def vigenere_encrypt(plaintext, key):
    result = []
    key = key.upper()
    for i, char in enumerate(plaintext.upper()):
        if char.isalpha():
            key_index = ord(key[i % len(key)]) - ord('A')
            encrypted_char = chr((ord(char) - ord('A') + key_index) % 26 + ord('A'))
            result.append(encrypted_char)
        else:
            result.append(char)
    return ''.join(result)

def vigenere_decrypt(ciphertext, key):
    result = []
    key = key.upper()
    for i, char in enumerate(ciphertext.upper()):
        if char.isalpha():
            key_index = ord(key[i % len(key)]) - ord('A')
            decrypted_char = chr((ord(char) - ord('A') - key_index + 26) % 26 + ord('A'))
            result.append(decrypted_char)
        else:
            result.append(char)
    return ''.join(result)

# Example
text = "ATTACKATDAWN"
key = "LEMON"
encrypted = vigenere_encrypt(text, key)
decrypted = vigenere_decrypt(encrypted, key)

print("Encrypted:", encrypted)  # Output: LXFOPVEFRNHR
print("Decrypted:", decrypted)  # Output: ATTACKATDAWN

def autokey_encrypt(plaintext, key):
    key_stream = key.upper() + plaintext.upper()
    result = []
    for i, char in enumerate(plaintext.upper()):
        if char.isalpha():
            shift = ord(key_stream[i]) - ord('A')
            encrypted_char = chr((ord(char) - ord('A') + shift) % 26 + ord('A'))
            result.append(encrypted_char)
        else:
            result.append(char)
    return ''.join(result)

def autokey_decrypt(ciphertext, key):
    key_stream = key.upper()
    result = []
    for i, char in enumerate(ciphertext.upper()):
        if char.isalpha():
            shift = ord(key_stream[i]) - ord('A')
            decrypted_char = chr((ord(char) - ord('A') - shift + 26) % 26 + ord('A'))
            result.append(decrypted_char)
            key_stream += decrypted_char
        else:
            result.append(char)
    return ''.join(result)

# Example
text = "ATTACKATDAWN"
key = "LEMON"
enc = autokey_encrypt(text, key)
dec = autokey_decrypt(enc, key)

print("Encrypted:", enc)
print("Decrypted:", dec)

function vigenereEncrypt(text, key) {
  text = text.toUpperCase();
  key = key.toUpperCase();
  let result = "";

  for (let i = 0, j = 0; i < text.length; i++) {
    let char = text[i];
    if (char >= 'A' && char <= 'Z') {
      let shift = key.charCodeAt(j % key.length) - 65;
      let encrypted = String.fromCharCode((char.charCodeAt(0) - 65 + shift) % 26 + 65);
      result += encrypted;
      j++;
    } else {
      result += char;
    }
  }
  return result;
}

function vigenereDecrypt(text, key) {
  text = text.toUpperCase();
  key = key.toUpperCase();
  let result = "";

  for (let i = 0, j = 0; i < text.length; i++) {
    let char = text[i];
    if (char >= 'A' && char <= 'Z') {
      let shift = key.charCodeAt(j % key.length) - 65;
      let decrypted = String.fromCharCode((char.charCodeAt(0) - 65 - shift + 26) % 26 + 65);
      result += decrypted;
      j++;
    } else {
      result += char;
    }
  }
  return result;
}

// Example
let message = "ATTACKATDAWN";
let keyword = "LEMON";
let encrypted = vigenereEncrypt(message, keyword);
let decrypted = vigenereDecrypt(encrypted, keyword);

console.log("Encrypted:", encrypted);  // LXFOPVEFRNHR
console.log("Decrypted:", decrypted);  // ATTACKATDAWN

function autokeyEncrypt(text, key) {
  text = text.toUpperCase();
  key = (key + text).toUpperCase();
  let result = "";

  for (let i = 0; i < text.length; i++) {
    let char = text[i];
    if (char >= 'A' && char <= 'Z') {
      let shift = key.charCodeAt(i) - 65;
      let encrypted = String.fromCharCode((char.charCodeAt(0) - 65 + shift) % 26 + 65);
      result += encrypted;
    } else {
      result += char;
    }
  }
  return result;
}

function autokeyDecrypt(text, key) {
  text = text.toUpperCase();
  key = key.toUpperCase();
  let result = "";

  for (let i = 0; i < text.length; i++) {
    let char = text[i];
    if (char >= 'A' && char <= 'Z') {
      let shift = key.charCodeAt(i) - 65;
      let decrypted = String.fromCharCode((char.charCodeAt(0) - 65 - shift + 26) % 26 + 65);
      result += decrypted;
      key += decrypted;
    } else {
      result += char;
    }
  }
  return result;
}

// Example
let plaintext = "ATTACKATDAWN";
let seed = "LEMON";
let enc = autokeyEncrypt(plaintext, seed);
let dec = autokeyDecrypt(enc, seed);

console.log("Encrypted:", enc);
console.log("Decrypted:", dec);