Block cipher challenge

Level: CTR mode

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Ciphertext (hexencoded), for easy copy-paste

7a15dcafebcfdbdb7e6d16fc6c2088c5326c008f95e56252899d6d31eb1c3be1bb9512079b2f7d2916cee8c30f76f2e97f763b02b38e429fbba672d9ce105c55

Ciphertext (hexencoded):

7a15dcafebcfdbdb7e6d16fc6c2088c5 326c008f95e56252899d6d31eb1c3be1 bb9512079b2f7d2916cee8c30f76f2e9 7f763b02b38e429fbba672d9ce105c55

Plaintext (hexencoded):

7a15dcafebcfdbdb7e6d16fc6c2088c5 7b22757365726e616d65223a20226775 657374222c202269735f677565737422 3a20747275657d090909090909090909

Plaintext:

z\x15\xdc\xaf\xeb\xcf\xdb\xdb~m\x16\xfcl \x88\xc5 {"username": "gu est", "is_guest" : true}\t\t\t\t\t\t\t\t\t

Final plaintext

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