HACKvent 2015: Day 6

Challenge

We are given the following string:

HR7DYQ3ON4TC6U2AFAZDGJK3J44TYXZUNRCTATK2GEZDGJR5JJUC6ULUFQZEI5L6HY======

Solution

We can deduce by the amount of padding that this is Base32 encoded string. We use an online decoder and get:

<~<Cno&/S@(23%[O9<_4lE0MZ123&=Jh/Qt,2Du~>

Then we try various other things to decode the above string. I will admit I failed to find a proper solution until I received some help from SlEePlEs5 on the hacking-lab IRC. Then I was able to realise that I had to decode the above string using ASCII85. I get:

UI15-g9C8-DnVI-W0Ne-83S3-Z8Qp

Great! This looks a lot like our nugget. I guess a simple Caesar cipher is required here and find the offset number to be 13 (as we want U->H for first character in our nugget). We apply the cipher (basically ROT13) and get our flag!

Flag:

HV15-t9P8-QaIV-J0Ar-83F3-M8Dc