Affine Cipher Encoder

AffineCipher.cs

using System;

public class Program
{
	static char[] lowerCaseLetters = new char[]{'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'};

    public static void Main(string[] args)
    {
        string text = AffineCipher("Affine", 1, 1);
        
        Console.WriteLine (text);
    }

    private static string AffineCipher(string oldText, int key1, int key2)
    {
        bool? isItUpper = false;
        char currentLetter = new char();
        string newText = ""; 
        char newLetter = new char(); 
        char[] letters = oldText.ToCharArray();

        foreach(char letter in letters) {
            if((int)letter >= 65 && (int)letter <= 90){
                isItUpper = true;

                currentLetter = Char.ToLower(letter);
            }
            else if((int)letter >= 97 && (int)letter <= 122){
                isItUpper = false;
                currentLetter = letter;
            }
            else{
                isItUpper = null;
            }

            int index = Array.IndexOf(lowerCaseLetters, currentLetter);

            if(isItUpper != null){
                if ((bool)isItUpper)
                { 
                    newLetter = FindNewLetter(index, key1, key2, true);
                }
                else if (!(bool)isItUpper)
                { 
                    newLetter = FindNewLetter(index, key1, key2, false);
                }
            }
            else{ 
                newLetter = letter;
            }

            newText += newLetter;
        }

        return newText;
    }

    private static char FindNewLetter(int index, int key1, int key2, bool isItUpper) 
    {
        char newLetter = new char();
        int newIndex = (key1 * index + key2) % 26;
        newLetter = lowerCaseLetters[newIndex];

        if(isItUpper)
            newLetter = char.ToUpper(newLetter);

        return newLetter;
    }
}