Two Tickets, One Number

Helpdesk workers call /usr/local/bin/ticket-next to get a ticket number. It reads the counter, adds one and writes it back, with nothing stopping two workers doing that at the same moment. During the morning rush the same number went out twice, more than once. Make the script safe, find every number that was issued twice, and use that list to unlock the flag.

The contract, ticket-next FILE:

• FILE holds one whole number N. Print N+1 on stdout and leave N+1 in FILE.
• Safe under concurrency: K copies started at the same moment on the same FILE print K different numbers, N+1 to N+K, and leave N+K.
• With the wrong number of arguments, a FILE that does not exist (do not create it), or a FILE that does not hold a whole number, exit non-zero and leave FILE unchanged.

Grading runs your script, including 30 copies at once on a fresh counter. Then:

1. In /var/lib/tickets/issued.log, find every ticket number that was issued (action=issue) more than once. A re-open (action=reopen) mentions an old ticket and is not a second issue. Write the numbers to /root/case/duplicates.txt, one per line.

2. /root/case/flag.enc was encrypted with openssl enc -aes-256-cbc -pbkdf2 -salt -pass pass:<KEY>, where KEY is those numbers sorted ascending and joined with commas (like 50012,50077,50310). Decrypt it and save the flag to /root/case/flag.txt.

See /root/case/brief.txt.