private void button1_Click(object sender, EventArgs e)
        {
            StringBuilder text = new StringBuilder(textBox2.Text);
            progressBar1.Maximum = text.Length;
            progressBar1.Value = 0;
            string key = textBox3.Text.ToUpper();
            int j = 0;

            for (int i = 0; i < text.Length; i++)
            {
                int newCode = text[i] + (key[j] - 64);

                if(char.IsUpper(text[i]) && newCode > 90)
                {
                    text[i] = (char)(64 + newCode - 90);
                }
                if(char.IsLower(text[i]) && newCode > 122)
                {
                    text[i] = (char)(96 + newCode - 122);
                }
                else
                {
                    text[i] = (char)newCode;
                }

                if (j == key.Length - 1)
                    j = 0;
                else
                    j++;

                progressBar1.Value++;
            }

            textBox1.Text = text.ToString();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            StringBuilder text = new StringBuilder(textBox2.Text);
            progressBar1.Maximum = text.Length;
            progressBar1.Value = 0;
            string key = textBox3.Text.ToUpper();
            int j = 0;

            for (int i = 0; i < text.Length; i++)
            {
                int newCode = text[i] - (key[j] - 64);

                if (char.IsUpper(text[i]) && newCode < 65)
                {
                    text[i] = (char)(91 - (65 - newCode));
                }
                if (char.IsLower(text[i]) && newCode < 97)
                {
                    text[i] = (char)(123- ( 97- newCode));
                }
                else
                {
                    text[i] = (char)newCode;
                }

                if (j == key.Length - 1)
                    j = 0;
                else
                    j++;

                progressBar1.Value++;
            }

            textBox1.Text = text.ToString();
        }