Block cipher challenge

Level: CTR mode

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Ciphertext (hexencoded), for easy copy-paste

31899cf9ab0443895d89290c79db85f2399a7c805b58dd70b1c39091a965bef11fa791233ff28d22c449d9dca677d8233683393fb38f5f0b5a8065dfd030a185

Ciphertext (hexencoded):

31899cf9ab0443895d89290c79db85f2 399a7c805b58dd70b1c39091a965bef1 1fa791233ff28d22c449d9dca677d823 3683393fb38f5f0b5a8065dfd030a185

Plaintext (hexencoded):

31899cf9ab0443895d89290c79db85f2 7b22757365726e616d65223a20226775 657374222c202269735f677565737422 3a20747275657d090909090909090909

Plaintext:

1\x89\x9c\xf9\xab\x04C\x89]\x89)\x0cy\xdb\x85\xf2 {"username": "gu est", "is_guest" : true}\t\t\t\t\t\t\t\t\t

Final plaintext

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