Generating Random Passwords

Question

When a user on our site loses his password and heads off to the Lost Password page we need to give him a new temporary password. I don't really mind how random this is, or if it matches all the "needed" strong password rules, all I want to do is give them a password that they can change later.

The application is a Web application written in C#. so I was thinking of being mean and going for the easy route of using part of a Guid. i.e.

Guid.NewGuid().ToString("d").Substring(1,8)

Suggesstions? thoughts?

Answer 1

>= .NET 8: Built-in solution

With .NET 8 a few nice convenience methods where added to RandomNumberGenerator.

One of them is RandomNumberGenerator.GetString(ReadOnlySpan<Char>, Int32) which solves exactly this problem and generates a random string consisting of a set of characters chosen from a set of characters provided as an argument. The characters will be chosen using using a cryptographically secure random number generator.

As you would expect this generates a Permutation with Repetition i. e.

• order of characters matters
• same character can be chosen more than once

Example

using System.Security.Cryptography;

// random numeric pin code consisting of 4 digits (combination lock)
var pin = RandomNumberGenerator.GetString(['0', '1', '2', '3', '4', '5', '6', '7', '8', '9'], 4);

// 8 random alphanumeric characters
var password = RandomNumberGenerator.GetString(['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'], 8)

Reference

Based on Microsoft official documentation on RandomNumberGenerator.GetString(ReadOnlySpan<char>, Int32).

---

Definition

• Namespace: System.Security.Cryptography
• Assembly: System.Security.Cryptography.dll
• Source: RandomNumberGenerator.cs

Creates a string populated with characters chosen at random from choices.

public static string GetString (ReadOnlySpan<char> choices, int
length); 

Parameters

• choices: ReadOnlySpan<Char> The characters to use to populate the string.
• length: Int32 The length of string to return.

Returns String:

A string populated with random choices.

Exceptions

• ArgumentException: choices is empty.

• ArgumentOutOfRangeException: length is not zero or a positive number.

---

Appendix: More complex password generation scenarios

If you want to create passwords that have additional constraints like e.g.:

• min. number of numeric characters
• min. number of lower- or uppercase characters
• etc.

please refer to another answer of mine to a similar question.

Answer 2

.NET 6+, low allocation password generator with

• at least one digit
• at least one letter
• all chars unique

requirements

public static String Generate(int length = 10, string symbols = "0123456789qwertyuioplkjhgfdsazxcvbnm")
{
    var password = "";

    do
    {
        password = String.Create(length, symbols, Generate);
    }
    while (!Verify(password));

    return password;
}

static void Generate(Span<char> password, string symbols)
{
    var availableChars = new HashSet<char>(symbols);

    Span<byte> randomBytes = stackalloc byte[password.Length];
    System.Security.Cryptography.RandomNumberGenerator.Fill(randomBytes);

    for (var i = 0; i < password.Length; i++)
    {
        var randomAvailableChar = availableChars.ElementAt(randomBytes[i] % availableChars.Count);
        password[i] = randomAvailableChar;
        availableChars.Remove(randomAvailableChar);
    }
}

static Boolean Verify(string password)
{
    return password.Any(ch => Char.IsDigit(ch)) // at least one digit
        && password.Any(ch => Char.IsLetter(ch)) // at least one letter
        && password.Length == password.Distinct().Count(); // all chars unique
}

Output examples:

80tez4fk1p
vniuhcak27
uezc9m1r45
e25my37wko
l4f5jr7ed6
ti8phxa41b
ojpw9lex32
q89xhfzo0d
v8ycx19607
63pbcrhkn5

Answer 3

protected void BtnRandomPasswordClick(object sender, EventArgs e)
{
    txtPassword.Text = CreateRandomPassword();
}

private string CreateRandomPassword()
{
    var chars = CreateRandomPasswordNumber() + CreateRandomPasswordUppercase() + CreateRandomPasswordLowercase() + CreateRandomPasswordSpecial();
    var stringChars = new char[12];
    var random1 = new Random();
    for (int i = 0; i < stringChars.Length; i++)
    {
        stringChars[i] = chars[random1.Next(chars.Length)];
    }
    var finalString = new String(stringChars);
    return finalString;
}

private string CreateRandomPasswordNumber()
{
    var chars = "0123456789";
    var stringChars = new char[30];
    var random1 = new Random();
    for (int i = 0; i < stringChars.Length; i++)
    {
        stringChars[i] = chars[random1.Next(chars.Length)];
    }
    var finalString = new String(stringChars);
    return finalString;
}

private string CreateRandomPasswordUppercase()
{
    var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    var stringChars = new char[30];
    var random1 = new Random();
    for (int i = 0; i < stringChars.Length; i++)
    {
        stringChars[i] = chars[random1.Next(chars.Length)];
    }
    var finalString = new String(stringChars);
    return finalString;
}

private string CreateRandomPasswordLowercase()
{
    var chars = "abcdefghijklmnopqrstuvwxyz";
    var stringChars = new char[30];
    var random1 = new Random();
    for (int i = 0; i < stringChars.Length; i++)
    {
        stringChars[i] = chars[random1.Next(chars.Length)];
    }
    var finalString = new String(stringChars);
    return finalString;
}

private string CreateRandomPasswordSpecial()
{
    var chars = "~`!@#$%^&*()!-+={[}]|\\:;\"',.?/";
    var stringChars = new char[30];
    var random1 = new Random();
    for (int i = 0; i < stringChars.Length; i++)
    {
        stringChars[i] = chars[random1.Next(chars.Length)];
    }
    var finalString = new String(stringChars);
    return finalString;
}

I found this method to be quite reliable after many tests.

