How to implement RSA encrypt and decrypt in Java

Question

I have the public and private key generation working. My next step is to create 2 more methods - encrypt and decrypt. I'm just not sure about how to implement the encrypt and decrypt. I have a few ideas, but nothing that seems to be a good solution. Any insights?

public class RSA
{
    private final static BigInteger one = new BigInteger("1");
    private final static SecureRandom random = new SecureRandom();

    // prime numbers
    private BigInteger p;
    private BigInteger q;

    // modulus
    private BigInteger n;

    // totient
    private BigInteger t;

    // public key
    private BigInteger e;

    // private key
    private BigInteger d;

    /**
     * Constructor for objects of class RSA
     */
    public RSA(int N)
    {
        p = BigInteger.probablePrime(N/2, random);
        q = BigInteger.probablePrime(N/2, random);

        // initialising modulus
        n = p.multiply(q);

        // initialising t by euclid's totient function (p-1)(q-1)
        t = (p.subtract(one)).multiply(q.subtract(one));

        // initialising public key ~ 65537 is common public key
        e = new BigInteger("65537");
    }

    public int generatePrivateKey()
    {
         d = e.modInverse(t);
         return d.intValue();
    }
}

Your private key generation function may fail. As an exercise, determine why. HINT: It will fail approximately with prob = 2/65537. Also, d.intValue() is a meaningless quantity. — President James K. Polk
– President James K. Polk, Commented Jan 1, 2011 at 2:51

bmargulies · Accepted Answer · 2011-01-01 02:12:18Z

4

Since you haven't really asked a specific question, I'll offer you a somewhat tangential answer.

DIY Crypto is famously a bit like DIY nuclear power.

I recommend reading bouncy castle if you want to learn about crypto coding, and using it rather than rolling your own.

answered Jan 1, 2011 at 2:12

bmargulies

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Comments

David R Tribble · Accepted Answer · 2011-01-03 16:49:09Z

0

Not sure what you're asking, but for RSA crypto, if your modulus is n bits wide, your public key will also be n bits wide. A simple int won't work.

See also

My own humble attempt at RSA encryption in Java:
http://david.tribble.com/src/java/tribble/crypto/RSACipher.java

answered Jan 3, 2011 at 16:49

David R Tribble

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