padding + input + message

import socket
from base64 import *
from hashlib import *
import string
def readuntil(conn, e):
    buf = bytes()
    while not buf.decode().endswith(e):
        buf += conn.recv(1)
    return buf
def payload(s):
    if len(s) < 43:
        p = socket.socket()
        p.connect(('localhost',33337))
        readuntil(p,': ')
        p.send( '\n')
        readuntil(p,': ')
        p.send( s + '\n')
        message = p.recv(1024).split(': ')[1].strip()
        p.close()
        return b64decode(message)
    else:
    p = socket.socket()
        p.connect(('localhost',33337))
        readuntil(p,': ')
        p.send( s[:43] +'\n')
        readuntil(p,': ')
        p.send( s[43:] + '\n')
        message = p.recv(1024).split(': ')[1].strip()
        p.close()
        return b64decode(message)
flag = ''
for k in range(0,5):
    for i in range(0,16):
        n = 21 - i
        real = n*'a'
        plain = payload(real)[0:32 + 16*k]
        for char in string.printable:
        guess = payload(n*'a' + flag + char)[0:32 + 16*k]
        if ( guess == plain ):
            flag+= char
            print flag
            break
print flag