@wapiflapi's solution for RektSA from https://qual.rtfm.re 2019

decrypt.py

"""
@wapiflapi's solution for RektSA from https://qual.rtfm.re 2019

This simply uses z3 which takes about a second longer than the challenge
timeout: Keeping the connection alive is enough to circumvent the timeout.

~/Projects/ctf/rtfm$ time ./decrypt.py
Congratulations: sigsegv{th1s_1s_4_cr1m3...4_mult1cr1m3!!!}

real    0m2,741s
user    0m3,340s
sys     0m0,417s

"""

import socket
import multiprocessing
import multiprocessing.pool

import gmpy2
import z3


def solve(e, N, r, phimod):
    """Solve equations copied directly from the challenge."""
    s, p, q, phi = z3.Solver(), z3.Int("p"), z3.Int("q"), z3.Int("phi")
    s.add(
        (p > 2**1023), (p < 2**1024),
        (q > 2**1023), (q < 2**1024),
        (p * q * r == N),
        (phimod == phi % 2**2050),
        (phi == (p - 1) * (q - 1) * (r - 1)),
    )
    s.check()
    return gmpy2.invert(e, s.model()[phi].as_long())


def main():
    """Keep-alive the connection while the result is computed."""
    s = socket.socket()
    s.connect(("qual-challs.rtfm.re", 9001))

    lines = s.recv(4096).decode("utf8").split("\n")
    defs = ([e.strip() for e in l.split(":")] for l in lines)
    vmap = {l[0]: int(l[1]) for l in defs if len(l) == 2 and l[1]}

    async_result = multiprocessing.pool.ThreadPool(
        processes=1
    ).apply_async(solve, args=(
        0x10001, vmap["N"], vmap["r"], vmap["phi"],
    ))

    while True:
        try:
            s.send((b"%d\r\n" % async_result.get(timeout=0.5)))
        except multiprocessing.context.TimeoutError:
            # Keep the connection alive so we do not timeout.
            s.send(b"0")  # '0' as a prefix is valid for int() parser.
        else:
            break

    print(s.recv(4096).decode("utf8"))


if __name__ == "__main__":
    main()

FTR, the original challenge by @SIben:

#!/usr/bin/python2.7

import sys
from secret import FLAG
from Crypto.Util.number import getPrime, bytes_to_long
from gmpy2 import invert

p = getPrime(1024)
q = getPrime(1024)
r = getPrime(1028)

e = 0x10001

N = p * q * r
phi = (p - 1) * (q - 1) * (r - 1)

print 'N:', N
print 'phi:', phi % 2**2050
print 'r:', r

print 'Give me d: '

sys.stdout.flush()

try:
    d = int(raw_input())
except:
    print 'Invalid input!'
    exit(1)

if d == invert(e, phi):
    print 'Congratulations: %s' % FLAG
else:
    print 'Nope!'
    exit(1)