Password Generator

PasswordGenerator.cs

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

public static class PasswordGenerator
{
  public static string Generate(int length)
  {
    return Generate(length, PasswordCharacters.All, null);
  }

  public static string GenerateDefaultNoSimilars(int length = 8)
  {
    return Generate(length, PasswordCharacters.AllLetters | PasswordCharacters.AlphaNumeric | PasswordCharacters.Numbers, true);
  }

  public static string Generate(int length, PasswordCharacters allowedCharacters, bool noSimilarChars = false)
  {
    var excludeCharacters = new List<char> { '1', 'i', 'I', 'l', 'L', '0', 'o', 'O' };
    var exclude = noSimilarChars ? excludeCharacters : null;

    return Generate(length, allowedCharacters, exclude);
  }

  public static string Generate(int length, PasswordCharacters allowedCharacters, IEnumerable<char> excludeCharacters)
  {
    var password = InternalGenerate(length, allowedCharacters, excludeCharacters, () => new char[length], (pw, ch, index) => pw[index] = ch);
    return new string(password);
  }

  public static SecureString GenerateSecure(int length)
  {
    return GenerateSecure(length, PasswordCharacters.All, null);
  }

  public static SecureString GenerateSecure(int length, PasswordCharacters allowedCharacters)
  {
    return GenerateSecure(length, allowedCharacters, null);
  }

  public static SecureString GenerateSecure(int length, PasswordCharacters allowedCharacters,
                                            IEnumerable<char> excludeCharacters)
  {
    var password = InternalGenerate(length, allowedCharacters, excludeCharacters, () => new SecureString(), (pw, ch, index) => pw.AppendChar(ch));
    password.MakeReadOnly();
    return password;
  }

  private static T InternalGenerate<T>(int length, PasswordCharacters allowedCharacters, IEnumerable<char> excludeCharacters, Func<T> initialValue, Action<T, char, int> appender)
  {
    if (length <= 0) throw new ArgumentOutOfRangeException(nameof(length), "Password length must be greater than zero");

    var randomBytes = new byte[length];
    var randomNumberGenerator = new RNGCryptoServiceProvider();
    randomNumberGenerator.GetBytes(randomBytes);

    var allowedCharactersString = GenerateAllowedCharactersString(allowedCharacters, excludeCharacters);
    var allowedCharactersCount = allowedCharactersString.Length;

    var password = initialValue();
    for (var i = 0; i < length; i++) appender(password, allowedCharactersString[randomBytes[i] % allowedCharactersCount], i);
    return password;
  }

  private static string GenerateAllowedCharactersString(PasswordCharacters characters, IEnumerable<char> excludeCharacters)
  {
    var allowedCharactersString = new StringBuilder();
    foreach (var type in AllowedPasswordCharacters)
    {
      if ((characters & type.Key) != type.Key) continue;
      if (excludeCharacters == null) allowedCharactersString.Append(type.Value);
      else allowedCharactersString.Append(type.Value.Where(c => !excludeCharacters.Contains(c)).ToArray());
    }
    return allowedCharactersString.ToString();
  }

  private static readonly Dictionary<PasswordCharacters, string> AllowedPasswordCharacters = new Dictionary<PasswordCharacters, string>(5)
  {
    { PasswordCharacters.LowercaseLetters, "abcdefghijklmnopqrstuvwxyz" },
    { PasswordCharacters.UppercaseLetters, "ABCDEFGHIJKLMNOPQRSTUVWXYZ" },
    { PasswordCharacters.Numbers, "0123456789" },
    { PasswordCharacters.Punctuations, @"~`!@#$%^&*()_-+={[}]|\:;""'<,>.?/" },
    { PasswordCharacters.Space, " " }
  };
}

[Flags]
    public enum PasswordCharacters
    {
        LowercaseLetters = 0x01,
        UppercaseLetters = 0x02,
        Numbers = 0x04,
        Punctuations = 0x08,
        Space = 0x10,
        AllLetters = LowercaseLetters | UppercaseLetters,
        AlphaNumeric = AllLetters | Numbers,
        All = AllLetters | Numbers | Punctuations | Space
    }