Answer 4

How about this?

// Add using System.Security.Cryptography;
Convert.ToBase64String(RandomNumberGenerator.GetBytes(8));

Answer 5

Here's my attempt cobbled together from the answers above which is pretty simple to follow and allows you to establish multiple character sets which are guaranteed to be included (needed for our complexity requirements).

using static System.Security.Cryptography.RandomNumberGenerator;
// Usage: GenerateRandomPassword(10, "abcdefghkmnpqrstwxyz", "ABCDEFGHJKMNPRTWXYZ", "2346789", "!*@#$%^&+=");
private static string GenerateRandomPassword(int len, params string[] types)
{
    if (types.Length > len) len = types.Length; // prevent errors
    if (len < 0 || types.Length < 1) return string.Empty;

    var chars = new char[len];
    var pos = 0;

    foreach (var type in types) // guarantee at least one char from each type
    {
        var charNum = GetInt32(0, type.Length);
        chars[pos++] = type[charNum];
    }

    while (pos < chars.Length)
    {
        var typeNum = GetInt32(0, types.Length);
        var type = types[typeNum];
        var charNum = GetInt32(0, type.Length);
        var charValue = type[charNum];
        if (chars.Contains(charValue)) continue; // prevent duplicates
        chars[pos++] = charValue;
    }

    return new string(chars.OrderBy(_ => GetInt32(0, len)).ToArray());
}

Answer 6

Here is a solution that uses RNGCryptoServiceProvider to mimic the functionality of Membership.GeneratePassword from the System.Web.Security namespace.

I needed a drop-in replacement for running it in Azure Function.

It can be tested here: https://dotnetfiddle.net/V0cNJw

public static string GeneratePassword(int length, int numberOfNonAlphanumericCharacters)
{
    const string allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789";
    const string nonAlphanumericChars = "!@#$%^&*()_-+=[{]};:<>|./?";
    var randNum = new byte[4];
    using (var rng = new RNGCryptoServiceProvider())
    {
        rng.GetBytes(randNum);
        var randomSeed = BitConverter.ToInt32(randNum, 0);
        var random = new Random(randomSeed);
        var chars = new char[length];
        var allowedCharCount = allowedChars.Length;
        var nonAlphanumericCharCount = nonAlphanumericChars.Length;
        var numNonAlphanumericCharsAdded = 0;
        for (var i = 0; i < length; i++)
        {
            if (numNonAlphanumericCharsAdded < numberOfNonAlphanumericCharacters && i < length - 1)
            {
                chars[i] = nonAlphanumericChars[random.Next(nonAlphanumericCharCount)];
                numNonAlphanumericCharsAdded++;
            }
            else
            {
                chars[i] = allowedChars[random.Next(allowedCharCount)];
            }
        }
        return new string(chars);
    }
}

Here is a version that runs on .Net 6.0+ Sandbox: https://dotnetfiddle.net/XqgTSg

public static string GeneratePassword(int length, int numberOfNonAlphanumericCharacters)
{
    const string allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789";
    const string nonAlphanumericChars = "!@#$%^&*()_-+=[{]};:<>|./?";
    var randNum = new byte[4];
    using (var rng = RandomNumberGenerator.Create())
    {
        rng.GetBytes(randNum);
        var randomSeed = BitConverter.ToInt32(randNum, 0);
        var random = new Random(randomSeed);
        var chars = new char[length];
        var allowedCharCount = allowedChars.Length;
        var nonAlphanumericCharCount = nonAlphanumericChars.Length;
        var numNonAlphanumericCharsAdded = 0;
        for (var i = 0; i < length; i++)
        {
            if (numNonAlphanumericCharsAdded < numberOfNonAlphanumericCharacters && i < length - 1)
            {
                chars[i] = nonAlphanumericChars[random.Next(nonAlphanumericCharCount)];
                numNonAlphanumericCharsAdded++;
            }
            else
            {
                chars[i] = allowedChars[random.Next(allowedCharCount)];
            }
        }
        return new string(chars);
    }
}

Answer 7

Since Random is not secure and RNGCryptoServiceProvider is obsolte I ended up doing this:

  // possible characters that password can have
  private const string passChars =
       "ABCDEFGHIJKLMNOPQRSTUVWXYZ" +
       "abcdefghijklmnopqrstuvwxyz" +
       "0123456789" +
       "!@#$%.-_"
       ;

    public static string GetRandomPassword(int length)
    {
        char[] p = new char[length];
        for (int i = 0; i < length; i++)
            p[i] = passChars[RandomNumberGenerator.GetInt32(0, passChars.Length)];
        return new string(p);
    }

Answer 8

Inspired by the answer from @kitsu.eb, but using RandomNumberGenerator instead of Random or RNGCryptoServiceProvider (deprecated in .NET 6), and added a few more special characters.

Optional parameter to exclude characters that will be escaped when using System.Text.Json.JsonSerializer.Serialize - for example & which is escaped as \u0026 - so that you can guarantee the length of the serialized string will match the length of the password.

