Miscellany

package kingsy101;

import java.math.BigInteger;
import java.util.Random;

public class Cipher {
  private BigInteger n;
  private BigInteger e;
  private BigInteger d;

  public Cipher () {
    BigInteger p = BigInteger.probablePrime (32, new Random ());
    BigInteger q = selectPrimeNotEqualTo (p);

    this.n = p.multiply (q);
    BigInteger pMinus1qMinus1 = p.subtract (BigInteger.ONE).multiply (
        q.subtract (BigInteger.ONE));
    this.e = p;

    while (!e.gcd (pMinus1qMinus1).equals (BigInteger.ONE)) {
      e = e.add (BigInteger.ONE);
    }

    this.d = e.modInverse (pMinus1qMinus1);

  }

  private static BigInteger selectPrimeNotEqualTo (BigInteger p) {
    BigInteger q;
    do {
      q = BigInteger.probablePrime (32, new Random ());
    } while (p == q);
    return q;
  }

  public byte[] encryptBlock (byte[] myarray) {
    BigInteger writeBigInt = new BigInteger (myarray);
    BigInteger encrypt = (writeBigInt.modPow (e, n));
    return encrypt.toByteArray ();
  }

  public byte[] decryptBlock (byte[] blockData) {
    BigInteger blockValue = new BigInteger (blockData);
    BigInteger decrypt = (blockValue.modPow (d, n));
    return decrypt.toByteArray ();
  }

}

// ------------------

package kingsy101;

import static org.junit.Assert.*;

import org.junit.Test;

public class TestCipher {

  @Test
  public void easyValue () {
    Cipher cipher = new Cipher ();

    byte[] sourceBlock = { 1, 2, 3, 4, 5, 6, 7, 8 };

    byte[] encBlock = cipher.encryptBlock (sourceBlock);
    byte[] decBlock = cipher.decryptBlock (encBlock);

    assertArrayEquals (sourceBlock, decBlock);
  }

  @Test
  public void easyValueEndingInZero () {
    Cipher cipher = new Cipher ();

    byte[] sourceBlock = { 1, 2, 3, 4, 5, 6, 7, 0 };

    byte[] encBlock = cipher.encryptBlock (sourceBlock);
    byte[] decBlock = cipher.decryptBlock (encBlock);

    assertArrayEquals (sourceBlock, decBlock);
  }

  @Test
  public void easyValueStartingInZero () {
    Cipher cipher = new Cipher ();

    byte[] sourceBlock = { 0, 1, 2, 3, 4, 5, 6, 7 };

    byte[] encBlock = cipher.encryptBlock (sourceBlock);
    byte[] decBlock = cipher.decryptBlock (encBlock);

    assertArrayEquals (new byte[] { 1, 2, 3, 4, 5, 6, 7 }, decBlock);
  }

  @Test
  public void valueExtractedFromRealInput () {
    Cipher cipher = new Cipher ();

    byte[] sourceBlock = { 84, 104, 105, 115, 32, 112, 108, 97 };

    byte[] encBlock = cipher.encryptBlock (sourceBlock);
    byte[] decBlock = cipher.decryptBlock (encBlock);

    assertArrayEquals (sourceBlock, decBlock);

  }
}