How can I generate a cryptographically secure random integer within a range?

Question

I have to generate a uniform, secure random integer within a given range for a program that generates passwords. Right now I use this:

RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
byte[] rand = new byte[4];
rng.GetBytes(rand);
int i = BitConverter.ToUInt16(rand, 0);
int result = i%max;   // max is the range's upper bound (the lower is 0)

Is this method safe to use for cryptographic purposes? If not, how should I do it?

Answer 1

You can have a look at the CryptoRandom class taken from niik/CryptoRandom.cs which is the original version by Stephen Toub and Shawn Farkas. In this class they implement several random generators that seem to be cryptographically secure.

I have used the following version in my projects for random integer generation.

public class RandomGenerator
{
    readonly RNGCryptoServiceProvider csp;

    public RandomGenerator()
    {
        csp = new RNGCryptoServiceProvider();
    }

    public int Next(int minValue, int maxExclusiveValue)
    {
        if (minValue >= maxExclusiveValue)
            throw new ArgumentOutOfRangeException("minValue must be lower than maxExclusiveValue");

        long diff = (long)maxExclusiveValue - minValue;
        long upperBound = uint.MaxValue / diff * diff;

        uint ui;
        do
        {
            ui = GetRandomUInt();
        } while (ui >= upperBound);
        return (int)(minValue + (ui % diff));
    }

    private uint GetRandomUInt()
    {
        var randomBytes = GenerateRandomBytes(sizeof(uint));
        return BitConverter.ToUInt32(randomBytes, 0);
    }

    private byte[] GenerateRandomBytes(int bytesNumber)
    {
        byte[] buffer = new byte[bytesNumber];
        csp.GetBytes(buffer);
        return buffer;
    }
}

Answer 2

There are two issues in the accepted answer.

• It does not dispose the disposable csp correctly
• When minvalue equals maxvalue, it throws an error (the standard random method does not)

Revised code

using System;
using System.Security.Cryptography;

namespace CovidMassTesting.Helpers
{
    /// <summary>
    /// Secure random generator
    ///
    /// <https://stackoverflow.com/questions/42426420/how-to-generate-a-cryptographically-secure-random-integer-within-a-range>
    ///
    /// </summary>
    public class RandomGenerator : IDisposable
    {
        private readonly RNGCryptoServiceProvider csp;

        /// <summary>
        /// Constructor
        /// </summary>
        public RandomGenerator()
        {
            csp = new RNGCryptoServiceProvider();
        }

        /// <summary>
        /// Get random value
        /// </summary>
        /// <param name="minValue"></param>
        /// <param name="maxExclusiveValue"></param>
        /// <returns></returns>
        public int Next(int minValue, int maxExclusiveValue)
        {
            if (minValue == maxExclusiveValue)
                return minValue;

            if (minValue > maxExclusiveValue)
            {
                throw new ArgumentOutOfRangeException($"{nameof(minValue)} must be lower than {nameof(maxExclusiveValue)}");
            }

            var diff = (long)maxExclusiveValue - minValue;
            var upperBound = uint.MaxValue / diff * diff;

            uint ui;
            do
            {
                ui = GetRandomUInt();
            } while (ui >= upperBound);

            return (int)(minValue + (ui % diff));
        }

        private uint GetRandomUInt()
        {
            var randomBytes = GenerateRandomBytes(sizeof(uint));
            return BitConverter.ToUInt32(randomBytes, 0);
        }

        private byte[] GenerateRandomBytes(int bytesNumber)
        {
            var buffer = new byte[bytesNumber];
            csp.GetBytes(buffer);
            return buffer;
        }

        private bool _disposed;

        /// <summary>
        /// Public implementation of Dispose pattern callable by consumers.
        /// </summary>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// Protected implementation of Dispose pattern.
        /// </summary>
        /// <param name="disposing"></param>
        protected virtual void Dispose(bool disposing)
        {
            if (_disposed)
            {
                return;
            }

            if (disposing)
            {
                // Dispose managed state (managed objects).
                csp?.Dispose();
            }

