How Dropbox Knows When You’re Sharing Copyrighted Stuff?
Hashing.
Before i dive in:
‘A’ is a criminal, he wants to rob a bank so he asked his friends ‘B’ and ‘C’ for help. They robbed the bank successfully but unfortunately ‘B’ had forgotten his gloves at home before the crime. The police officer got his fingerprint and after a few days from fingerprint matching, the police catch’em all. Same thing for files, every tiny file has its unique fingerprint from characters and numbers like “ah4213jn234bmn53b” and even if you change the location or name or things like that the fingerprint not change at all - if you wanna see install this https://support.microsoft.com/en-us/help/841290/availability-and-description-of-the-file-checksum-integrity-verifier-utility and have little fun - The dropbox has a big table with the fingerprints of the origin files and when you upload a file, a hash is generated and if its fingerprint exist the site blocked the uploaded one.
It’s the same idea to store your password and username.