Summary

Author: @JohnHammond

Maybe you already know about base64, but what if we took it up a notch?

Steps

To start this challenge I ran file against the basefff1 file and see that its is baseffff1: Unicode text, UTF-8 text, with no line terminators. Next, previewing the file I see these characters:

After many failed decoding attempts, I went back to the challenge description and started to work on what FFFF+1 means. Eventually, I found that FFFF is a hex value for 65535. Here is the recipe in CyberChef

Next, FFFF+1 or 65535+1 would be 65536. Looking back at the challenge name, it would be renamed to Base65536. Looking online I found CyberChef doesn’t have this option yet, but I was able to locate Base65536 Encoder/Decoder.

Using this tool, I was able to decode the flag: flag{716abce880f09b7cdc7938eddf273648}