gistfile1.pde

String alphabet = "abcdefghijklmnopqrstuvwxyz";

void setup() {
  String plaintext = "happy monday, everyone!";
  String ciphertext = encrypt(plaintext, 1);
  println(ciphertext);
}
void draw() {
}

String encrypt(String msg, int k) {
  String encryptedMsg = "";
  for (int i = 0; i < msg.length(); i++) {
    char ltr = msg.charAt(i);
    int plainIndex = alphabet.indexOf(ltr);
    if (plainIndex == -1) {
      encryptedMsg = encryptedMsg + ltr;
    } else {
      int cipherIndex = plainIndex + k;
      if (cipherIndex > 26) {
        cipherIndex = cipherIndex % 26;
      }
      char cipherLtr = alphabet.charAt(cipherIndex);
      encryptedMsg = encryptedMsg + cipherLtr;
    }
  }
  return encryptedMsg;
}