For .NET Core 3.0 and above.

public static class PasswordGenerator
{
    const string lower = "abcdefghijklmnopqrstuvwxyz";
    const string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const string number = "1234567890";
    const string special = "!@#$%^&*()[]{},.:`~_-=+"; // excludes problematic characters like ;'"/\
    const string specialJsonSafe = "!@#$%^*()[]{},.:~_-="; // excludes problematic characters like ;'"/\ and &`+

    const int lowerLength = 26; // lower.Length
    const int upperLength = 26; // upper.Length;
    const int numberLength = 10; // number.Length;
    const int specialLength = 23; // special.Length;
    const int specialJsonSafeLength = 20; // specialJsonSafe.Length;

    public static string Generate(int length = 96, bool jsonSafeSpecialCharactersOnly = false)
    {
        Span<char> result = length < 1024 ? stackalloc char[length] : new char[length].AsSpan();

        for (int i = 0; i < length; ++i)
        {
            switch (RandomNumberGenerator.GetInt32(4))
            {
                case 0:
                    result[i] = lower[RandomNumberGenerator.GetInt32(0, lowerLength)];
                    break;
                case 1:
                    result[i] = upper[RandomNumberGenerator.GetInt32(0, upperLength)];
                    break;
                case 2:
                    result[i] = number[RandomNumberGenerator.GetInt32(0, numberLength)];
                    break;
                case 3:
                    if (jsonSafeSpecialCharactersOnly)
                    {
                        result[i] = specialJsonSafe[RandomNumberGenerator.GetInt32(0, specialJsonSafeLength)];
                    }
                    else
                    {
                        result[i] = special[RandomNumberGenerator.GetInt32(0, specialLength)];
                    }
                    break;
            }
        }

        return result.ToString();
    }
}

Answer 9

Pretty easy way to require one from each group using Random and linq-to-objects.

• Randomize each group
• Select random amount from first group
• Select remaining random amounts from following groups

 

Random rand = new Random();
int min = 8;
int max = 16;
int totalLen = rand.Next(min, max);
int remainingGroups = 4;
                
string[] allowedLowerChars = "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".Split(',');           
string [] allowedUpperChars = "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".Split(',');
string [] allowedNumbers = "1,2,3,4,5,6,7,8,9,0".Split(',');
string [] allowedSpecialChars = "!,@,#,$,%,&,?".Split(',');


var password = allowedLowerChars.OrderBy(c => rand.Next()).Take(rand.Next(1, totalLen-remainingGroups--)).ToList();
password.AddRange(allowedUpperChars.OrderBy(c => rand.Next()).Take(rand.Next(1, totalLen-password.Count-remainingGroups--)).ToList());
password.AddRange(allowedNumbers.OrderBy(c => rand.Next()).Take(rand.Next(1, totalLen-password.Count-remainingGroups--)).ToList());
password.AddRange(allowedSpecialChars.OrderBy(c => rand.Next()).Take(totalLen-password.Count).ToList());
password = password.OrderBy(c => rand.Next()).ToList(); // randomize groups

Answer 10

I don't like the passwords that Membership.GeneratePassword() creates, as they're too ugly and have too many special characters.

This code generates a 10 digit not-too-ugly password.

string password = Guid.NewGuid().ToString("N").ToLower()
                      .Replace("1", "").Replace("o", "").Replace("0","")
                      .Substring(0,10);

Sure, I could use a Regex to do all the replaces but this is more readable and maintainable IMO.

Answer 11

How to Generate the Random Password in C#.
Output : (https://prnt.sc/11fac8v)
Run : https://onlinegdb.com/HJe5OHBLu

    private static Random random = new Random();
    public static void Main()
    {
        Console.WriteLine("Random password with length of 8 character.");
        Console.WriteLine("===========================================");
        Console.WriteLine("Capital latters : 2");
        Console.WriteLine("Number latters : 2");
        Console.WriteLine("Special latters : 2");
        Console.WriteLine("Small latters : 2");
        Console.WriteLine("===========================================");
        Console.Write("The Random Password : ");
        Console.WriteLine(RandomStringCap(2) + RandomStringNum(2) + RandomStringSpe(2) + RandomStringSml(2));
        Console.WriteLine("===========================================");
    }
    public static string RandomStringCap(int length)
    {
        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        return new string(Enumerable.Repeat(chars, length)
                         .Select(s => s[random.Next(s.Length)]).ToArray());
    }
    public static string RandomStringNum(int length)
    {
        const string chars = "0123456789";
        return new string(Enumerable.Repeat(chars, length)
                         .Select(s => s[random.Next(s.Length)]).ToArray());
    }
    public static string RandomStringSml(int length)
    {
        const string chars = "abcdefghijklmnopqrstuvwxyz";
        return new string(Enumerable.Repeat(chars, length)
                         .Select(s => s[random.Next(s.Length)]).ToArray());
    }
    public static string RandomStringSpe(int length)
    {
        const string chars = "!@#$%^&*_-=+";
        return new string(Enumerable.Repeat(chars, length)
                         .Select(s => s[random.Next(s.Length)]).ToArray());
    }

