The link leads to a site with info about time-based side-channel analysis and a password box:
The challenge is straightforward: Find the password using calculation speed from the server.
0000000000 we got the message
Password verification failed. Processing time 275 microseconds. We kept changing the first number until we got a spike, so at
8000000000 we got
Password verification failed. Processing time 499 microseconds.
We continued this method until we got
8105237467 and the following new box:
We could have written code to do this, but why bother? 😜
The password was checked digit by digit. That means that the moment it sees a charactar that shouldn't be there it returns an error. Using this we knew to look for a longer processing time each try and finally get the flag: