the rot13 is essentially the caesar transform applied with an offset of 13

rot13.cs

static class Rot13
    {
	/// <summary>
	/// Performs the ROT13 character rotation.
	/// </summary>
	public static string Transform(string value)
	{
	    char[] array = value.ToCharArray();
	    for (int i = 0; i < array.Length; i++)
	    {
		int number = (int)array[i];

		if (number >= 'a' && number <= 'z')
		{
		    if (number > 'm')
		    {
			number -= 13;
		    }
		    else
		    {
			number += 13;
		    }
		}
		else if (number >= 'A' && number <= 'Z')
		{
		    if (number > 'M')
		    {
			number -= 13;
		    }
		    else
		    {
			number += 13;
		    }
		}
		array[i] = (char)number;
	    }
	    return new string(array);
	}
    }