3 replies to this topic

[Purkinje]

First off, the problem I am about to post has been fixed. I could post my updated code, but I wanted to know why I was having this problem, and what the best way to solve it would be if it arises again.

I'm working on an RSA algorithm for school, and I need to generate n to be a certain length (300 digits to be exact). I found that the length was different when I called it again later. Let me post a condensed version of my program that displays my problem.

import java.math.BigInteger;

import java.util.Random;


public class post {

	private static BigInteger n;

	private static BigInteger phi;


	public static BigInteger generateN (int bitLength) {

		BigInteger p;

		BigInteger q;

		BigInteger pq;

		int pqLength;

		String pqCheck = "255";

		Random r = new Random();


		p = new BigInteger ( BigInteger.probablePrime ( bitLength, r ).toString() );

		q = new BigInteger ( BigInteger.probablePrime ( bitLength, r ).toString() );

		phi = (p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE)));

		pq = new BigInteger ( p.multiply ( q ).toString() );

		pqLength = pq.toString().length();

		System.out.println ( "pq's initial length = " + pqLength );


		if ( pqLength == 300  && pq.toString().substring(0,3).compareTo(pqCheck) > 0 ) {

			pqLength = pq.toString().length();

			System.out.println("pq's final length = " + pq.toString().length());

			System.out.println ( "pq's length is correct! ");

			return pq;

		}


		else if ( pqLength > 300 ) {

			System.out.println ( "pq's length = " + pqLength + ".");

			System.out.println("Adjusting pq's size . . .");

			generateN ( bitLength - 1 );

		}


		else if ( pqLength < 300 ) {

			System.out.println ( "pq's length = " + pqLength + ".");

			System.out.println("Adjusting pq's size . . .");

			generateN ( bitLength + 1 );

		}

		return pq;

	}




	public static void main (String[] args) {

		n = generateN(500);

		System.out.println("n = " + n);

		System.out.println("n length: " + n.toString().length());




	}

}

Here's the output:

pq's initial length = 301

pq's length = 301.

Adjusting pq's size . . .

pq's initial length = 301

pq's length = 301.

Adjusting pq's size . . .

pq's initial length = 300

pq's final length = 300

pq's length is correct!

n = 8329309603832552605779583004111676117460428566431326966642828167503750649563

80535960505216672704803077357421811675713198519942740954310354688260927978852650

64394907705824412066832605959708311583720908765772363409810060959977345989655475

43031211942699693596749477723293046696554262266476407957866108867

n length: 301

As you can see, it says that pq's length is 300, but n's length is 301. The numbers are clearly not identical, and I don't know why.

Where am I going wrong in this? Why is n not the value I assume that I assign it?

Thanks for the help.

[wim DC]

	public static BigInteger generateN (int bitLength) {

		BigInteger p;

		BigInteger q;

		BigInteger pq;

		int pqLength;

		String pqCheck = "255";

		Random r = new Random();


		p = new BigInteger ( BigInteger.probablePrime ( bitLength, r ).toString() );

		q = new BigInteger ( BigInteger.probablePrime ( bitLength, r ).toString() );

		phi = (p.subtract(BigInteger.ONE).multiply(q.subtract(BigInteger.ONE)));

		pq = new BigInteger ( p.multiply ( q ).toString() );

		pqLength = pq.toString().length();

		System.out.println ( "pq's initial length = " + pqLength );


		if ( pqLength == 300  && pq.toString().substring(0,3).compareTo(pqCheck) > 0 ) {

			pqLength = pq.toString().length();

			System.out.println("pq's final length = " + pq.toString().length());

			System.out.println ( "pq's length is correct! ");

			return pq;

		}


		else if ( pqLength > 300 ) {

			System.out.println ( "pq's length = " + pqLength + ".");

			System.out.println("Adjusting pq's size . . .");

			[B][COLOR="red"]return [/COLOR][/B]generateN ( bitLength - 1 );

		}

		[COLOR="red"][B]else {[/B][/COLOR]

			System.out.println ( "pq's length = " + pqLength + ".");

			System.out.println("Adjusting pq's size . . .");

			[B][COLOR="red"]return [/COLOR][/B]generateN ( bitLength + 1 );

		}

                [STRIKE][B][COLOR="red"]return pq;[/COLOR][/B][/STRIKE]

	}

There, I fixed it :)

[Purkinje]

If I get rid of the last return pq at the end, I get the following error:

post.java:42: missing return statement

        }

        ^

1 error

I think it still works if I leave that 'return pq;' in there at line 42?

[sourlemon]

Regarding the error you got, did you change the last statement to "else" instead of "if else"?

Taking a quick scan at your code, the possible problem that I see is that you're not returning the correct value. When you check if pq is less or more than 300, you didn't collect the result after calling generateN again. So you can use win DC method to return it. Else, you need to say
pq = generateN ( bitLength + 1 );

then return it at the end of the method.