Block cipher challenge

Level: CTR mode

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Ciphertext (hexencoded), for easy copy-paste

b543f8245a9b9510716745a1eadf29098c1928198c22a0f6fc8c9a61c3c10da2fa565e97f75fbe8eeef24e97d7bb6d7593b776ba8ab4468f16caee96f6b5386a

Ciphertext (hexencoded):

b543f8245a9b9510716745a1eadf2909 8c1928198c22a0f6fc8c9a61c3c10da2 fa565e97f75fbe8eeef24e97d7bb6d75 93b776ba8ab4468f16caee96f6b5386a

Plaintext (hexencoded):

b543f8245a9b9510716745a1eadf2909 7b22757365726e616d65223a20226775 657374222c202269735f677565737422 3a20747275657d090909090909090909

Plaintext:

\xb5C\xf8$Z\x9b\x95\x10qgE\xa1\xea\xdf)\t {"username": "gu est", "is_guest" : true}\t\t\t\t\t\t\t\t\t

Final plaintext

{"username": "guest", "is_guest": true}