easy_f(x)_sever

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来源：2020 RCTF

Quick Start

题目如下：

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sha256(XXXX+Kn3wfqcrtz0IsfpD) == 
c89df26a64ea25b728268d6a5118c277895a91bf699e3f134b18b4db89887c8a
Give me XXXX:

提供文件代码如下：

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import os,random,sys,string
from hashlib import sha256
import SocketServer
import signal
from Crypto.Util.number import *
from flag import FLAG

Nbits = 768
Pbits = 512

    
class Task(SocketServer.BaseRequestHandler):
    def proof_of_work(self):
        random.seed(os.urandom(8))
        proof = ''.join([random.choice(string.ascii_letters+string.digits) for _ in xrange(20)])
        digest = sha256(proof).hexdigest()
        self.request.send("sha256(XXXX+%s) == %s\n" % (proof[4:],digest))
        self.request.send('Give me XXXX:')
        x = self.request.recv(10)
        x = x.strip()
        if len(x) != 4 or sha256(x+proof[4:]).hexdigest() != digest: 
            return False
        return True
    def recvnum(self, sz):
        try:
            r = sz
            res =""
            while r>0:
                res += self.request.recv(r)
                if res.endswith('\n'):
                    r = 0
                else:
                    r = sz - len(res)
            res = res.strip()
            t = int(res)
        except:
            res = ''
            t = 0
        return t


    def dosend(self, msg):
        try:
            self.request.sendall(msg + '\n')
        except:
            pass
            
    def f(self, k, m):
        r = 0
        x = getRandomInteger(Nbits)
        for i in range(len(k)):
            r = (r + k[i] * pow (x, i, m)) % m
        self.dosend("f(%d)=%d" % (x, r))
        return x, r

    def handle(self):
        signal.alarm(300)
        if not self.proof_of_work():
            return
        signal.alarm(300)
        M = getPrime(Nbits)
        self.dosend("M=%d" % M)
        check = getRandomInteger(Nbits) % M
        K = [check]
        for i in range(Pbits):
            K = K + [getRandomInteger(Nbits)]
        
        self.dosend("How many f(x) do you want?")
        num = self.recvnum(1024)
        if num > 1024:
            self.dosend("Nonono")
        else :
            for i in range(num):
                self.f(K, M)
            answer = self.recvnum(1024)
            if answer == check:
                self.dosend("%s" % FLAG)
            else:
                self.dosend("Nonono")
        self.request.close()

class ForkingServer(SocketServer.ForkingTCPServer, SocketServer.TCPServer):
    pass

if __name__ == "__main__":
    HOST, PORT = '0.0.0.0', 2333
    print (HOST)
    print (PORT)
    server = ForkingServer((HOST, PORT), Task)
    server.allow_reuse_address = True
    server.serve_forever()