Encrypt and Decrypt text using alphabet shifting method

Decrypt

            char[] alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();

            char[] cypertext = (txtCyperText.Text.ToUpper()).ToCharArray();

            string decryptText = "";

            foreach (char item in cypertext)

            {

                for (int i = 0; i < alpha.Length; i++)

                {

                    if (item == 'A')

                    {

                        decryptText = decryptText + "X";

                        break;

                    }

                    else if (item == 'B')

                    {

                        decryptText = decryptText + "Y";

                        break;

                    }

                    else if (item == 'C')

                    {

                        decryptText = decryptText + "Z";

                        break;

                    }

                    else if (item == alpha[i])

                    {

                        decryptText = decryptText + alpha[i - 3];

                        break;

                    }

                }

            }

            txtDecryptText.Text = decryptText;

Encrypt

            char[] alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();

            char[] decriptText = (txtDecryptText.Text.ToUpper()).ToCharArray();

            string encryptText = "";

            foreach (char item in decriptText)

            {

                for (int i = 0; i < alpha.Length; i++)

                {

                    if (item == 'X')

                    {

                        encryptText = encryptText + "A";

                        break;

                    }

                    else if (item == 'Y')

                    {

                        encryptText = encryptText + "B";

                        break;

                    }

                    else if (item == 'Z')

                    {

                        encryptText = encryptText + "C";

                        break;

                    }

                    else if (item == alpha[i])

                    {

                        encryptText = encryptText + alpha[i + 3];

                        break;

                    }

                }               

            }

            txtEncryptText.Text = encryptText;