from flask import Flask, make_response, request
from Crypto.Util.number import *
from Crypto.Cipher import ChaCha20_Poly1305
import secrets
import requests
import hashlib
CHACHA_KEY = secrets.token_bytes(32)
app = Flask(__name__)

encrypted_text = []

@app.route('/api', methods=['POST'])
def api():
    try:
        pkt = request.headers.get('pkt')
        msg = bytes.fromhex(pkt)
        nonce = b"random_text_Lorem_ipsum_sim_ahmet_next_ales"
        nonce = hashlib.sha256(msg[:32] + nonce[:32]).digest()[:12]
        cipher = ChaCha20_Poly1305.new(key=CHACHA_KEY, nonce=nonce)
        ct, tag = cipher.encrypt_and_digest(msg)
        ctk = ct + tag + nonce
        
        encrypted_text.append(ctk.hex())
        return make_response(ctk.hex())
    except:
        return make_response("Error")

@app.route('/resource', methods=['POST'])
def send_resource(): 
    try:
        pkt = request.headers.get('pkt')
        ip = request.headers.get('ip')
        if pkt in encrypted_text:
            return make_response("You already try that!")

        msg = bytes.fromhex(pkt)
        ct = msg[:-28]
        tag = msg[-28:-12]
        nonce = msg[-12:]
        cipher = ChaCha20_Poly1305.new(key=CHACHA_KEY, nonce=nonce)
        try:
            pt = cipher.decrypt_and_verify(ct, tag)
        except (ValueError, KeyError):
            return make_response("Decryption failed")

        if pt:
            try:
                url = "http://" + ip
                resource = open("resource.txt","r").read().strip()
                requests.post(url, data=resource, timeout=1)
                encrypted_text.add(pkt)
                return make_response('Success')
            except Exception as e:
                return make_response("Error sending resource")
    except Exception as e:
        return make_response("Error")
    
if __name__ == '__main__':
    app.run(debug=False, host='0.0.0.0', port=5002)

So this is a server where is a ChaCha20_Poly1305 algorithm,
If i use P1 = [64Bytes + ‘A’] in hexa and P2=[64Bytes + ‘C’] to encrypt datas I get C1 =ciphertext and C2=ciphertext

For the server to give me the resource i need to provide it a ciphertext forged by me without being added on server in encrypted_text list.

I figured out that if I do (C1⊕C2) = I will get zeros with a ‘2’ + tag(which i need to sign datas from ciphertext) + nonce which will be reused cause i know to do it so

How do I find the tag from this problem? it is from poly1305 algorithm that needs CHACHA_KEY