Tags: steganography 

Rating: 5.0

This challenge gives us a png file which shows a coin with a man inside.

#### Quick solution:
zsteg -a coolcoin.png | grep -Poi UMDCTF-{.*?}

#### More advances solution:
using [https://tineye.com/](http://), to search what this picture may present, the results lead us to the [wikipedia page](https://en.wikipedia.org/wiki/Eratosthenes) of Eratosthenes, who was a greek mathematician.

after searching in google his name together with steganograpy, we are being led to python [stegano](https://pypi.org/project/stegano/), which has an option to hide and **reveal** messages from png files with the **Sieve of Eratosthenes**.

using the tool on the picture:
$ stegano-lsb-set reveal -i coolcoin.png --generator eratosthenes

and we recieve the flag!