$ unzip J.zip
$ binwalk J
DECIMAL HEXADECIMAL DESCRIPTION
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3912330 0x3BB28A ARJ archive data, header size: 22472, version 1, minimum version to extract: 1, compression method: stored, file type: binary, original name: "1", original file date: 1970-01-01 00:00:00, compressed file size: 538968064, uncompressed file size: 1441792, os: MS-DOS
Since MS-DOS stores data in 16bits little endian, append -el to strings for searching, let's try some keywords:
$ strings -el J | rg "SEC"
$ strings -el J | rg "CON"
And will find something like,
<CON{.txt
Then try
$ strings -el J | rg ".txt"
...
<SEC.txt
<SEC.txt
<SEC.txt
<SEC.txt
<CON{.txt
<CON{.txt
<CON{.txt
<F0r.txt
<F0r.txt
<tktksec.txt
<tktksec.txt
<tktksec.txt
<F0r.txt
<ensic.txt
<ensic.txt
<ensic.txt
<s.txt
<s.txt
<s.txt
<_usnjrnl.txt
<_usnjrnl.txt
<_usnjrnl.txt
<2018}.txt
<2018}.txt
The flag is SECCON{F0rensics_usnjrnl2018}