How can I create a password?

Question

I want to give maybe a million password to some users that should be like:

1. It must have at least 6 characters
2. It must have digits and also letters

Answer 1

Here's one that I wrote a while back:

package com.stackoverflow.does.my.code.for.me;

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.ArrayList;
import java.util.List;

public class PasswordUtil {

    /** Minimum password length = 6 */
    public static final int MIN_PASSWORD_LENGTH = 6;
    /** Maximum password length = 8 */
    public static final int MAX_PASSWORD_LENGTH = 8;

    /** Uppercase characters A-Z */
    public static final char[] UPPERS = new char[26];
    /** Lowercase characters a-z */
    public static final char[] LOWERS = new char[26];
    /**
     * Printable non-alphanumeric characters, excluding space.
     */
    public static final char[] SPECIALS = new char[32];
    public static final char[] DIGITS = new char[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    static {
        // Static initializer block for populating arrays
        int U = 'A';
        int l = 'a';
        int d = '0';
        for (int i = 0; i < 26; i++) {
            UPPERS[i] = (char) (U + i);
            LOWERS[i] = (char) (l + i);
            if (i < 10) {
                DIGITS[i] = (char) (d + i);
            }
        }
        int p = 0;
        for (int s = 33; s < 127; s++) {
            char specialChar = (char) 32;

            if (s >= 'a' && s <= 'z')
                s = 'z' + 1; // jump over 'a' to 'z'
            else if (s >= 'A' && s <= 'Z')
                s = 'Z' + 1; // jump over 'A' to 'Z'
            else if (s >= '0' && s <= '9')
                s = '9' + 1; // jump over '0' to '9'

            specialChar = (char) s;
            SPECIALS[p] = specialChar;
            p++;
        }
    }

    public String generatePassword() {
        List<char[]> activeSets = new ArrayList<char[]>(4);
        List<char[]> inactiveSets = new ArrayList<char[]>(4);

        activeSets.add(UPPERS);
        activeSets.add(LOWERS);
        activeSets.add(SPECIALS);
        activeSets.add(DIGITS);

        SecureRandom random = new SecureRandom();

        int passwordLength = 5 + random.nextInt(3);
        StringBuffer password = new StringBuffer(passwordLength + 1);

        for (int p = 0; p <= passwordLength; p++) {
            char[] randomSet = null;
            if (activeSets.size() > 1) {
                int rSet = random.nextInt(activeSets.size());
                randomSet = activeSets.get(rSet);
                inactiveSets.add(randomSet);
                activeSets.remove(rSet);
            } else {
                randomSet = activeSets.get(0);
                inactiveSets.add(randomSet);
                activeSets.clear();
                activeSets.addAll(inactiveSets);
                inactiveSets.clear();
            }
            int rChar = random.nextInt(randomSet.length);
            char randomChar = randomSet[rChar];
            password.append(randomChar);
        }

        return password.toString();
    }
}

Answer 2

Implemented a PasswordBuilder. Supports passwrod limit, must have characters and how much of them, char ranges and a list of them.

Usage Example:

System.out.println(new PasswordBuilder().addCharsOption("!@#$%&*()_-+=[]{}\\|:/?.,><", 1).addRangeOption('A', 'Z', 1).addRangeOption('a', 'z', 0).addRangeOption('0', '9', 1).build());

Example result: QU1GY7p+j+-PUW+_

System.out.println(new PasswordBuilder().addCharsOption("!@#$%&*()_-+=[]{}\\|:/?.,><", 1).addRangeOption('A', 'Z', 1).addRangeOption('a', 'z', 0).addRangeOption('0', '9', 1).setSize(5).build());

Example result: %,4NX

Implementation:

//Version=1.0
//Source=https://www.dropbox.com/s/3a4uyrd2kcqdo28/PasswordBuilder.java?dl=0
//Dependencies=java:7 com.google.guava:guava:18.0 commons-lang:commons-lang:2.6

import com.google.common.primitives.Chars;
import org.apache.commons.lang.ArrayUtils;

import java.security.SecureRandom;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * Created by alik on 5/26/16.
 */
public class PasswordBuilder {
    private int size = 16;
    private List<Character> options = new ArrayList<>();
    private Map<List<Character>, Integer> musts = new java.util.LinkedHashMap<>();
    private SecureRandom secureRandom = new SecureRandom();

