create a function that takes key and plain text as inputs and outputs a cipher text using hill cipher algorithm

public string hillCipher(string key, string plainText) 
    {
        string cipherText = "";
        int[] keyInt = key.Select(c => (int)c).ToArray();
        int[] plainTextInt = plainText.Select(c => (int)c).ToArray();
        int[,] keyMatrix = new int[2, 2];
        int[,] plainTextMatrix = new int[2, 1];
        keyMatrix[0, 0] = keyInt[0] - 65;
        keyMatrix[0, 1] = keyInt[1] - 65;
        keyMatrix[1, 0] = keyInt[2] - 65;
        keyMatrix[1, 1] = keyInt[3] - 65;
        plainTextMatrix[0, 0] = plainTextInt[0] - 65;
        plainTextMatrix[1, 0] = plainTextInt[1] - 65;
        int[,] cipherTextMatrix = new int[2, 1];
        cipherTextMatrix[0, 0] = (keyMatrix[0,