How the "my fine is" meme could have gone horribly wrong

The "my fine is" meme asks people to calculate the total fine for various personal indiscretions that they've engaged in and post the aggregate result publicly.

Smoking pot, for example, nets you a $10 fine whereas having sex in church sets you back $25. You add up the various fines for all the offenses you committed to reach the value of your final fine and publish it publicly (e.g., on Facebook or your blog).

The idea is that the higher your final fine, the naughtier you've been and yet no one knows exactly what you've done so your secrets are safe.

A little harmless fun, surely.

But if the originator of the meme had been less scrupulous or better versed in mathematics (and, more than likely, both), it could have led to a lot of potentially embarrassing admissions. How?

  1. Use only prime numbers for the fine amounts (e.g., $3, $5, $7, $19, $43, etc.)
  2. Instead of asking people to sum the total, ask them to find the product of their answers (i.e., multiply them).

So, say that smoking pot had a $3 fine and having sex in church set you back $5. If you had engaged in both these offenses, your fine would be $15.

If you posted this publicly, thinking that the aggregate amount hid your true dealings, you would be in for a rude awakening at some point since multiplying two prime numbers results in a composite number that is divisible by those two numbers alone (as well as 1 and itself). So there's only one way that $15 could be read as: you smoked pot and had sex in church.

(It would still be up to people's imaginations, however, if you did both at the same time.)

Similarly, if you had partaken in the $3, $5, and $7 offenses, your total fine would be $105. A seemingly innocent number but one that can only be divided by those three numbers, their various composites, 1, and itself. So publishing that single number would have been essentially the same as owning up to each of the separate offenses publicly in a list.

All this to say that someone could have used this meme, in a slightly altered format, as a widespread social engineering hack to get people to reveal their most personal details while lulled into thinking that they were publishing an aggregate and rather less revealing number.