Lesson 12

static void Main(string[] args)
        {
            Console.WriteLine("Choose action:\n1.Encryption \n2.Decryption");
            int choice = int.Parse(Console.ReadLine());

            Console.WriteLine("Submit text");
            string text = Console.ReadLine();

            Console.WriteLine("Submit key (1-31)");
            int key = int.Parse(Console.ReadLine());
            if (key < 0)
            {
                key = 0;
            }

            if (choice == 1)
            {
                string encryptedText = Encryption(text, key);
                Console.WriteLine($"Encrypted text: {encryptedText}");
            }
            else
            {
                string decryptedText = Decryption(text, key);
                Console.WriteLine($"Decrypted text: {decryptedText}");
            }

            Console.ReadLine();

        }
        public static string Encryption(string text, int key)
        {
            string alphabet = "abcdefghijklmnopqrstuvwxyz";
            string encryptedText = "";

            for (int counter = 0; counter < text.Length; counter++)
            {

                char character = text[counter];

                int characterNumber = alphabet.IndexOf(character);

                if (characterNumber == -1)
                {
                    encryptedText += character;
                }
                else
                {

                    int newCharacterNumber = (characterNumber + key) % alphabet.Length;
                    char newCharacter = alphabet[newCharacterNumber];
                    encryptedText += newCharacter;
                }
            }

            return encryptedText;
        }

        public static string Decryption(string text, int key)
        {
            string alphabet = "abcdefghijklmnopqrstuvwxyz";
            string decryptedText = "";

            for (int counter = 0; counter < text.Length; counter++)
            {

                char character = text[counter];

                int characterNumber = alphabet.IndexOf(character);

                if (characterNumber == -1)
                {
                    decryptedText += character;
                }
                else
                {
                    int newCharacterNumber = (characterNumber + (alphabet.Length - key)) % alphabet.Length;
                    char newCharacter = alphabet[newCharacterNumber];
                    decryptedText += newCharacter;
                }
            }

            return decryptedText;
        }