Complete Random Password Generation Algorithm

Hi Friends,
Few days back I had a requirement of a routine/method which will generate a completely random password but a very strong password too.


Requirements : -
Generated password must contain
a) At least one Capital letter (i.e. A to Z - ASCII value- 65 to 90)
b) At least one special character (from set of - '@','!','#','$','^','&','*','~')
c) At least one numeric character (i.e. 0 to 9 - ASCII value - 48 to 57)
d) and remaining characters to small letters (i.e. a to z - ASCII value - 97-122)


There positions to be random and each time it should output a random, different, distinct password.
So here is my code implementation - (in JAVA)
But logic can be changed easily for any language.


Code :

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.security.DigestInputStream;
import java.security.DigestOutputStream;
import java.security.MessageDigest;
import java.util.Arrays;
import java.util.Random;

/**
 * @author Pramod Khare
 * Purpose : To generate Random password with required strength
 */
public class Main {

 public static void main(String arg[]) throws Exception {
  System.out.println("Password = " + generateResetPassword());
 }

 public static String generateResetPassword() {
  String finalPwd = null;
  char[] num;
  char[] spchar;
  char[] caps;
  Random rand = new Random();
  int totalLength = rand.nextInt(5) + 8;
  int spcharlen = rand.nextInt(2) + 1; // Max - 2 , Min -1
  int capslen = rand.nextInt(2) + 1; // Max - 2 , Min -1
  int numlen = rand.nextInt(2) + 1; // Max - 2 , Min -1

  int extracharlen = spcharlen + capslen + numlen;

  int[] capsPOS = new int[capslen]; // Position index array
  int[] spcharPOS = new int[spcharlen];
  int[] numPOS = new int[numlen];

  // calculate indexes for chars
  for (int i = 0; i < capslen; i++) {
   int temp = (rand.nextInt(totalLength - 1) + 1);
   for (int t = 0; t < i; t++) {
    if (temp == capsPOS[t]) {
     temp = (rand.nextInt(totalLength - 1) + 1);
     t = -1;
    }
   }
   capsPOS[i] = temp; // from 1 to n-1
  }
  for (int i = 0; i < spcharlen; i++) {
   int temp = (rand.nextInt(totalLength - 1) + 1);
   for (int j = 0, t = 0; capslen > j || t < i; j++, t++) {
    if ((t < i && temp == spcharPOS[t])
      || (capslen > j && temp == capsPOS[j])) {
     temp = (rand.nextInt(totalLength - 1) + 1);
     j = -1;
     t = -1;
    }
   }
   spcharPOS[i] = temp;
  }
  for (int i = 0; i < numlen; i++) {
   int temp = (rand.nextInt(totalLength - 1) + 1);
   for (int j = 0, k = 0, t = 0; (capslen > j || k < spcharlen || t < i); j++, k++, t++) {
    if ((j < capslen && temp == capsPOS[j])
      || (k < spcharlen && temp == spcharPOS[k])
      || (t < i && temp == numPOS[t])) {
     temp = (rand.nextInt(totalLength - 1) + 1);
     j = -1;
     k = -1;
     t = -1;
    }
   }
   numPOS[i] = temp;
  }

  char[] spchars = { '@', '!', '#', '$', '^', '&', '*', '~' }; // 0-7 = 8
  // a-z - 97-122
  // A-Z - 65-90
  // 0-9 - 48-57
  // Generate Random Characters

  char[] extrachars = new char[extracharlen]; // hold actual chars
  int[] extracharsPOS = new int[extracharlen]; // hold there positions
  int e = 0;
  for (int i = 0; i < capslen; i++) {
   extrachars[e] = (char) (rand.nextInt(26) + 65);
   extracharsPOS[e++] = capsPOS[i];
  }
  for (int i = 0; i < spcharlen; i++) {
   extrachars[e] = spchars[rand.nextInt(8)];
   extracharsPOS[e++] = spcharPOS[i];
  }
  for (int i = 0; i < numlen; i++) {
   extrachars[e] = (char) (rand.nextInt(10) + 48);
   extracharsPOS[e++] = numPOS[i];
  }
  System.out.println("Total Extra Chars = " + e);
  for (int i = 0; i < extracharsPOS.length; i++) {
   System.out.println("POS - " + extracharsPOS[i] + "\t Char - "
     + extrachars[i]);
  }
  // To do - randomise char order too.. currently caps,spachars,nums is
  // fixed order,
  // randomise it according to their actual positions
  // While sorting change char positions too
  // Arrays.sort(extracharsPOS);

  // For Complete Randomness of chars -
  int temp = 0;
  for (int i = 0; i < extracharsPOS.length; i++) {
   for (int j = (i + 1); j < extracharsPOS.length; j++) {
    if (extracharsPOS[i] > extracharsPOS[j]) {
     temp = extracharsPOS[i];
     extracharsPOS[i] = extracharsPOS[j];
     extracharsPOS[j] = temp;

     // Changing Value Array positions accordingly too
     temp = (int) extrachars[i];
     extrachars[i] = extrachars[j];
     extrachars[j] = (char) temp;
    }
   }
  }

  System.out.println("totalLength = " + totalLength);
  System.out.println("capslen = " + capslen);
  System.out.println("spcharlen = " + spcharlen);
  System.out.println("numlen = " + numlen);

  String pwd = "";
  for (int m = 0, f = 0; m < totalLength; m++) {
   if (f < extrachars.length && extracharsPOS[f] == m) {
    pwd += extrachars[f];
    f++;
   } else {
    pwd += (char) (rand.nextInt(26) + 97);
   }
  }
  return pwd;
 }
}