(Challenge 3) Hidden

We get an image:

If we inspect the image metadata using any number of online tools, or the command-line exifdata, we can find some interesting-looking metadata.

$ exiftool final.png
ExifTool Version Number         : 11.70
File Name                       : final.png
Directory                       : .
File Size                       : 5.0 kB
...
Modify Date                     : 2019:08:17 11:50:12
Comment                         : rot(QCV{9J5Y_7q3a3_jL7DjuuH_15})
Image Size                      : 200x200
Megapixels                      : 0.040

That QCV{9J5Y_7q3a3_jL7DjuuH_15} looks almost like a flag, except the flag prefix looks wrong. The rot part in front implies that we should try applying a ROT operation (a.k.a, a Caesar Cipher) to it. After brute-forcing all the combinations, we can find the version that starts with HTM{, from applying ROT17.

Then we can easily just read off the flag as HTM{9A5P_7h3r3_aC7UallY_15}.