String scrambler

StringScrambler

    class StringScrambler
    {
        private readonly char[] _scrambleLetters;
        private Random rand = new Random();

        public StringScrambler(char[] scrambleLetters)
        {
            if (scrambleLetters == null) throw new ArgumentNullException("scrambleLetters");
            _scrambleLetters = scrambleLetters;
        }

        public string Scramble(string source, float percent)
        {
            var lettersToScramble = (int) (source.Length*percent);
            var builder = new StringBuilder(source);

            for (int i = lettersToScramble; i < source.Length; i++)
                builder[i] = GetRandomChar(builder[i]);

            return builder.ToString();
        }

        private char GetRandomChar(char c)
        {
            if (_scrambleLetters.Contains(c))
            {
                int index = _scrambleLetters.ToList().IndexOf(c) + rand.Next(6);
                if (index > _scrambleLetters.Length - 1)
                    index = 0;
                return _scrambleLetters[index];
            }
            return _scrambleLetters[rand.Next(_scrambleLetters.Length)];
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var letters = new[]
            {
                
                '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', '[', ']', '(', ')', '.', '-',
                ','
            };

            while (true)
            {
                Console.Write(">");
                string input = Console.ReadLine();

                float current = 0;
                float duration = 50 * input.Length;

                float stepDuration = (input.Length/duration) * 1000f;

                while (current < duration)
                {
                    if (input.Length == 0) break;

                    current += stepDuration;
                    Thread.Sleep((int)stepDuration);
                    Console.Clear();
                    Console.WriteLine(current / duration);
                    Console.WriteLine(new StringScrambler(letters).Scramble(input, current / duration));
                }
            }
        }
    }