    public PasswordBuilder() {
    }

    public PasswordBuilder setSize(int size) {
        this.size = size;
        return this;
    }

    public PasswordBuilder addRangeOption(char from, char to, int mustCount) {
        List<Character> option = new ArrayList<>(to - from + 1);
        for (char i = from; i < to; ++i) {
            option.add(i);
        }
        return addOption(option, mustCount);
    }

    public PasswordBuilder addCharsOption(String chars, int mustCount) {
            return addOption(Chars.asList(chars.toCharArray()), mustCount);
    }

    public PasswordBuilder addOption(List<Character> option, int mustCount) {
        this.options.addAll(option);
        musts.put(option, mustCount);
        return this;

    }

    public String build() {
        validateMustsNotOverflowsSize();
        Character[] password = new Character[size];

        // Generate random from musts
        for (Map.Entry<List<Character>, Integer> entry : musts.entrySet()) {
            for (int i = 0; i < entry.getValue(); i++) {
                int charIndex = secureRandom.nextInt(entry.getKey().size());
                char c = entry.getKey().get(charIndex);
                addChar(password, c);
            }
        }

        // Generate from overall
        for (int i = 0; i < password.length; i++) {
            if (password[i] != null) continue;
            password[i] = options.get(secureRandom.nextInt(options.size()));
        }
        return new String(ArrayUtils.toPrimitive(password));
    }

    private void addChar(Character[] password, char c) {
        int i;
        for (i = secureRandom.nextInt(password.length); password[i] != null; i = secureRandom.nextInt(password.length)) {
        }
        password[i] = c;
    }

    private void validateMustsNotOverflowsSize() {
        int overallMusts = 0;
        for (Integer mustCount : musts.values()) {
            overallMusts += mustCount;
        }
        if (overallMusts > size) {
            throw new RuntimeException("Overall musts exceeds the requested size of the password.");
        }
    }

    public static void main(String[] args) {
        System.out.println(new PasswordBuilder().addCharsOption("!@#$%&*()_-+=[]{}\\|:/?.,><", 1).addRangeOption('A', 'Z', 1).addRangeOption('a', 'z', 0).addRangeOption('0', '9', 1).build());
        System.out.println(new PasswordBuilder().addCharsOption("!@#$%&*()_-+=[]{}\\|:/?.,><", 1).addRangeOption('A', 'Z', 1).addRangeOption('a', 'z', 0).addRangeOption('0', '9', 1).setSize(5).build());
    }
}

Answer 3

A bit late, but I usually use the following code:

private static final int PASSWORD_SIZE = 16;
private static final String VALID_SPECIAL_CHARACTERS = "!@#$%&*()_-+=[]{}\\|:/?.,><"; // Note the double \ as escape

private static String createPassword() {
    SecureRandom random = new SecureRandom();
    StringBuilder password = new StringBuilder();
    while (password.length() < PASSWORD_SIZE) {
        char character = (char) random.nextInt(Character.MAX_VALUE);
        if ((character >= 'a' && character <= 'z') || (character >= 'A' && character <= 'Z') || (character >= '0' && character <= '9') || VALID_SPECIAL_CHARACTERS.contains(String.valueOf(character))) {
            password.append(character);
        }
    }
    return password.toString();
}

There is no guarantee that there will always be a number, special character, lower-case and upper-case character in the password. This could be enforced by first adding a character and a digit, however this would create passwords that are a bit more predictable.

Answer 4

This will be the easiest one :)

String char_group = "abcdefghijklmnopqrstuvwxyz";
String digit_group = "123456789";

// first choose a len of pwd
Random ran = new Random();
int pwd_len = ran.nextInt(50); //50 is the max length of password,say
// check that pwd_len is not less than 6
// do the check here

// finally create the password..
StringBuffer pwd = new StringBuffer();
Random RNG = new Random();
for (int i = 0; i < pwd_len ; i++) {
    int randomNum = RNG.nextInt(100);
    char c = '';
    // Here 25% is the ratio of mixing digits
    // in the password, you can change 4 to any
    // other value to change the depth of mix
    // or you can even make it random. 
    if (randomNum % 4 == 0) {
        c = digit_group[randomNum  % digit_group.length];
    } else {
        c = char_group[randomNum  % char_group.length];
    }
    pwd.append(c);
}
return pwd.toString();