Mr-Worldwide picoCTF 2019 Solution

Description

1. Step 1Read the coordinate list
   Open the file. It contains a list of GPS coordinates (latitude, longitude pairs). Each coordinate points to a specific location on Earth.
   bash

   cat message.txt

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   GPS coordinates use the WGS84 datum. Latitude ranges from -90 (South Pole) to +90 (North Pole). Longitude ranges from -180 to +180, with 0 at the Prime Meridian through Greenwich, England. Positive longitude is East, negative is West.

2. Step 2Geocode each coordinate to a city name
   For each coordinate pair, use Google Maps (paste coordinates into the search bar) or a geocoding API to find the nearest city. Note the first letter of each city name.
   python

   python3 << 'EOF'
   # Using geopy for reverse geocoding
   from geopy.geocoders import Nominatim
   geolocator = Nominatim(user_agent="picoctf")
   
   coords = [
       # (lat, lon),
       # paste coordinates here
   ]
   
   for lat, lon in coords:
       location = geolocator.reverse(f"{lat}, {lon}")
       print(location.address)
   EOF

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   Reverse geocoding converts coordinates to a human-readable address. The Nominatim service (powered by OpenStreetMap) is free for low-volume use. Google Maps Geocoding API is more accurate but requires an API key.

   For this challenge, the important part is the city name, not the full address. Focus on the major city closest to each coordinate.

3. Step 3Take first letters to spell the flag
   Collect the first letter of each city name in order. These letters form the flag content. Wrap the result in picoCTF{...}.
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   This encoding technique is called an acrostic - using the first letter of each word or item to spell a hidden message. It has been used in literature, art, and espionage for centuries.

Flag

Geocode each coordinate to a city and take the first letter of each city name - they spell out the flag.