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Posted 5 Jun 2002


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A C# Password Generator

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17 Aug 20033 min read
An article on implementing a simple password generator class in C#


This article illustrates how to create a very simple password generator using C#. Password generators are useful in many applications:

  • Registration/membership systems for Web sites
  • Auto-creation of passwords according to a specified rule
  • Securing application-specific data

The PasswordGenerator class is fairly simple. It exposes several properties that control how the password will be generated.

  • Exclusions: Specifies the set of characters to exclude in password generation.
  • Minimum: Specifies the minimum length of the generated password.
  • Maximum: Specifies the maximum length of the generated password.
  • ConsecutiveCharacters: Controls generation of consecutive characters in the generated password.
  • RepeatingCharacters: Controls generation of repeating characters in the generated password.
  • ExcludeSymbols: Excludes symbols from the set of characters used to generate the password

After setting your desired properties, call the Generate() method to create your new password.

namespace WorkingCode.CodeProject.PwdGen
    using System;
    using System.Security.Cryptography;
    using System.Text;

    public class PasswordGenerator
        public PasswordGenerator() 
            this.Minimum               = DefaultMinimum;
            this.Maximum               = DefaultMaximum;
            this.ConsecutiveCharacters = false;
            this.RepeatCharacters      = true;
            this.ExcludeSymbols        = false;
            this.Exclusions            = null;

            rng = new RNGCryptoServiceProvider();
        protected int GetCryptographicRandomNumber(int lBound, int uBound)
            // Assumes lBound >= 0 && lBound < uBound
            // returns an int >= lBound and < uBound
            uint urndnum;   
            byte[] rndnum = new Byte[4];   
            if (lBound == uBound-1)  
                // test for degenerate case where only lBound can be returned
                return lBound;
            uint xcludeRndBase = (uint.MaxValue -
                urndnum = System.BitConverter.ToUInt32(rndnum,0);      
            } while (urndnum >= xcludeRndBase);   
            return (int)(urndnum % (uBound-lBound)) + lBound;

        protected char GetRandomCharacter()
            int upperBound = pwdCharArray.GetUpperBound(0);

            if ( true == this.ExcludeSymbols )
                upperBound = PasswordGenerator.UBoundDigit;

            int randomCharPosition = GetCryptographicRandomNumber(
                pwdCharArray.GetLowerBound(0), upperBound);

            char randomChar = pwdCharArray[randomCharPosition];

            return randomChar;
        public string Generate()
            // Pick random length between minimum and maximum   
            int pwdLength = GetCryptographicRandomNumber(this.Minimum,

            StringBuilder pwdBuffer = new StringBuilder();
            pwdBuffer.Capacity = this.Maximum;

            // Generate random characters
            char lastCharacter, nextCharacter;

            // Initial dummy character flag
            lastCharacter = nextCharacter = '\n';

            for ( int i = 0; i < pwdLength; i++ )
                nextCharacter = GetRandomCharacter();

                if ( false == this.ConsecutiveCharacters )
                    while ( lastCharacter == nextCharacter )
                        nextCharacter = GetRandomCharacter();

                if ( false == this.RepeatCharacters )
                    string temp = pwdBuffer.ToString();
                    int duplicateIndex = temp.IndexOf(nextCharacter);
                    while ( -1 != duplicateIndex )
                        nextCharacter = GetRandomCharacter();
                        duplicateIndex = temp.IndexOf(nextCharacter);

                if ( ( null != this.Exclusions ) )
                    while ( -1 != this.Exclusions.IndexOf(nextCharacter) )
                        nextCharacter = GetRandomCharacter();

                lastCharacter = nextCharacter;

            if ( null != pwdBuffer )
                return pwdBuffer.ToString();
                return String.Empty;
        public string Exclusions
            get { return this.exclusionSet;  }
            set { this.exclusionSet = value; }

        public int Minimum
            get { return this.minSize; }
                this.minSize = value;
                if ( PasswordGenerator.DefaultMinimum > this.minSize )
                    this.minSize = PasswordGenerator.DefaultMinimum;

        public int Maximum
            get { return this.maxSize; }
                this.maxSize = value;
                if ( this.minSize >= this.maxSize )
                    this.maxSize = PasswordGenerator.DefaultMaximum;

        public bool ExcludeSymbols
            get { return this.hasSymbols; }
            set	{ this.hasSymbols = value;}

        public bool RepeatCharacters
            get { return this.hasRepeating; }
            set	{ this.hasRepeating = value;}

        public bool ConsecutiveCharacters
            get { return this.hasConsecutive; }
            set	{ this.hasConsecutive = value;}

        private const int DefaultMinimum = 6;
        private const int DefaultMaximum = 10;
        private const int UBoundDigit    = 61;

        private RNGCryptoServiceProvider    rng;
        private int 			minSize;
        private int 			maxSize;
        private bool			hasRepeating;
        private bool			hasConsecutive;
        private bool            hasSymbols;
        private string          exclusionSet;
        private char[] pwdCharArray = "abcdefghijklmnopqrstuvwxyzABCDEFG" +
            "HIJKLMNOPQRSTUVWXYZ0123456789`~!@#$%^&*()-_=+[]{}\\|;:'\",<" + 

The previous version of this class was intended for use in a much larger project (I will be publishing articles on various components of that project soon.). Due to many factors, that code was rushed and proved to be inefficient. In fact, I wish I could go back in time and unpublish it! While this version is definitely better, there is still room for improvement. The generation algorithm can still be optimized. Also, it would be interesting to use regular expressions to both define and validate the passwords we wish to generate. I would have done this, but it's been a long time since I wrote a parser. Maybe for the next version...

