Hackvent 2019: Hidden 4

Challenge

HV19.H4 Hidden Four

Solution

During the Day 14 challenge HV19.14 Achtung das Flag, our final flag looks quite interesting:

HV19{s@@jSfx4gPcvtiwxPCagrtQ@,y^p-za-oPQ^a-z\x20\n^&&s[(.)(..)][\2\1]g;s%4(...)%"p$1t"%ee}

The hints in the __DATA__ segment point us in the right direction:

Only perl can parse Perl!
Run me in Perl!

So we evaluate the string as Perl code like so:

# Hackvent 2019 - Hidden 4
# Mo Beigi (https://mobeigi.com)

eval 's@@jSfx4gPcvtiwxPCagrtQ@,y^p-za-oPQ^a-z\x20\n^&&s[(.)(..)][\2\1]g;s%4(...)%"p$1t"%ee';

This prints out the content of our hidden flag: Squ4ring the Circle

Flag:

HV19{Squ4ring the Circle}