input: Write a function to create a Python application for encryption and decryption of strings using the AES cipher. The function should import the necessary libraries, set a key, and define two methods: `encrypt` for encoding a plaintext string and `decrypt` for decoding a ciphertext string.
  assert encrypt(b'Hello, World!') == b'\xe3\xb1\xdf\xe0\x9c\x88\xf899p\x9e\xd0\\\xcf\x851'
  assert decrypt(b'\xe3\xb1\xdf\xe0\x9c\x88\xf899p\x9e\xd0\\\xcf\x851') == b'Hello, World!'
  assert encrypt(b'') == b'T\x8aR\xf9%H\xed\x98Bzz\xfcaC\x8a\xe7'
  assert decrypt(b'T\x8aR\xf9%H\xed\x98Bzz\xfcaC\x8a\xe7') == b''
output: from Crypto.Cipher import AES
  from Crypto.Util.Padding import pad, unpad
  import os
  
  key = os.urandom(16)
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_CBC)
      ciphertext = cipher.encrypt(pad(plaintext, AES.block_size))
      return cipher.iv + ciphertext
  
  def decrypt(ciphertext):
      iv = ciphertext[:16]
      cipher = AES.new(key, AES.MODE_CBC, iv)
      plaintext = unpad(cipher.decrypt(ciphertext[16:]), AES.block_size)
      return plaintext