Rating:

# 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

___

## 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](src/code60.py)

Original writeup (https://github.com/WesternCyber/CTF-WriteUp/blob/master/2016/Internetwache/Code/Code60.md).