Answer 12

check this code... I added the .remove(length) to improve anaximander's response

            public string GeneratePassword(int length)
            {
                using(RNGCryptoServiceProvider cryptRNG = new RNGCryptoServiceProvider();)
               {
                      byte[] tokenBuffer = new byte[length];
                      cryptRNG.GetBytes(tokenBuffer);
                      return Convert.ToBase64String(tokenBuffer).Remove(length);
                }
                          
            }

Answer 13

I created this class that uses RNGCryptoServiceProvider and it is flexible. Example:

var generator = new PasswordGenerator(minimumLengthPassword: 8,
                                      maximumLengthPassword: 15,
                                      minimumUpperCaseChars: 2,
                                      minimumNumericChars: 3,
                                      minimumSpecialChars: 2);
string password = generator.Generate();

Answer 14

If you want to make use of the cryptographically secure random number generation used by System.Web.Security.Membership.GeneratePassword but also want to restrict the character set to alphanumeric characters, you can filter the result with a regex:

static string GeneratePassword(int characterCount)
{
    string password = String.Empty;
    while(password.Length < characterCount)
        password += Regex.Replace(System.Web.Security.Membership.GeneratePassword(128, 0), "[^a-zA-Z0-9]", string.Empty);
    return password.Substring(0, characterCount);
}

Answer 15

This package allows you to generate a random password while fluently indicating which characters it should contain (if needed):

https://github.com/prjseal/PasswordGenerator/

Example:

var pwd = new Password().IncludeLowercase().IncludeUppercase().IncludeSpecial();
var password = pwd.Next();

Answer 16

public string GenerateToken(int length)
{
    using (RNGCryptoServiceProvider cryptRNG = new RNGCryptoServiceProvider())
    {
        byte[] tokenBuffer = new byte[length];
        cryptRNG.GetBytes(tokenBuffer);
        return Convert.ToBase64String(tokenBuffer);
    }
}

(You could also have the class where this method lives implement IDisposable, hold a reference to the RNGCryptoServiceProvider, and dispose of it properly, to avoid repeatedly instantiating it.)

It's been noted that as this returns a base-64 string, the output length is always a multiple of 4, with the extra space using = as a padding character. The length parameter specifies the length of the byte buffer, not the output string (and is therefore perhaps not the best name for that parameter, now I think about it). This controls how many bytes of entropy the password will have. However, because base-64 uses a 4-character block to encode each 3 bytes of input, if you ask for a length that's not a multiple of 3, there will be some extra "space", and it'll use = to fill the extra.

If you don't like using base-64 strings for any reason, you can replace the Convert.ToBase64String() call with either a conversion to regular string, or with any of the Encoding methods; eg. Encoding.UTF8.GetString(tokenBuffer) - just make sure you pick a character set that can represent the full range of values coming out of the RNG, and that produces characters that are compatible with wherever you're sending or storing this. Using Unicode, for example, tends to give a lot of Chinese characters. Using base-64 guarantees a widely-compatible set of characters, and the characteristics of such a string shouldn't make it any less secure as long as you use a decent hashing algorithm.

Answer 17

I use this code for generate password with balance composition of alphabet, numeric and non_alpha_numeric chars.

public static string GeneratePassword(int Length, int NonAlphaNumericChars)
    {
        string allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789";
        string allowedNonAlphaNum = "!@#$%^&*()_-+=[{]};:<>|./?";
        string pass = "";
        Random rd = new Random(DateTime.Now.Millisecond);
        for (int i = 0; i < Length; i++)
        {
            if (rd.Next(1) > 0 && NonAlphaNumericChars > 0)
            {
                pass += allowedNonAlphaNum[rd.Next(allowedNonAlphaNum.Length)];
                NonAlphaNumericChars--;
            }
            else
            {
                pass += allowedChars[rd.Next(allowedChars.Length)];
            }
        }
        return pass;
    }

Answer 18

I'll add another ill-advised answer to the pot.

I have a use case where I need random passwords for machine-machine communication, so I don't have any requirement for human readability. I also don't have access to Membership.GeneratePassword in my project, and don't want to add the dependency.

I am fairly certain Membership.GeneratePassword is doing something similar to this, but here you can tune the pools of characters to draw from.

public static class PasswordGenerator
{
    private readonly static Random _rand = new Random();

    public static string Generate(int length = 24)
    {
        const string lower = "abcdefghijklmnopqrstuvwxyz";
        const string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        const string number = "1234567890";
        const string special = "!@#$%^&*_-=+";

        // Get cryptographically random sequence of bytes
        var bytes = new byte[length];
        new RNGCryptoServiceProvider().GetBytes(bytes);

        // Build up a string using random bytes and character classes
        var res = new StringBuilder();
        foreach(byte b in bytes)
        {
            // Randomly select a character class for each byte
            switch (_rand.Next(4))
            {
                // In each case use mod to project byte b to the correct range
                case 0:
                    res.Append(lower[b % lower.Count()]);
                    break;
                case 1:
                    res.Append(upper[b % upper.Count()]);
                    break;
                case 2:
                    res.Append(number[b % number.Count()]);
                    break;
                case 3:
                    res.Append(special[b % special.Count()]);
                    break;
            }
        }
        return res.ToString();
    }
}

And some example output:

PasswordGenerator.Generate(12)
"pzY=64@-ChS$"
"BG0OsyLbYnI_"
"l9#5^2&adj_i"
"#++Ws9d$%O%X"
"IWhdIN-#&O^s"

To preempt complaints about the use of Random: The primary source of randomness is still the crypto RNG. Even if you could deterministically preordain the sequence coming out of Random (say it only produced 1) you still wouldn't know the next char that would be picked (though that would limit the range of possibilities).

