ROT13

ROT13.cs

public static string EncodeROT13(string input)
{
    return string.Join("", input.Select(x => char.IsLetter(x) ? (x >= 65 && x <= 77) || (x >= 97 && x <= 109) ? (char)(x + 13) : (char)(x - 13) : x));
}

public static string DecodeRot13(string input)
    {
        if (string.IsNullOrEmpty(input))
            return input;

        StringBuilder result = new StringBuilder(input.Length);

        foreach (char c in input)
        {
            if (c >= 'a' && c <= 'z')
            {
                result.Append((char)('a' + (c - 'a' + 13) % 26));
            }
            else if (c >= 'A' && c <= 'Z')
            {
                result.Append((char)('A' + (c - 'A' + 13) % 26));
            }
            else
            {
                result.Append(c);
            }
        }

        return result.ToString();
    }