPublicExponent();
    RSAPublicKey pub = (RSAPublicKey) pair.getPublic();
    return e.equals(pub.getPublicExponent()) && 
      pvt.getModulus().equals(pub.getModulus());
  }
  else {
    throw new IllegalArgumentException("Not a CRT RSA key.");
  }
}