Java RSA Key Sizes

Question

I am trying to learn how to sign messages using the RSA algorithm with SHA256 in Java. When I generated a 2048-bit KeyPair, I found that both the public and private key were 294 bytes. Here is my code for finding the size of the keys:

import java.security.*;

public class RSATesting
{    
    public static void main(String[] args) throws Exception
    {
        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
        generator.initialize(2048, new SecureRandom());
        KeyPair pair = generator.generateKeyPair();

        byte[] publicKeyBytes=pair.getPublic().getEncoded();
        System.out.println(publicKeyBytes.length);
        byte[] privateKeyBytes=pair.getPublic().getEncoded();
        System.out.println(privateKeyBytes.length);
    }
}

Why are the keys not 256 bytes?

Thanks

Answer 1

RSA keys aren't like AES keys.

AES keys are some random bytes. RSA keys are numbers. The RSA modulus (N) defines the lenght.

Your key is 294 bytes long, because of getEncoded();. It returns a formatted key and not the real lenght.