I need help with "I know Mag1k"

So, i’ve been stuck for some time in this challenge and although i think i know what the vulnerability is, i don’t know how to exploit it
Basically while doing reconnaissance of how the site works, i stumbled with the cookie “iknowmag1k”, since it changes everytime i do log in on the website and it was encrypted with base64(which means it could be decoded) i guessed that what defines which user is logged in is inside this cookie, so my strategy was “deocde it, change it to something admin related, encode it and reload the page”, but then when i started doing that the result of the decoded string was a seemingly random string.
My guess is that the resulting decoded string is encrypted with something else, but my knowledge with cryptography is not the best (and i don’t really know how to search for it on the web) , can someone point me in the right direction or give some tips on how to advance with the challenge
(If the question is too much of a spoiler, tell me that i’ll remove it, but please DM with what i asked for so i can move on with my life)