LHC was a short & sweet challenge in Backdoor CTF. It had a nice ‘aha-Erlebnis’ moment.
During the CTF, the organizers dropped this challenge. The description mentioned that the flag was hidden in a data file. This data file was kindly provided by the Large Hadron Collider and was 2049 GB large.
I’ll let that sink in.
Two-thousand and forty nine gigabytes. Downloading it would take more than twenty days. The flag was in the middle of the file, but that still meant downloading more than a terabyte of data.
I fired up
curl and looked at the download:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Yup, 2200000000000 bytes of data. But wait!
curl has the option to resume a broken download; that meant that I could control where the download would start. I issued this command and started searching the output for the flag:
1 2 3
This gave me part of the flag; the description said it was in the middle of the datafile, so I subtracted another 1000 bytes:
1 2 3
Simple, really, once you know the trick.