Code60.md

Internetwache 2016 : It's Prime Time! (60)

Category: code | Points: 60 | Name: It's Prime Time! | Solves: 388 | Description:

We all know that prime numbers are quite important in cryptography. Can you help me to find some?

Service: 188.166.133.53:11059

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Write-up

Part Zero

We were given a service which we connect using python sockets.

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('188.166.133.53',11059))
data = s.recv(2048)
print data

And we get the first part

Hi, you know that prime numbers are important, don't you? Help me calculating the next prime!
Level 1.: Find the next prime number after 8:

Part One

The quesion is split by spaces, so it seemed easiest to use .split on the string to convert it to an array.

data = s.recv(2048)
question = data.split(' ')
print question

Part Two

Using Bertrand's postulate for finding the next prime number, it solves easily

x = int(question[8][:1])
y = 2
for x in range(int(question[8][:-2]) + 1, 2 * int(question[8][:-2])):
    if x % 2 != 0:
        for y in range(2, x / 2 + 1):
            if x % y == 0:
                break
        if x % y == 0:
            continue
        else:
            print "Sending " + str(x)
            s.send(str(x) + "\n")
            break

We get the flag after 100 solves we get the flag:

IW{Pr1m3s_4r3_!mp0rt4nt}

See full script here