            _disposed = true;
        }
    }
}

Usage

/// <summary>
        /// Generates a random password,
        /// respecting the given strength requirements.
        /// </summary>
        /// <param name="opts">A valid PasswordOptions object
        /// containing the password strength requirements.</param>
        /// <returns>A random password</returns>
        public static string GenerateRandomPassword(PasswordOptions opts = null)
        {
            if (opts == null) opts = new PasswordOptions()
            {
                RequiredLength = 10,
                RequiredUniqueChars = 4,
                RequireDigit = true,
                RequireLowercase = true,
                RequireNonAlphanumeric = true,
                RequireUppercase = true
            };

            string[] randomChars = new[] {
                "ABCDEFGHJKLMNOPQRSTUVWXYZ",    // Uppercase
                "abcdefghijkmnopqrstuvwxyz",    // Lowercase
                "0123456789",                   // Digits
                "!@$?_-"                        // Non-alphanumeric
            };

            using RandomGenerator rand = new RandomGenerator();
            List<char> chars = new List<char>();

            if (opts.RequireUppercase)
                chars.Insert(rand.Next(0, chars.Count),
                    randomChars[0][rand.Next(0, randomChars[0].Length)]);

            if (opts.RequireLowercase)
                chars.Insert(rand.Next(0, chars.Count),
                    randomChars[1][rand.Next(0, randomChars[1].Length)]);

            if (opts.RequireDigit)
                chars.Insert(rand.Next(0, chars.Count),
                    randomChars[2][rand.Next(0, randomChars[2].Length)]);

            if (opts.RequireNonAlphanumeric)
                chars.Insert(rand.Next(0, chars.Count),
                    randomChars[3][rand.Next(0, randomChars[3].Length)]);

            for (int i = chars.Count; i < opts.RequiredLength
                || chars.Distinct().Count() < opts.RequiredUniqueChars; i++)
            {
                string rcs = randomChars[rand.Next(0, randomChars.Length)];
                chars.Insert(rand.Next(0, chars.Count),
                    rcs[rand.Next(0, rcs.Length)]);
            }

            return new string(chars.ToArray());
        }

Answer 3

In .NET 6, RNGCryptoServiceProvider used in the previous answers is now obsolete.

For cryptographic random numbers, simply use RandomNumberGenerator static methods, such as:

var byteArray = RandomNumberGenerator.GetBytes(24);

Answer 4

From your code I can see, you want to get a random integer number from an interval.

There is a new cryptographic random number generator included in .NET (since versions Core 3.0, Core 3.1, .NET 5, .NET 6, .NET 7 RC 1 and .NET Standard 2.1).

As jws mentioned, the formerly used class RNGCryptoServiceProvider is deprecated.

You can use this helper method. It can also easily replace the unsafe System.Random's .Next method:

/// <summary>
/// Generate a secure random number
/// </summary>
/// <param name="fromInclusive">Random number interval (min, including this number)</param>
/// <param name="toExclusive">Random number interval (max, excluding this number)</param>
/// <returns></returns>
private int RandomNumber(int fromInclusive, int toExclusive)
=> System.Security.Cryptography.RandomNumberGenerator.GetInt32(fromInclusive, toExclusive);

To simulate rolling a dice, use it like

var getNumber = RandomNumber(1, 7); // including 1, excluding 7 => 1 .. 6

If you prefer to use it "the old way" with .Next(), you can create a class like so (note that instead of a seed there are the fromInclusive and toExclusive parameters instead):

public class SecureRandom
{
    private int fromInc, toExcl;
    public SecureRandom(int toExclusive = 2) => Init(0, toExclusive);
    public SecureRandom(int fromInclusive, int toExclusive) 
           => Init(fromInclusive, toExclusive);
    private void Init(int fromInclusive, int toExclusive)
    {
        fromInc = fromInclusive; toExcl = toExclusive;
    }
    
    public int Next() => RandomNumber(fromInc, toExcl);

    public static int RandomNumber(int fromInclusive, int toExclusive)
    => System.Security.Cryptography.RandomNumberGenerator.GetInt32(fromInclusive, toExclusive);
}

Example:

// always the same interval in a loop:
var rnd = new SecureRandom(1, 7); 
for (int i = 0; i < 100; i++)
{
    Console.WriteLine(rnd.Next()); // roll the dice 100 times
}

// alternative usage (without creating an instance):
Console.WriteLine(SecureRandom.RandomNumber(1, 7));

Note:

• This version doesn't require to get an instance from the cryptographic class any more - you just call it to get the next random number.

• There's also an overload of GetInt32(...) which takes one argument for the maximum exclusive value, which starts from the minimum value 0. If you need that, feel free to update the code and create another overloaded method for the static function RandomNumber.