I've received a letter... Uh, Mr. Smith?
By @afcidk
From the provided image, we can guess that this challenge is related to Adam7, which is an interlacing scheme for PNG images. Adam7 has seven passes, so I decide to generate those seven subimages first, and see if there is any clues.
I wrote a simple script to transform the original sandstorm.png to seven images using Adam7 algorithm.
Level 1:
Level 2:
Level 3:
Level 4:
Level 5:
Level 6:
Level 7:
The flag is encoded to QR code in Level-1 subimage, SECCON{p0nlMpzlCQ5AHol6}.