public static void Main()
string plaintext = "The ASCII code represents characters.";
for (int i = 0; i < plaintext.Length; i++)
if (plaintext[i] == '.' || plaintext[i] == ' ' )
ciphertext += plaintext[i];
ciphertext += (char)(((plaintext[i] + 3 - plaintext[i]) % 26) + plaintext[i]);
Console.WriteLine(ciphertext);