UIUCTF 2023 - First Class Mail
First class mail
First Class Mail is an easy challenge for the UIUCTF 2023 challenge. It’s an OSINT challenge that I think has an interesting solution. There were several OSINT challenges, this one I learned something new.
The only file provided is a photo, and the challenge is to obtain the zipcode of where this photo was taken.
Downloading the photo and reviewing the photo doesn’t show anything too obvious. No specific enough regional products or buildings. The meta data does show latitude/longitude, but as the challenge says, that’s not the correct place.
In the photo, among the various products is a letter’s bottom right code. It took some googling, but this is the POSTNET code. This code does translate to the zip code.
On the wiki there is a guide on how to decode these. https://en.wikipedia.org/wiki/POSTNET . You can also use the dcode.fr site to decode this for you using 1s and 0s for tall and short lines. https://www.dcode.fr/barcode-postnet
Simple and straight forward challenge. Looked at this photo and tried multiple ways before discovering postnet codes. Overall, fun challenge, that if you knew envelope layouts, is a quick 50 points. If you didn’t, fun thing to learn that I’m sure other CTFs may use as well.