Generating a strong password in C#?

Question

I just tried the following in linqpad:

System.Web.Security.Membership.GeneratePassword(25, 10)

This is the password I got:

[XTJ_67g.i/ag1rL)6_Yv>*+%

Or, if that's not secure enough, try this:

System.Web.Security.Membership.GeneratePassword(128, 100)

which got me the following when running it three times:

|c^.:?m)#q+(]V;}[Z(})/?-;$]+@!|^/8*_9.$&.&!(?=^!Wx?[@%+&-@b;)>N;&+*w[>$2+_$%l;+h+#zhs^{e?&=*(}X_%|:}]]}*X[+)Er%J/-=;Q0{:+=%c7:^$

/:_)hxF+*){2|;(>:*N^+!_&|}B.$})?[V=[+v({-:-@9-Z$j?.[-}(@MHx+}(}Mz_S(7#4}{..>@G|!+++{+C=|_}=+r^@&$0;L*|kz-;$++/N3$=}?;%&]]*/^#^!+

:*{]-x^$g{|?*))_=B@^.#%L;g|+)#[nq}?y(_(m;]S^I$*q=l-[_/?}&-!k^(+[_{Z|&:^%!_)!=p%=)=wYd-#.UP$%s1{*l%+[%?!c+7=@=.;{+M)!^}&d/]{];(&}

this took way less than a second, btw. The framework is your friend.

See http://msdn.microsoft.com/en-us/library/system.web.security.membership.generatepassword.aspx

Not sure where I found this but here's a class to generate high entropy, truly random strings that can be used as passwords.

using System.Security.Cryptography;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

public class PasswordGenerator
{
    public int MinimumLengthPassword { get; private set; }
    public int MaximumLengthPassword { get; private set; }
    public int MinimumLowerCaseChars { get; private set; }
    public int MinimumUpperCaseChars { get; private set; }
    public int MinimumNumericChars { get; private set; }
    public int MinimumSpecialChars { get; private set; }

    public static string AllLowerCaseChars { get; private set; }
    public static string AllUpperCaseChars { get; private set; }
    public static string AllNumericChars { get; private set; }
    public static string AllSpecialChars { get; private set; }
    private readonly string _allAvailableChars;

    private readonly RandomSecureVersion _randomSecure = new RandomSecureVersion();
    private int _minimumNumberOfChars;

    static PasswordGenerator()
    {
        // Define characters that are valid and reject ambiguous characters such as ilo, IO and 1 or 0
        AllLowerCaseChars = GetCharRange('a', 'z', exclusiveChars: "ilo");
        AllUpperCaseChars = GetCharRange('A', 'Z', exclusiveChars: "IO");
        AllNumericChars = GetCharRange('2', '9');
        AllSpecialChars = "!@#%*()$?+-=";

    }

    public PasswordGenerator(
        int minimumLengthPassword = 15,
        int maximumLengthPassword = 20,
        int minimumLowerCaseChars = 2,
        int minimumUpperCaseChars = 2,
        int minimumNumericChars = 2,
        int minimumSpecialChars = 2)
    {
        if (minimumLengthPassword < 15)
        {
            throw new ArgumentException("The minimumlength is smaller than 15.",
                "minimumLengthPassword");
        }

        if (minimumLengthPassword > maximumLengthPassword)
        {
            throw new ArgumentException("The minimumLength is bigger than the maximum length.",
                "minimumLengthPassword");
        }

        if (minimumLowerCaseChars < 2)
        {
            throw new ArgumentException("The minimumLowerCase is smaller than 2.",
                "minimumLowerCaseChars");
        }

        if (minimumUpperCaseChars < 2)
        {
            throw new ArgumentException("The minimumUpperCase is smaller than 2.",
                "minimumUpperCaseChars");
        }

        if (minimumNumericChars < 2)
        {
            throw new ArgumentException("The minimumNumeric is smaller than 2.",
                "minimumNumericChars");
        }

        if (minimumSpecialChars < 2)
        {
            throw new ArgumentException("The minimumSpecial is smaller than 2.",
                "minimumSpecialChars");
        }

        _minimumNumberOfChars = minimumLowerCaseChars + minimumUpperCaseChars +
                                minimumNumericChars + minimumSpecialChars;

        if (minimumLengthPassword < _minimumNumberOfChars)
        {
            throw new ArgumentException(
                "The minimum length of the password is smaller than the sum " +
                "of the minimum characters of all catagories.",
                "maximumLengthPassword");
        }

        MinimumLengthPassword = minimumLengthPassword;
        MaximumLengthPassword = maximumLengthPassword;

        MinimumLowerCaseChars = minimumLowerCaseChars;
        MinimumUpperCaseChars = minimumUpperCaseChars;
        MinimumNumericChars = minimumNumericChars;
        MinimumSpecialChars = minimumSpecialChars;

        _allAvailableChars =
            OnlyIfOneCharIsRequired(minimumLowerCaseChars, AllLowerCaseChars) +
            OnlyIfOneCharIsRequired(minimumUpperCaseChars, AllUpperCaseChars) +
            OnlyIfOneCharIsRequired(minimumNumericChars, AllNumericChars) +
            OnlyIfOneCharIsRequired(minimumSpecialChars, AllSpecialChars);
    }

    private string OnlyIfOneCharIsRequired(int minimum, string allChars)
    {
        return minimum > 0 || _minimumNumberOfChars == 0 ? allChars : string.Empty;
    }

    public string Generate()
    {
        var lengthOfPassword = _randomSecure.Next(MinimumLengthPassword, MaximumLengthPassword);

