app.py

from flask import Flask, request, render_template
from Crypto.Cipher import AES
from Crypto.Util import Counter
import os

FLAG = os.environ.get("FLAG") or "PLEASE_SET_A_FLAG"
KEY = os.environ.get("KEY") or os.urandom(32)

app = Flask(__name__)

class Encryptor(object):
    def __init__(self, key):
        self.key = key

    def encrypt(self, plaintext):
        nonce = os.urandom(15)
        self._reset(nonce)
        key = AES.new(self.key, AES.MODE_CTR, counter=self.counter)
        return nonce.hex() + key.encrypt(plaintext.encode()).hex()

    def decrypt(self, ciphertext):
        ciphertext = bytes.fromhex(ciphertext)
        self._reset(ciphertext[:15])
        key = AES.new(self.key, AES.MODE_CTR, counter=self.counter)
        return key.decrypt(ciphertext[15:]).decode()

    def _reset(self, nonce):
        self.counter = Counter.new(nbits=8, prefix=nonce, initial_value=0, little_endian=False, allow_wraparound=True)

encryptor = Encryptor(KEY)

@app.route('/encrypt', methods=['POST'])
def encrypt():
    plaintext = request.form.get('plaintext')

    if not plaintext or len(plaintext) == 0:
        return "Something wents wrong"

    return encryptor.encrypt(FLAG + plaintext)

@app.route('/decrypt', methods=['POST'])
def decrypt():
    ciphertext = request.form.get('ciphertext')

    plaintext = encryptor.decrypt(ciphertext)
    if not plaintext.startswith(FLAG):
        return "Something wents wrong"
    else:
        return plaintext[len(FLAG):]

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

if __name__ == '__main__':
    app.run(debug=True, port=5000)