In previous articles, I have used the NAnt tool as my build solution. Unfortunately, that team has not produced a stable release that integrates NUnit 2.0. I COULD get the source from the CVS tree, but I'm way too lazy for that. Instead, I have decided to go back to Visual Studio .NET as my development environment. I'm also getting used to test-driven development with NUnit 2.0. If you aren't using this tool for unit testing, I highly recommend you give it a try Its use of attributes and reflection to specify test suites, test fixtures and tests, and is quite remarkable and easy to use. I've included my unit test fixture with the source code. Also, try the NUnit Addin for Visual Studio .NET; it's very handy for running your tests within the IDE.

The demo project is a simple Windows Forms UI that allows one to configure the password generator's properties. I must say that while VS.NET is fairly complete and powerful, I just don't like the feel of the forms designer. However, it definitely does the job.

Many thanks to Mike Asher and Julian Roberts for their feedback on the first version of the password generator. Julian was kind enough to test the code in an ASP.NET project and confirmed that it performs much better. Also, I reverted to my old C++ bracing style just to make Nish happy...hope you appreciate the sacrifice! :-)

Change Log

Version 1.2

  • Updated for .NET Framework 1.1
  • Removed FirstCharacter and LastCharacter properties; Exclusions works across all characters
  • Replaced Password property with Generate() method
  • Removed PwdMaskFlags; use Exclusions property and/or ExcludeSymbols property
  • Used RNGCryptoServiceProvider instead of Random for random number generation
  • Updated demo application

Perfection (in design) is achieved not when there is nothing more to add, but rather when there is nothing more to take away. - Antoine de Saint-Exupéry

Version 1.1

  • Improved password generation algorithm

Version 1.0

  • Initial version


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Kevin Stewart
Web Developer
United States United States
Kevin is a software development manager for a small consumer-oriented company in SoHo, NY. He is technology and platform agnostic having worked on PCs, Macintosh and various forms of Unix and Linux. His programming knowledge includes several languages, including C/C++, Java and C#. In the rare moments when his head is not buried in the latest tech book purchase from Amazon, Kevin enjoys spending time with his wife Donna and their dog Kirby.

Comments and Discussions

QuestionLicense for use of the above code Pin
monicanahar10-Apr-14 1:59
Membermonicanahar10-Apr-14 1:59 
GeneralBug with pwdCharArray Pin
quandary2-Aug-12 5:29
Memberquandary2-Aug-12 5:29 
GeneralNice work Pin
Nueman3-Feb-11 4:55
MemberNueman3-Feb-11 4:55 
QuestionBug in Generate() method Pin
Bishop, Geoff15-Dec-08 9:48
MemberBishop, Geoff15-Dec-08 9:48 
QuestionBug when exact password size requested Pin
Bishop, Geoff15-Dec-08 9:09
MemberBishop, Geoff15-Dec-08 9:09 
AnswerRe: Bug when exact password size requested Pin
Rolando CC24-Feb-09 7:34
professionalRolando CC24-Feb-09 7:34 
GeneralException thrown Pin
thom_ek29-May-08 2:49
Memberthom_ek29-May-08 2:49 
GeneralPronounceable passwords Pin
yetanotherchris16-Nov-07 11:29
Memberyetanotherchris16-Nov-07 11:29 
GeneralRe: Pronounceable passwords Pin
Kevin Stewart16-Nov-07 18:30
MemberKevin Stewart16-Nov-07 18:30 
GeneralOverriding the constructor Pin
Spam Basket9-Mar-07 8:36
MemberSpam Basket9-Mar-07 8:36 
GeneralHere's a VB Implementation... Pin
code-frog31-Jan-07 7:59
professionalcode-frog31-Jan-07 7:59 
GeneralRe: Here's a VB Implementation... Pin
Kevin Stewart7-Feb-07 14:28
MemberKevin Stewart7-Feb-07 14:28 
GeneralRe: Here's a VB Implementation... Pin
code-frog8-Feb-07 2:38
professionalcode-frog8-Feb-07 2:38 
GeneralRe: Here's a VB Implementation... Pin
code-frog8-Feb-07 2:40
professionalcode-frog8-Feb-07 2:40 
GeneralI threw you a 5... Pin
code-frog30-Oct-06 18:43
professionalcode-frog30-Oct-06 18:43 
GeneralRe: I threw you a 5... Pin
Kevin Stewart3-Nov-06 2:02
MemberKevin Stewart3-Nov-06 2:02 
QuestionCopyright Release? Pin
tgiphil4-Oct-05 18:14
Membertgiphil4-Oct-05 18:14 
AnswerRe: Copyright Release? Pin
Kevin Stewart5-Oct-05 4:42
MemberKevin Stewart5-Oct-05 4:42 
GeneralNice class Pin
Jan R Hansen17-May-05 20:46
MemberJan R Hansen17-May-05 20:46 
GeneralRe: Nice class Pin
Dan Crowell27-Jun-05 12:11
MemberDan Crowell27-Jun-05 12:11 
GeneralpwdCharArray Pin
azTracker121-Mar-05 8:41
MemberazTracker121-Mar-05 8:41 
GeneralInfinite Loop using RepeatCharacters as false Pin
frihani7-Feb-05 11:34
Memberfrihani7-Feb-05 11:34 
GeneralAlternative Pin
smallguy16-Jun-04 8:04
Membersmallguy16-Jun-04 8:04 
GeneralRe: Alternative Pin
Kevin Stewart16-Jun-04 8:30
MemberKevin Stewart16-Jun-04 8:30 
GeneralRe: Alternative Pin
smallguy16-Jun-04 11:01
Membersmallguy16-Jun-04 11:01 

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