import java.security.InvalidAlgorithmParameterException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.security.spec.RSAKeyGenParameterSpec;

import static java.lang.String.format;

public class Main {
  private static int KEY_SIZE = 2048;

  public static void main(String[] args) throws Exception {
    genKey1();
    genKey2();
  }

  public static void genKey1() throws NoSuchAlgorithmException, InvalidAlgorithmParameterException {
    KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
    RSAKeyGenParameterSpec params = new RSAKeyGenParameterSpec(KEY_SIZE, RSAKeyGenParameterSpec.F4);
    keyGen.initialize(params, new FakeRandom());
    KeyPair keyPair = keyGen.generateKeyPair();
    print(keyPair);
  }

  public static void genKey2() throws NoSuchAlgorithmException {
    KeyPairGenerator keyGen = KeyPairGenerator.getInstance("RSA");
    keyGen.initialize(KEY_SIZE, new FakeRandom());
    KeyPair keyPair = keyGen.generateKeyPair();
    print(keyPair);
  }

  public static void print(KeyPair key) throws NoSuchAlgorithmException {
    MessageDigest prng = MessageDigest.getInstance("SHA-256");
    byte[] pub = prng.digest(key.getPublic().getEncoded());
    for (int i = 0; i < pub.length; i++) {
      System.out.print(format("%02x", pub[i]));
    }
    System.out.println();
  }

  public static class FakeRandom extends SecureRandom {
    private byte[] state;
    private int counter;

    public FakeRandom() {
      state = new byte[32];
      for (byte i=0; i<32; i++) {
        state[i] = i;
      }
    }

    @Override
    public void nextBytes(byte[] bytes) {
      System.err.println(format("in FakeRandom: %d %d", bytes.length, counter++));
      try {
        int offset = 0;
        while (true) {
          MessageDigest prng = MessageDigest.getInstance("SHA-256");
          state = prng.digest(state);
          for (int i = 0; i < state.length; i++) {
            if (offset == bytes.length) {
              return;
            }
            bytes[offset++] = state[i];
          }
        }
      } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(e);
      }
    }
  }
}