Q/Create software that can encrypt and decrypt using a 2 * 2 Hill cipher

HillCipher.py

import string
import numpy
from sympy  import Matrix

def HillEncrypt(message : string, key: numpy.ndarray):
    message = message.replace(" ", "")

    if len(message)%2 != 0:
        message += 'a'

    indexMessageArray = GetIndexArrayFromString(message)

    rowNumber = int(len(message)/2)
    messageMatrix = numpy.array(indexMessageArray).reshape(rowNumber,2)

    resultMatrix = messageMatrix.dot(key)
    resultMatrix = resultMatrix%26

    return GetStringFromIndexArray(resultMatrix.flatten())



def HillDecrypt(cipher : string, key : numpy.ndarray):
    cipher = cipher.replace(" ", "")

    indexCipherArray = GetIndexArrayFromString(cipher)

    rowNumber = int(len(cipher)/2)
    cipherMatrix = numpy.array(indexCipherArray).reshape(rowNumber,2)

    keyInverse = numpy.array(Matrix(key).inv_mod(26))

    resultMatrix = cipherMatrix.dot(keyInverse)
    resultMatrix = resultMatrix%26

    return GetStringFromIndexArray(resultMatrix.flatten())



def GetIndexArrayFromString(input: string):
    lettersIndexedArray = []
    for letter in input:
        lettersIndexedArray.append(GetIndexFromLetter(letter))
    return lettersIndexedArray

def GetStringFromIndexArray(leterIndexes : list):
    result = ""
    for index in leterIndexes :
        result += GetLetterFromIndex(int(index))
    return result


def GetIndexFromLetter(letter):
    return string.ascii_uppercase.index(letter.upper()) 

def GetLetterFromIndex(index):
    return string.ascii_uppercase[index]


message = "meet me at eight"
key = numpy.array(
    [[9,4],
    [5,7]])

cipher = HillEncrypt(message, key)
decryptedMessage = HillDecrypt(cipher, key)

print(cipher) #print YYBTYYRDYULVPY
print(decryptedMessage) #print MEETMEATEIGHTA