27p

C47360ry: Cryp706r4phy

P01n75: 144

D35cr1p710n: V3n3z 73573r n07r3 ch1ffr3ur un1v3r53l ! N0u5 u71l150n5 d35 73chn0l06135 d3 p01n73, 64r4n7135 1nv10l48l35 ! P0ur pr3uv3, n0u5 v0u5 d0nn0n5 l3 fl46 ch1ffré 37 j4m415 v0u5 n3 p0urr3z l3 r37r0uv3r. nc ch4ll3n635.3c5c-734mfr4nc3.fr 2000

F1l35: N0n3

TL;DR

A 53rv3r 61v35 u5 7h3 fl46 3ncryp73d w17h AES-GSM 4nd 4ll0w5 u5 70 53nd 17 4 57r1n6 7h47 17 r37urn5 70 u5 3ncryp73d. S3nd1n6 4 null 8y73 4ll0w5 u5 70 x0r 7h3 r37urn3d k3y w17h 7h3 3ncryp73d fl46 70 r3c0v3r 17.

M37h0d0l06y

A f1r57 c0nn3c710n 70 7h3 53rv3r 61v35 u5 7h3 fl46 3ncryp73d w17h AES-GSM 4nd 45k5 u5 70 53nd 17 4 57r1n6 70 3ncryp7 17.

>_ nc challenges.ecsc-teamfrance.fr 2000

Welcome to our state-of-the-art encryption service!
We use PBKDF2 and AES-GCM!
As an example, here is the encrypted flag: 7b656d3993152e8f04f8273ca1509e27a3e39249cf4784e23b81d5f2524fee75f6b28a6a07a128e4880e770bc70b32bd7d5f37bb5eba76d38edb8d1964733b
Now, enter your text: 

Ou7 0f cur10517y w3 53nd 4 57r1n6 w17h 7h3 fl46 f0rm47 ECSC{.

Now, enter your text: ECSC{givemetheflag

Here is your ciphertext: 7b656d399316709c51ad727af0079c72f7e2f5849a0dc411487c8845081b3fb04601

W3 n071c3 7h47 7h3 f1r57 7 ch4r5 0f 7h3 c1ph3r73x7 r37urn3d 4r3 7h3 54m3 45 7h3 3ncryp73d fl46 4nd wh3n w3 53nd 53v3r4l 71m35 7h3 54m3 57r1n6 w3 637 7h3 54m3 r35ul7.

In73r3571n6 !

I7 15 455um3d 7h47 7h3 k3y u53d f0r 3ncryp710n 15 4lw4y5 7h3 54m3.

I7 15 p05518l3 70 f1nd 17 8y 53nd1n6 4 null 8y73 70 m4k3 4 x0r 0f 7h3 k3y 0n 7h3 fl46.

W3 53nd null 8y735 50 7h47 7h3y 4r3 l0n63r 7h4n 7h3 fl46:

>_ cat get_key.py

#!/usr/bin/env python3

from pwn import *

HOST = 'challenges.ecsc-teamfrance.fr'
PORT = 2000

m = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"

r = remote(HOST, PORT)

rep = r.recvuntil('Now, enter your text: ').decode('UTF-8')
print(rep)

rep = m
print(rep)

r.send(rep+"\n")
rep = r.recv().decode("UTF-8")
print(rep)

r.close()

>_ ./get_key.py
[+] Opening connection to challenges.ecsc-teamfrance.fr on port 2000: Done
Welcome to our state-of-the-art encryption service!
We use PBKDF2 and AES-GCM!
As an example, here is the encrypted flag: 7b656d3993152e8f04f8273ca1509e27a3e39249cf4784e23b81d5f2524fee75f6b28a6a07a128e4880e770bc70b32bd7d5f37bb5eba76d38edb8d1964733b

Now, enter your text: 
\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00

Here is your ciphertext: 3e263e7ae87119ea34c0170e9862fa1e9685a37afe22b4d509b5e4936b778a16c0d1bb0b669018d6bf374f33fe763835b874bfe06b31923fd2a58905290add80aaed75d00a519f33037203e1801cb26f8fd01b427f566d14a75e344320f6f42355b18c6435c829c0

[*] Closed connection to challenges.ecsc-teamfrance.fr port 2000

N0w 7h47 w3 h4v3 7h3 k3y, 4ll w3 h4v3 70 d0 15 m4k3 4 x0r.

>_ cat xor.py

#!/usr/bin/env python3
import binascii

enc_flag = binascii.unhexlify("7b656d3993152e8f04f8273ca1509e27a3e39249cf4784e23b81d5f2524fee75f6b28a6a07a128e4880e770bc70b32bd7d5f37bb5eba76d38edb8d1964733b")
key = binascii.unhexlify("3e263e7ae87119ea34c0170e9862fa1e9685a37afe22b4d509b5e4936b778a16c0d1bb0b669018d6bf374f33fe763835b874bfe06b31923fd2a58905290add80aaed75d00a519f33037203e1801cb26f8fd01b427f566d14a75e344320f6f42355b18c6435c829c0")

#Xor func
flag = bytearray(len(enc_flag))
for i in range(len(enc_flag)):
    flag[i] = enc_flag[i] ^ key[i]

print (flag.decode("ISO-8859-1"))

>_ ./xor.py
ECSC{d7e080292d95f131e07241a98dc6c1aa10279889}
Å+[5äì\\~Myæ

FLAG_IS:

ECSC{d73080292d95f131307241498dc6c14410279889}