Checksums for checks

September 29th, 2009 by ftobia

Here’s a really cool instance of life hacking I heard the other day from Brian Rowe:

First, an introduction. When you’re at a restaurant, and you pay by credit card, how do you decide what to tip? That is, how do you choose an exact amount that’s within the acceptable range? Two common strategies are 1) to choose a round number for a tip and add it to the (not round) total, or 2) choose a value for the tip that will make the final amount a round number.

I have heard that one way is less secure, though I can’t remember which, because the person running the bill can input another amount and you wouldn’t know just from checking your credit card statement. I don’t know how credible this threat is. But now I happen to have a solution.

You can use a checksum for your checks. Leave a final amount such that the last digit is equal to the sum of all the digits preceding it. For example, instead of paying $42, leave a tip such that the amount you pay is $42.06. This method is straightforward, awesome, and helps curtail credit card fraud.

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One Response to “Checksums for checks”

  1. Crystal Says:

    The way that I heard it (from my sistah) is that leaving payment + tip equal to a round number is less secure, because if the person then says you paid any round number, you’ll be less likely to notice the discrepancy. For example, you say you want to pay a total of $42, but they put in $47 to give themselves a better tip or whatever, are you really going to remember what you wrote down? But if you intended to pay $42.29 and it shows up as $47 on your statement, then you’d be like “hey, wait a minute…” Then again, if you intended to pay $42.29 and it showed up as $47.29, would you notice that either? And how much of a threat is this anyways? I feel like people don’t trust people in the service industry enough… silly humans.

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