const String PLAIN_TEXT = "ppgrocy.ddns.info,211,login,Password,/path/";
const String KEY = "SlEPYCE";

void setup() {
  Serial.begin(9600);
  String encryption_result = vigenere_cipher(PLAIN_TEXT, KEY, true);
  String decryption_result = vigenere_cipher(encryption_result, KEY, false);
  Serial.print("Plain Text: ");
  Serial.println(PLAIN_TEXT);
  Serial.print("Encryption: ");
  Serial.println(encryption_result);
  Serial.print("Decryption: ");
  Serial.println(decryption_result);
  Serial.println();
}

void loop() {}

String vigenere_cipher(String text, String key, bool is_encryption) {
  String result;
  int ascii, text_index, key_index, result_index;
  char character;
  for(int i = 0; i < text.length(); i++) {
    ascii = (int)text[i];
    if(ascii >= 64) {
      text_index = ((int)text[i] % 32) - 1;
      key_index = ((int)key[i % key.length()] % 32) - 1;
      if(is_encryption)
      	result_index = (text_index + key_index) % 26;
      else
      	result_index = (text_index - key_index + 26) % 26;

      if(ascii >= 'a')
        character = result_index + 'a';
      else
        character = result_index + 'A';
    } else
      character = ascii;
    
    result += character;
  }
  return result;
}