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Keysar sounds like caesar cipher, but caesar ciphers dont have keys? Turns out it was encrypted using the Keyed caesar cipher

Plugging everything into http://rumkin.com/tools/cipher/caesar-keyed.php we will get the flag.

Original writeup (https://github.com/darkvoid32/CTF-writeups/tree/master/%C3%A5ngstromCTF%202020/Keysar).