        // Get the required number of characters of each catagory and 
        // add random charactes of all catagories
        var minimumChars = GetRandomString(AllLowerCaseChars, MinimumLowerCaseChars) +
                        GetRandomString(AllUpperCaseChars, MinimumUpperCaseChars) +
                        GetRandomString(AllNumericChars, MinimumNumericChars) +
                        GetRandomString(AllSpecialChars, MinimumSpecialChars);
        var rest = GetRandomString(_allAvailableChars, lengthOfPassword - minimumChars.Length);
        var unshuffeledResult = minimumChars + rest;

        // Shuffle the result so the order of the characters are unpredictable
        var result = unshuffeledResult.ShuffleTextSecure();
        return result;
    }

    private string GetRandomString(string possibleChars, int lenght)
    {
        var result = string.Empty;
        for (var position = 0; position < lenght; position++)
        {
            var index = _randomSecure.Next(possibleChars.Length);
            result += possibleChars[index];
        }
        return result;
    }

    private static string GetCharRange(char minimum, char maximum, string exclusiveChars = "")
    {
        var result = string.Empty;
        for (char value = minimum; value <= maximum; value++)
        {
            result += value;
        }
        if (!string.IsNullOrEmpty(exclusiveChars))
        {
            var inclusiveChars = result.Except(exclusiveChars).ToArray();
            result = new string(inclusiveChars);
        }
        return result;
    }
}

internal static class Extensions
{
    private static readonly Lazy<RandomSecureVersion> RandomSecure =
        new Lazy<RandomSecureVersion>(() => new RandomSecureVersion());
    public static IEnumerable<T> ShuffleSecure<T>(this IEnumerable<T> source)
    {
        var sourceArray = source.ToArray();
        for (int counter = 0; counter < sourceArray.Length; counter++)
        {
            int randomIndex = RandomSecure.Value.Next(counter, sourceArray.Length);
            yield return sourceArray[randomIndex];

            sourceArray[randomIndex] = sourceArray[counter];
        }
    }

    public static string ShuffleTextSecure(this string source)
    {
        var shuffeldChars = source.ShuffleSecure().ToArray();
        return new string(shuffeldChars);
    }
}

internal class RandomSecureVersion
{
    //Never ever ever never use Random() in the generation of anything that requires true security/randomness
    //and high entropy or I will hunt you down with a pitchfork!! Only RNGCryptoServiceProvider() is safe.
    private readonly RNGCryptoServiceProvider _rngProvider = new RNGCryptoServiceProvider();

    public int Next()
    {
        var randomBuffer = new byte[4];
        _rngProvider.GetBytes(randomBuffer);
        var result = BitConverter.ToInt32(randomBuffer, 0);
        return result;
    }

    public int Next(int maximumValue)
    {
        // Do not use Next() % maximumValue because the distribution is not OK
        return Next(0, maximumValue);
    }

    public int Next(int minimumValue, int maximumValue)
    {
        var seed = Next();

        //  Generate uniformly distributed random integers within a given range.
        return new Random(seed).Next(minimumValue, maximumValue);
    }
}

Consume in your code thusly:

var generator = new PasswordGenerator();
string password = generator.Generate();
Console.WriteLine(password);

To address your question about that formula:

The formula is saying that a password of length L drawn from an alphabet of N symbols is equivalent to a password of length H drawn from an alphabet of two symbols. So if you have, say, 64 symbols (say abc...xyzABC...XYZ01...89_!) and the password is 10 characters long, then that gives you equivalent security to a password 10 log2 64 = 60 characters long drawn from the alphabet "ab".

A "log" is the inverse operation of exponentiation. Two to the sixth power gives you sixty-four, therefore the "log two" of sixty-four gives you six.

I don't know if this will help you, but this is what I use when I want to generate a random password which is also strong. It's quick and simple to implement/understand and isn't as much of an overkill as the one through the membership provider above...

    private string Token(byte Length) {
        char[] Chars = new char[] {
            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
            'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9'
        };
        string String = string.Empty;
        Random Random = new Random();

        for (byte a = 0; a < Length; a++) {
            String += Chars[Random.Next(0, 61)];
        };

        return (String);
    }

Why not just fill an array with some characters and pick on random a number of them. You can divide them in groups to be sure that are include letters numbers and special characters.

You will also have to pick a proper length and how much of every group of characters to include and that's it. I don't think you need some sophisticated formulas.

For systems that don't allow user-generated passwords it's very easy, actually: Any password is as secure as it's long. Not counting, of course, people who tack post-its to monitors, etc.

You probably want to maximize the set of characters from which the password is generated. But restricting the generated passwords greatly reduces the search space and therefore makes the password less secure. Again, this only holds if the user can't choose their own password.

If you deal with both generated and user-created passwords, then all bets are off, obviously. You then probably want to generate the passwords in a way that it uses as many characters from different classes as possible, resembling a strong user-chosen password. Ideally it should conform to the same constraints that the user-created password has to pass as well (if any).