One simple extension would be to add weighting to the different character sets, which could be as simple as upping the max value and adding fall-through cases to increase weight.

switch (_rand.Next(6))
{
    // Prefer letters 2:1
    case 0:
    case 1:
        res.Append(lower[b % lower.Count()]);
        break;
    case 2:
    case 3:
        res.Append(upper[b % upper.Count()]);
        break;
    case 4:
        res.Append(number[b % number.Count()]);
        break;
    case 5:
        res.Append(special[b % special.Count()]);
        break;
}

For a more humanistic random password generator I once implemented a prompt system using the EFF dice-word list.

Answer 19

public static string GeneratePassword(int passLength) {
        var chars = "abcdefghijklmnopqrstuvwxyz@#$&ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        var random = new Random();
        var result = new string(
            Enumerable.Repeat(chars, passLength)
                      .Select(s => s[random.Next(s.Length)])
                      .ToArray());
        return result;
    }

Answer 20

 Generate random password of specified length with 
  - Special characters   
  - Number
  - Lowecase
  - Uppercase

  public static string CreatePassword(int length = 12)
    {
        const string lower = "abcdefghijklmnopqrstuvwxyz";
        const string upper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        const string number = "1234567890";
        const string special = "!@#$%^&*";

        var middle = length / 2;
        StringBuilder res = new StringBuilder();
        Random rnd = new Random();
        while (0 < length--)
        {
            if (middle == length)
            {
                res.Append(number[rnd.Next(number.Length)]);
            }
            else if (middle - 1 == length)
            {
                res.Append(special[rnd.Next(special.Length)]);
            }
            else
            {
                if (length % 2 == 0)
                {
                    res.Append(lower[rnd.Next(lower.Length)]);
                }
                else
                {
                    res.Append(upper[rnd.Next(upper.Length)]);
                }
            }
        }
        return res.ToString();
    }

Answer 21

validChars can be any construct, but I decided to select based on ascii code ranges removing control chars. In this example, it is a 12 character string.

string validChars = String.Join("", Enumerable.Range(33, (126 - 33)).Where(i => !(new int[] { 34, 38, 39, 44, 60, 62, 96 }).Contains(i)).Select(i => { return (char)i; }));
string.Join("", Enumerable.Range(1, 12).Select(i => { return validChars[(new Random(Guid.NewGuid().GetHashCode())).Next(0, validChars.Length - 1)]; }))

Answer 22

Added some supplemental code to the accepted answer. It improves upon answers just using Random and allows for some password options. I also liked some of the options from the KeePass answer but did not want to include the executable in my solution.

private string RandomPassword(int length, bool includeCharacters, bool includeNumbers, bool includeUppercase, bool includeNonAlphaNumericCharacters, bool includeLookAlikes)
{
    if (length < 8 || length > 128) throw new ArgumentOutOfRangeException("length");
    if (!includeCharacters && !includeNumbers && !includeNonAlphaNumericCharacters) throw new ArgumentException("RandomPassword-Key arguments all false, no values would be returned");

    string pw = "";
    do
    {
        pw += System.Web.Security.Membership.GeneratePassword(128, 25);
        pw = RemoveCharacters(pw, includeCharacters, includeNumbers, includeUppercase, includeNonAlphaNumericCharacters, includeLookAlikes);
    } while (pw.Length < length);

    return pw.Substring(0, length);
}

private string RemoveCharacters(string passwordString, bool includeCharacters, bool includeNumbers, bool includeUppercase, bool includeNonAlphaNumericCharacters, bool includeLookAlikes)
{
    if (!includeCharacters)
    {
        var remove = new string[] { "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" };
        foreach (string r in remove)
        {
            passwordString = passwordString.Replace(r, string.Empty);
            passwordString = passwordString.Replace(r.ToUpper(), string.Empty);
        }
    }

    if (!includeNumbers)
    {
        var remove = new string[] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);
    }

    if (!includeUppercase)
        passwordString = passwordString.ToLower();

    if (!includeNonAlphaNumericCharacters)
    {
        var remove = new string[] { "!", "@", "#", "$", "%", "^", "&", "*", "(", ")", "-", "_", "+", "=", "{", "}", "[", "]", "|", "\\", ":", ";", "<", ">", "/", "?", "." };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);
    }

    if (!includeLookAlikes)
    {
        var remove = new string[] { "(", ")", "0", "O", "o", "1", "i", "I", "l", "|", "!", ":", ";" };
        foreach (string r in remove)
            passwordString = passwordString.Replace(r, string.Empty);
    }

    return passwordString;
}

This was the first link when I searched for generating random passwords and the following is out of scope for the current question but might be important to consider.

• Based upon the assumption that System.Web.Security.Membership.GeneratePassword is cryptographically secure with a minimum of 20% of the characters being Non-Alphanumeric.
• Not sure if removing characters and appending strings is considered good practice in this case and provides enough entropy.
• Might want to consider implementing in some way with SecureString for secure password storage in memory.

