Random string generation

[Anna Hays]

Hi, can Java generate a string of random charactors? I need a password generation. Can someone give me an idea of what's the proper way of doing this please? Thanks.

[Michael Dunn]

This might be one way

class Testing
{
  public Testing()
  {
    final int PASSWORD_LENGTH = 8;
    StringBuffer sb = new StringBuffer();
    for (int x = 0; x < PASSWORD_LENGTH; x++)
    {
      sb.append((char)((int)(Math.random()*26)+97));
    }
    System.out.println(sb.toString());
  }
  public static void main(String[] args){new Testing();}
}

[Steve Deadsea]

I wrote libraries for doing this:
http://ostermiller.org/utils/RandPass.html

[somkiat puisungnoen]

Originally posted by Anna Kafei:
Hi, can Java generate a string of random charactors? I need a password generation. Can someone give me an idea of what's the proper way of doing this please? Thanks.

1. Specified your data for random.
2. Specified length for generate password.
3. Create random utilities , you can use Math.random() method or java.util.Random class.

[Ian Darwin]

Everybody has their own idea; here's one I've used on a couple of projects.

 
package com.darwinsys.security;
 
/**
 * Cheap, lightweight, low-security password generator. See also:
 * java.security.*;
 */
public class PassPhrase {
    /**
     * Minimum length for a decent password
     */
    public static final int MIN_LENGTH = 10;
 
    /**
     * The random number generator.
     */
    protected static java.util.Random r = new java.util.Random();
 
    /* Set of characters that is valid. Must be printable, memorable,
     * and "won't break HTML" (i.e., not '<', '>', '&', '=', ...).
     * or break shell commands (i.e., not '<', '>', '$', '!', ...).
     * I, L and O are good to leave out, as are numeric zero and one.
     */
    protected static char[] goodChar = {
        // Comment out next two lines to make upper-case-only, then
        // use String toUpper() on the user's input before validating.
        'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'j', 'k', 'm', 'n',
        'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'J', 'K', 'M', 'N',
        'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
        '2', '3', '4', '5', '6', '7', '8', '9',
        '+', '-', '@',
    };
 
    /* Generate a Password object with a random password. */
    public static String getNext() {
        return getNext(MIN_LENGTH);
    }
 
    /* Generate a Password object with a random password. */
    public static String getNext(int length) {
        if (length < 1) {
            throw new IllegalArgumentException(
                    "Ridiculous password length " + length);
        }
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < length; i++) {
            sb.append(goodChar[r.nextInt(goodChar.length)]);
        }
        return sb.toString();
    }
}
 

But since I never like to ship code without testing it:

 package regress;
 
import junit.framework.TestCase;
 
import com.darwinsys.security.PassPhrase;
 
/**
 * Test the PassPhrase class.
 */
public class PassPhraseTest extends TestCase {
 
    /*
     * Class under test for String getNext()
     */
    public void testGetNext() {
        String s = PassPhrase.getNext();
        System.out.println(s);
        assertNotNull(s);
        assertEquals(PassPhrase.MIN_LENGTH, s.length());
    }
 
    /*
     * Class under test for String getNext(int)
     */
    public void testGetNextint() {
        final int[] lengths = {1, 5, 20, 200};
        for (int i = 0; i & lt; lengths.length
        ;
        i++) {
            int j = lengths[i];
            String s = PassPhrase.getNext(j);
            System.out.println(s);
            assertNotNull(s);
            assertEquals(j, s.length());
        }
    }
 
    public void testGetNextZero() {
        try {
            PassPhrase.getNext(0);
            fail("Did not trap length zero request");
        } catch (IllegalArgumentException ex) {
            System.out.println("OK: Caught expected exception");
 
        }
    }
}

Cheers

Ian

[fixed some formatting - Jim]
[ October 13, 2004: Message edited by: Jim Yingst ]

[Stan James]

If you have a long enough password space another good technique is to take a large dictionary and randomly choose two words. These are much easier to remember than a string of random letters & numbers. I still recall my original CompuServe password from the 80s was "water boldly".