Using the file command i can see that this is an Android Backup file, not encrypted

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┌─[phire@parrot]─[~/Sync/Docs/Hacking/htb/Challenges/Cat]
└──╼ $file cat.ab
cat.ab: Android Backup, version 5, Compressed, Not-Encrypted

A quick google search shows that there is a linux command to extract this file

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( printf "\x1f\x8b\x08\x00\x00\x00\x00\x00" ; tail -c +25 cat.ab ) |  tar xfvz -

After looking through the files i take a look at the pictures folder. One of the images has the flag written on a document, after zooming in.