Answer 23

public string Sifre_Uret(int boy, int noalfa)
{

    //  01.03.2016   
    // Genel amaçlı şifre üretme fonksiyonu


    //Fonskiyon 128 den büyük olmasına izin vermiyor.
    if (boy > 128 ) { boy = 128; }
    if (noalfa > 128) { noalfa = 128; }
    if (noalfa > boy) { noalfa = boy; }


    string passch = System.Web.Security.Membership.GeneratePassword(boy, noalfa);

    //URL encoding ve Url Pass + json sorunu yaratabilecekler pass ediliyor.
    //Microsoft Garanti etmiyor. Alfa Sayısallar Olabiliyorimiş . !@#$%^&*()_-+=[{]};:<>|./?.
    //https://msdn.microsoft.com/tr-tr/library/system.web.security.membership.generatepassword(v=vs.110).aspx


    //URL ve Json ajax lar için filtreleme
    passch = passch.Replace(":", "z");
    passch = passch.Replace(";", "W");
    passch = passch.Replace("'", "t");
    passch = passch.Replace("\"", "r");
    passch = passch.Replace("/", "+");
    passch = passch.Replace("\\", "e");

    passch = passch.Replace("?", "9");
    passch = passch.Replace("&", "8");
    passch = passch.Replace("#", "D");
    passch = passch.Replace("%", "u");
    passch = passch.Replace("=", "4");
    passch = passch.Replace("~", "1");

    passch = passch.Replace("[", "2");
    passch = passch.Replace("]", "3");
    passch = passch.Replace("{", "g");
    passch = passch.Replace("}", "J");


    //passch = passch.Replace("(", "6");
    //passch = passch.Replace(")", "0");
    //passch = passch.Replace("|", "p");
    //passch = passch.Replace("@", "4");
    //passch = passch.Replace("!", "u");
    //passch = passch.Replace("$", "Z");
    //passch = passch.Replace("*", "5");
    //passch = passch.Replace("_", "a");

    passch = passch.Replace(",", "V");
    passch = passch.Replace(".", "N");
    passch = passch.Replace("+", "w");
    passch = passch.Replace("-", "7");





    return passch;



}

Answer 24

Here Is what i put together quickly.

    public string GeneratePassword(int len)
    {
        string res = "";
        Random rnd = new Random();
        while (res.Length < len) res += (new Func<Random, string>((r) => {
            char c = (char)((r.Next(123) * DateTime.Now.Millisecond % 123)); 
            return (Char.IsLetterOrDigit(c)) ? c.ToString() : ""; 
        }))(rnd);
        return res;
    }

Answer 25

This is a lot larger, but I think it looks a little more comprehensive: http://www.obviex.com/Samples/Password.aspx

///////////////////////////////////////////////////////////////////////////////
// SAMPLE: Generates random password, which complies with the strong password
//         rules and does not contain ambiguous characters.
//
// To run this sample, create a new Visual C# project using the Console
// Application template and replace the contents of the Class1.cs file with
// the code below.
//
// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND,
// EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE IMPLIED
// WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A PARTICULAR PURPOSE.
// 
// Copyright (C) 2004 Obviex(TM). All rights reserved.
// 
using System;
using System.Security.Cryptography;

/// <summary>
/// This class can generate random passwords, which do not include ambiguous 
/// characters, such as I, l, and 1. The generated password will be made of
/// 7-bit ASCII symbols. Every four characters will include one lower case
/// character, one upper case character, one number, and one special symbol
/// (such as '%') in a random order. The password will always start with an
/// alpha-numeric character; it will not start with a special symbol (we do
/// this because some back-end systems do not like certain special
/// characters in the first position).
/// </summary>
public class RandomPassword
{
    // Define default min and max password lengths.
    private static int DEFAULT_MIN_PASSWORD_LENGTH  = 8;
    private static int DEFAULT_MAX_PASSWORD_LENGTH  = 10;

    // Define supported password characters divided into groups.
    // You can add (or remove) characters to (from) these groups.
    private static string PASSWORD_CHARS_LCASE  = "abcdefgijkmnopqrstwxyz";
    private static string PASSWORD_CHARS_UCASE  = "ABCDEFGHJKLMNPQRSTWXYZ";
    private static string PASSWORD_CHARS_NUMERIC= "23456789";
    private static string PASSWORD_CHARS_SPECIAL= "*$-+?_&=!%{}/";

    /// <summary>
    /// Generates a random password.
    /// </summary>
    /// <returns>
    /// Randomly generated password.
    /// </returns>
    /// <remarks>
    /// The length of the generated password will be determined at
    /// random. It will be no shorter than the minimum default and
    /// no longer than maximum default.
    /// </remarks>
    public static string Generate()
    {
        return Generate(DEFAULT_MIN_PASSWORD_LENGTH, 
                        DEFAULT_MAX_PASSWORD_LENGTH);
    }

    /// <summary>
    /// Generates a random password of the exact length.
    /// </summary>
    /// <param name="length">
    /// Exact password length.
    /// </param>
    /// <returns>
    /// Randomly generated password.
    /// </returns>
    public static string Generate(int length)
    {
        return Generate(length, length);
    }

