Hidden in Plain Sight (Forensics)
tl;dr read the file before it's been decrypted
In this task, we're only given a mega download link.(https://mega.nz/#!6N0gmRLK!VsN9gLdiYbxMVTA-AdsRjsvezMpdEqiR9ngwrS6gR7k)
The hint is: you need to know the basic principle behind how MEGA works in order to solve this challenge. Flag.txt is the flag, but it is hidden... in plain sight ;)
Mega doesn't store keys to decrypt your data, you are given them with the url, in our case it's AdsRjsvezMpdEqiR9ngwrS6gR7k. The files are download to a sandbox using FileSystem API.
After the download is complete, the file is then decrypted using aes and passed to the normal download folder.
The downloaded flag looks like gibberish: }Żv#ĘÖ›{QřČxJ�ZzŃ\��M÷2Ž¸�IĆ&N˛<z´ŕ´„ĽĹ*‹�—�ýk�/Ăõ
Let's then try viewing the flag before it's been decrypted by using Chrome developer tools.
Bingo!