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How Facebook Knows When Its Ads Influence Your Offline Purchases

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Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#11

Post by dmurray »

moab wrote: post #8
dmurray wrote: post #6

> The trick is that the hash of a phone number captured on Facebook will look just like the hash of the same phone number captured in a brick and mortar store, so the two companies can match the numbers without actually trading them. This just isn't possible. How many cents would it cost to brute-force hash all legal American phone numbers?

Why on earth would you have to brute force all numbers? You can do this optimally: just hash the numbers you see (or get requests for).

So that I can cold call someone who might be interested in my product, even though DataLogix only told me the hash of their phone number.

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#13

Post by dmurray »

logicallee wrote: post #10
dmurray wrote: post #6

> The trick is that the hash of a phone number captured on Facebook will look just like the hash of the same phone number captured in a brick and mortar store, so the two companies can match the numbers without actually trading them. This just isn't possible. How many cents would it cost to brute-force hash all legal American phone numbers?

This is correct. As a zero-knowledge proof it is, sadly, trivially broken. (Not just in a theoretical sense. As you mention there just isn't enough entropy going into the hash. It's like asking for your age and gender, but only providing a hash to some other party so they can verify that you are who you say you are, without your divulging what information you were just given. That doesn't work - you would leak everyt…

If there is, the 'someone else' would still have to do some rate limiting. Otherwise, once again, I can just run this algorithm for all possible phone numbers.

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#14

Post by dao- »

"It has happened to you, and it has probably happened more than once."

Umm, no. Why on earth would I give my phone number or email address to random shops? This is crazy. I surely don't want merchants to contact me "about other stuff I might want to buy." This assumption sold as a certainty in this article is somewhat disturbing. Do others from the US feel the same way? (I'm from Germany. Different culture?)

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#16

Post by berelig »

dao- wrote: post #14

"It has happened to you, and it has probably happened more than once." Umm, no. Why on earth would I give my phone number or email address to random shops? This is crazy. I surely don't want merchants to contact me "about other stuff I might want to buy." This assumption sold as a certainty in this article is somewhat disturbing. Do others from the US feel the same way? (I'm from Germany. Different culture?)

I can only think of a couple of retailers that have asked me for an email address here in Canada (coincidentally both were US companies).

I do get asked for my postal code fairly often but that's hardly a personal detail.

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#17

Post by logicallee »

dmurray wrote: post #13
logicallee wrote: post #10

Earlier quoted context omitted.

This is correct. As a zero-knowledge proof it is, sadly, trivially broken. (Not just in a theoretical sense. As you mention there just isn't enough entropy going into the hash. It's like asking for your age and gender, but only providing a hash to some other party so they can verify that you are who you say you are, without your divulging what information you were just given. That doesn't work - you would leak everyt…

If there is, the 'someone else' would still have to do some rate limiting. Otherwise, once again, I can just run this algorithm for all possible phone numbers.

No, no. Read my link :) The whole point is that the definition of 'zero-knowledge' is that (if you use a zero knowledge interactive protocol) you learn no further knowledge.

You are right that the hash-passing is not zero-knowledge. Rather than just verify whether it's in a set, you've accidentally revealed a number. So if you have a set of phone numbers and I have 1 to check if it's in that set, then checking hashes doesn't work - I would end up divulging my number if I used it.

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#19

Post by breitling »

chiph wrote: post #12

Who gives their phone number to store clerks? "Sorry, it's unlisted."

As hard as it is to believe, I'm sure plenty of people give out their phone numbers when asked. I have personally witnessed many many people (including my wife) give out their number at checkout without hesitation. I guess the fact that this is continued practice means that it actually works.

This may vary by geography...my observations are from Canada.

Re: How Facebook Knows When Its Ads Influence Your Offline Purchases

#20

Post by dao- »

berelig wrote: post #16
dao- wrote: post #14

"It has happened to you, and it has probably happened more than once." Umm, no. Why on earth would I give my phone number or email address to random shops? This is crazy. I surely don't want merchants to contact me "about other stuff I might want to buy." This assumption sold as a certainty in this article is somewhat disturbing. Do others from the US feel the same way? (I'm from Germany. Different culture?)

I can only think of a couple of retailers that have asked me for an email address here in Canada (coincidentally both were US companies). I do get asked for my postal code fairly often but that's hardly a personal detail.

Same here regarding the postal code, that's fairly common in Germany.
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