    /// <summary>
    /// Generates a random password.
    /// </summary>
    /// <param name="minLength">
    /// Minimum password length.
    /// </param>
    /// <param name="maxLength">
    /// Maximum password length.
    /// </param>
    /// <returns>
    /// Randomly generated password.
    /// </returns>
    /// <remarks>
    /// The length of the generated password will be determined at
    /// random and it will fall with the range determined by the
    /// function parameters.
    /// </remarks>
    public static string Generate(int   minLength,
                                  int   maxLength)
    {
        // Make sure that input parameters are valid.
        if (minLength <= 0 || maxLength <= 0 || minLength > maxLength)
            return null;

        // Create a local array containing supported password characters
        // grouped by types. You can remove character groups from this
        // array, but doing so will weaken the password strength.
        char[][] charGroups = new char[][] 
        {
            PASSWORD_CHARS_LCASE.ToCharArray(),
            PASSWORD_CHARS_UCASE.ToCharArray(),
            PASSWORD_CHARS_NUMERIC.ToCharArray(),
            PASSWORD_CHARS_SPECIAL.ToCharArray()
        };

        // Use this array to track the number of unused characters in each
        // character group.
        int[] charsLeftInGroup = new int[charGroups.Length];

        // Initially, all characters in each group are not used.
        for (int i=0; i<charsLeftInGroup.Length; i++)
            charsLeftInGroup[i] = charGroups[i].Length;

        // Use this array to track (iterate through) unused character groups.
        int[] leftGroupsOrder = new int[charGroups.Length];

        // Initially, all character groups are not used.
        for (int i=0; i<leftGroupsOrder.Length; i++)
            leftGroupsOrder[i] = i;

        // Because we cannot use the default randomizer, which is based on the
        // current time (it will produce the same "random" number within a
        // second), we will use a random number generator to seed the
        // randomizer.

        // Use a 4-byte array to fill it with random bytes and convert it then
        // to an integer value.
        byte[] randomBytes = new byte[4];

        // Generate 4 random bytes.
        RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
        rng.GetBytes(randomBytes);

        // Convert 4 bytes into a 32-bit integer value.
        int seed = BitConverter.ToInt32(randomBytes, 0);

        // Now, this is real randomization.
        Random  random  = new Random(seed);

        // This array will hold password characters.
        char[] password = null;

        // Allocate appropriate memory for the password.
        if (minLength < maxLength)
            password = new char[random.Next(minLength, maxLength+1)];
        else
            password = new char[minLength];

        // Index of the next character to be added to password.
        int nextCharIdx;

        // Index of the next character group to be processed.
        int nextGroupIdx;

        // Index which will be used to track not processed character groups.
        int nextLeftGroupsOrderIdx;

        // Index of the last non-processed character in a group.
        int lastCharIdx;

        // Index of the last non-processed group.
        int lastLeftGroupsOrderIdx = leftGroupsOrder.Length - 1;

        // Generate password characters one at a time.
        for (int i=0; i<password.Length; i++)
        {
            // If only one character group remained unprocessed, process it;
            // otherwise, pick a random character group from the unprocessed
            // group list. To allow a special character to appear in the
            // first position, increment the second parameter of the Next
            // function call by one, i.e. lastLeftGroupsOrderIdx + 1.
            if (lastLeftGroupsOrderIdx == 0)
                nextLeftGroupsOrderIdx = 0;
            else
                nextLeftGroupsOrderIdx = random.Next(0, 
                                                     lastLeftGroupsOrderIdx);

            // Get the actual index of the character group, from which we will
            // pick the next character.
            nextGroupIdx = leftGroupsOrder[nextLeftGroupsOrderIdx];

            // Get the index of the last unprocessed characters in this group.
            lastCharIdx = charsLeftInGroup[nextGroupIdx] - 1;

            // If only one unprocessed character is left, pick it; otherwise,
            // get a random character from the unused character list.
            if (lastCharIdx == 0)
                nextCharIdx = 0;
            else
                nextCharIdx = random.Next(0, lastCharIdx+1);

            // Add this character to the password.
            password[i] = charGroups[nextGroupIdx][nextCharIdx];

            // If we processed the last character in this group, start over.
            if (lastCharIdx == 0)
                charsLeftInGroup[nextGroupIdx] = 
                                          charGroups[nextGroupIdx].Length;
            // There are more unprocessed characters left.
            else
            {
                // Swap processed character with the last unprocessed character
                // so that we don't pick it until we process all characters in
                // this group.
                if (lastCharIdx != nextCharIdx)
                {
                    char temp = charGroups[nextGroupIdx][lastCharIdx];
                    charGroups[nextGroupIdx][lastCharIdx] = 
                                charGroups[nextGroupIdx][nextCharIdx];
                    charGroups[nextGroupIdx][nextCharIdx] = temp;
                }
                // Decrement the number of unprocessed characters in
                // this group.
                charsLeftInGroup[nextGroupIdx]--;
            }

