Need help understanding AES

I have been working on a hacking competition called picoCTF, and there is one question called "Secure Logon" that requires me to login using the username of admin and somehow login. The hint said that there is a insecure type of AES in which we can hack. However, all I was able to understand up to this point is that there is a base 64 encryption of the cookie. Can someone please explain to be how I can possibly figure out how to login and get the flag? The page is http://2018shell2.picoctf.com:46026/ and the code for the encryption is attached below.

from flask import Flask, render_template, request, url_for, redirect, make_response, flash
import json
from hashlib import md5
from base64 import b64decode
from base64 import b64encode
from Crypto import Random
from Crypto.Cipher import AES

app = Flask(__name__)
app.secret_key = 'seed removed'
flag_value = 'flag removed'

BLOCK_SIZE = 16  # Bytes
pad = lambda s: s + (BLOCK_SIZE - len(s) % BLOCK_SIZE) * \
                chr(BLOCK_SIZE - len(s) % BLOCK_SIZE)
unpad = lambda s: s[:-ord(s[len(s) - 1:])]


@app.route("/")
def main():
    return render_template('index.html')

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.form['user'] == 'admin':
        message = "I'm sorry the admin password is super secure. You're not getting in that way."
        category = 'danger'
        flash(message, category)
        return render_template('index.html')
    resp = make_response(redirect("/flag"))

    cookie = {}
    cookie['password'] = request.form['password']
    cookie['username'] = request.form['user']
    cookie['admin'] = 0
    print(cookie)
    cookie_data = json.dumps(cookie, sort_keys=True)
    encrypted = AESCipher(app.secret_key).encrypt(cookie_data)
    print(encrypted)
    resp.set_cookie('cookie', encrypted)
    return resp

@app.route('/logout')
def logout():
    resp = make_response(redirect("/"))
    resp.set_cookie('cookie', '', expires=0)
    return resp

@app.route('/flag', methods=['GET'])
def flag():
  try:
      encrypted = request.cookies['cookie']
  except KeyError:
      flash("Error: Please log-in again.")
      return redirect(url_for('main'))
  data = AESCipher(app.secret_key).decrypt(encrypted)
  data = json.loads(data)

  try:
     check = data['admin']
  except KeyError:
     check = 0
  if check == 1:
      return render_template('flag.html', value=flag_value)
  flash("Success: You logged in! Not sure you'll be able to see the flag though.", "success")
  return render_template('not-flag.html', cookie=data)

class AESCipher:
    """
    Usage:
        c = AESCipher('password').encrypt('message')
        m = AESCipher('password').decrypt(c)
    Tested under Python 3 and PyCrypto 2.6.1.
    """

    def __init__(self, key):
        self.key = md5(key.encode('utf8')).hexdigest()

    def encrypt(self, raw):
        raw = pad(raw)
        iv = Random.new().read(AES.block_size)
        cipher = AES.new(self.key, AES.MODE_CBC, iv)
        return b64encode(iv + cipher.encrypt(raw))

    def decrypt(self, enc):
        enc = b64decode(enc)
        iv = enc[:16]
        cipher = AES.new(self.key, AES.MODE_CBC, iv)
        return unpad(cipher.decrypt(enc[16:])).decode('utf8')

if __name__ == "__main__":
    app.run()

Any help of explanation would be highly appreciated!!!