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I tried this for some time, I was looking for unlisted videos. Just generate a random valid link and then check if it gives a video or not. I found exactly 0

• dr-detroit 5y ago
  Of course. They are using some modulo arithmetic: 1. Start from the rightmost digit (i.e. check digit) 2. Multiply every second digit by 2 (i.e. digit at even positions) 3. If the result in step 2 is more than one digit, add them up (E.g. 12: 1+2 = 3) 4. Add the resulting digits to digits at the odd positions