finxol.io
1---
2title: 404CTF Write-Up À l'aube d'un échange
3description: Write-up for the OSINT challenge "À l'aube d'un échange" @ 404CTF 2022
4image: /blog/404ctf.png
5date: 2022-06-08
6authors:
7 - name: finxol
8tags:
9 - writeup
10 - 404ctf
11published: true
12---
13
14## 404CTF
15
16The [404CTF](https://404ctf.fr) is a CTF organized by the Direction Générale de la Sécurité Extérieure (DGSE), Télécom SudParis and
17its association Hackademint.
18This 2022 edition marked the double anniversary of "the 80th anniversary of the BCRA, the secret service of the Free France and
19the 40th anniversary of its heir, the DGSE".
20
21### Description
22
23*This is a translation of the original description in French.*
24
25New recruit! We need you around here.
26One of our agents has just intercepted a short telephone conversation between two Hallebarde agents.
27An important exchange of confidential documents is to take place and to indicate the location of the meeting,
28one of the enemy agents has sent the following picture to his colleague with the following message:
29
30<blockquote
31 style="border-left: 4px solid #e0e0e0; padding: 0 0 0 1rem;border-radius: 0.1rem; margin: 1rem 2rem;line-height: 1.5rem"
32>
33 What a beautiful sunrise, isn't it? I'll be waiting in the street between the building in the foreground and those in the background.
34 See you tonight, 10pm.
35</blockquote>
36
37We have less than a day to find out the name of the street and prevent the exchange!
38
39Flag format: 404CTF{md5 of the full street name}.<br>
40The street name must be in lower case, include the type of street (e.g. avenue, street, boulevard...),
41without accents, without abbreviations, and all spaces must be replaced by dashes.
42For example: if the street is Avenue de Saint-Mandé in Paris, the correct flag is `404CTF{129af9edde5659143536427f9a5f659a}`.
43
44Author : **Artamis**
45
46
47
48
49## Solution
50
51Before starting this OSINT investigation, lets analyse the image provided.
52
53We can immediately check that there is no useful exif data.
54According to the instructions, it is a rising sun, so we can assume that the picture was taken facing east.
55Three prominent buildings can also be clearly seen in the background.
56
57Before going further, we will assume that this is a French city and look for a list of the tallest buildings in France.
58
59We then come across the Wikipedia page on [France's tallest skyscrapers](https://fr.wikipedia.org/wiki/Liste_des_plus_hauts_gratte-ciel_de_France).
60Looking at the images associated with the towers, we notice that the third one looks remarkably similar to the one in our photo.
61
62
63
64By simply reading the Wikipedia description of the Tour Incity, we find a link to the Part-Dieu district page.
65Fortunately for us, the viewpoint of the description image is very similar to the one of our photo.
66The description also states that this photo was taken "from Fourvière".
67
68
69
70We can now start to search with [Google Earth](https://earth.google.com/web/@45.7589869,4.82472116,199.71703945a,1402.6169008d,35y,9.91010942h,69.22260348t,0r) for places around Fourvière or further west,
71probably higher than the rest of the city.
72We can then see that the Fourvière district is located on a hill.
73
74
75
76Exploring the surroundings, we soon find a building on the East side of the hill which looks like the one in the background of our photo.
77
78
79
80The street below this building is called the *Montée Saint Barthélémy*.
81We can then format and hash this street name with `echo -n "montee-saint-barthelemy" | md5sum`,
82which gives us the flag `404CTF{eb66c65861da9fe667f26667b3427d2c}`.