            // If we processed the last group, start all over.
            if (lastLeftGroupsOrderIdx == 0)
                lastLeftGroupsOrderIdx = leftGroupsOrder.Length - 1;
            // There are more unprocessed groups left.
            else
            {
                // Swap processed group with the last unprocessed group
                // so that we don't pick it until we process all groups.
                if (lastLeftGroupsOrderIdx != nextLeftGroupsOrderIdx)
                {
                    int temp = leftGroupsOrder[lastLeftGroupsOrderIdx];
                    leftGroupsOrder[lastLeftGroupsOrderIdx] = 
                                leftGroupsOrder[nextLeftGroupsOrderIdx];
                    leftGroupsOrder[nextLeftGroupsOrderIdx] = temp;
                }
                // Decrement the number of unprocessed groups.
                lastLeftGroupsOrderIdx--;
            }
        }

        // Convert password characters into a string and return the result.
        return new string(password);
     }
}

/// <summary>
/// Illustrates the use of the RandomPassword class.
/// </summary>
public class RandomPasswordTest
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main(string[] args)
    {
        // Print 100 randomly generated passwords (8-to-10 char long).
        for (int i=0; i<100; i++)
            Console.WriteLine(RandomPassword.Generate(8, 10));
    }
}
//
// END OF FILE
///////////////////////////////////////////////////////////////////////////////

Answer 26

I've always been very happy with the password generator built-in to KeePass. Since KeePass is a .Net program, and open source, I decided to dig around the code a bit. I ended up just referncing KeePass.exe, the copy provided in the standard application install, as a reference in my project and writing the code below. You can see how flexible it is thanks to KeePass. You can specify length, which characters to include/not include, etc...

using KeePassLib.Cryptography.PasswordGenerator;
using KeePassLib.Security;


public static string GeneratePassword(int passwordLength, bool lowerCase, bool upperCase, bool digits,
        bool punctuation, bool brackets, bool specialAscii, bool excludeLookAlike)
    {
        var ps = new ProtectedString();
        var profile = new PwProfile();
        profile.CharSet = new PwCharSet();
        profile.CharSet.Clear();

        if (lowerCase)
            profile.CharSet.AddCharSet('l');
        if(upperCase)
            profile.CharSet.AddCharSet('u');
        if(digits)
            profile.CharSet.AddCharSet('d');
        if (punctuation)
            profile.CharSet.AddCharSet('p');
        if (brackets)
            profile.CharSet.AddCharSet('b');
        if (specialAscii)
            profile.CharSet.AddCharSet('s');

        profile.ExcludeLookAlike = excludeLookAlike;
        profile.Length = (uint)passwordLength;
        profile.NoRepeatingCharacters = true;

        KeePassLib.Cryptography.PasswordGenerator.PwGenerator.Generate(out ps, profile, null, _pool);

        return ps.ReadString();
    }

Answer 27

I created this method similar to the available in the membership provider. This is usefull if you don't want to add the web reference in some applications.

It works great.

public static string GeneratePassword(int Length, int NonAlphaNumericChars)
    {
        string allowedChars = "abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNOPQRSTUVWXYZ0123456789";
        string allowedNonAlphaNum = "!@#$%^&*()_-+=[{]};:<>|./?";
        Random rd = new Random();

        if (NonAlphaNumericChars > Length || Length <= 0 || NonAlphaNumericChars < 0)
            throw new ArgumentOutOfRangeException();

            char[] pass = new char[Length];
            int[] pos = new int[Length];
            int i = 0, j = 0, temp = 0;
            bool flag = false;

            //Random the position values of the pos array for the string Pass
            while (i < Length - 1)
            {
                j = 0;
                flag = false;
                temp = rd.Next(0, Length);
                for (j = 0; j < Length; j++)
                    if (temp == pos[j])
                    {
                        flag = true;
                        j = Length;
                    }

                if (!flag)
                {
                    pos[i] = temp;
                    i++;
                }
            }

            //Random the AlphaNumericChars
            for (i = 0; i < Length - NonAlphaNumericChars; i++)
                pass[i] = allowedChars[rd.Next(0, allowedChars.Length)];

            //Random the NonAlphaNumericChars
            for (i = Length - NonAlphaNumericChars; i < Length; i++)
                pass[i] = allowedNonAlphaNum[rd.Next(0, allowedNonAlphaNum.Length)];

            //Set the sorted array values by the pos array for the rigth posistion
            char[] sorted = new char[Length];
            for (i = 0; i < Length; i++)
                sorted[i] = pass[pos[i]];

            string Pass = new String(sorted);

            return Pass;
    }

Answer 28

On my website I use this method:

    //Symb array
    private const string _SymbolsAll = "~`!@#$%^&*()_+=-\\|[{]}'\";:/?.>,<";

    //Random symb
    public string GetSymbol(int Length)
    {
        Random Rand = new Random(DateTime.Now.Millisecond);
        StringBuilder result = new StringBuilder();
        for (int i = 0; i < Length; i++)
            result.Append(_SymbolsAll[Rand.Next(0, _SymbolsAll.Length)]);
        return result.ToString();
    }

Edit string _SymbolsAll for your array list.

Answer 29

public string CreatePassword(int length)
{
        const string valid = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        StringBuilder res = new StringBuilder();
        Random rnd = new Random();
        while (0 < length--)
        {
            res.Append(valid[rnd.Next(valid.Length)]);
        }
        return res.ToString();
}

This has a good benefit of being able to choose from a list of available characters for the generated password (e.g. digits only, only uppercase or only lowercase etc.)

Answer 30

There's always System.Web.Security.Membership.GeneratePassword(int length, int numberOfNonAlphanumericCharacters).