RSGCryptoService.cs

using System.Linq;
using System.Security.Cryptography;

namespace Cryptography
{
    public interface IRSGCryptoService
    {
        /// <summary>
        ///     Generates a cryptographically secure string of specified length
        /// </summary>
        /// <param name="length">Length of the string to be generated</param>
        /// <returns>Generated string</returns>
        string GenerateString(int length);
    }

    /// <summary>
    ///     A cryptographically secure random string generator (RSG)
    /// </summary>
    public class RSGCryptoService : IRSGCryptoService
    {
        private readonly char[] charPool;

        /// <summary>
        ///     A cryptographically secure random string generator (RSG)
        /// </summary>
        public RSGCryptoService()
        {
            this.charPool = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789".ToArray();
        }

        /// <summary>
        ///     A cryptographically secure random string generator (RSG)
        /// </summary>
        /// <param name="charPool">
        ///     The pool of characters to use
        ///     when generating a string, overrides
        ///     the default which contains chars [a-zA-Z0-9]
        /// </param>
        public RSGCryptoService(char[] charPool)
        {
            this.charPool = charPool.Distinct().ToArray();
        }

        /// <summary>
        ///     Generates a cryptographically secure string of specified length
        /// </summary>
        /// <param name="length">Length of the string to be generated</param>
        /// <returns>Generated string</returns>
        public string GenerateString(int length)
        {
            int maxByteVal = GetMaxByteVal();
            string generatedString = "";

            using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
            {
                for (int i = 0; i < length; i++)
                {
                    byte[] bits = new byte[1];

                    do
                    {
                        rng.GetNonZeroBytes(bits);
                    } while (bits[0] > maxByteVal);

                    generatedString += charPool[bits[0] % charPool.Length];
                }
            }
            return generatedString;
        }

        #region Utilities

        private int GetMaxByteVal()
        {
            int multiplier = 256 / charPool.Length;
            int maxVal = multiplier * charPool.Length;
            return maxVal;
        }

        #endregion
    }

}