Zero-knowledge advertising in a post-iOS20 world

If the explosion of LLMs wasn’t enough to suggest that local search engines weren’t going to become a reality, then products like Rewind surely solidify the idea. Cool productivity app, sure. But the implications for data privacy and digital advertising are huge.

If you haven’t used the product or seen the demo, Rewind allows users to search for anything they have seen or heard from their computer — making it the “Search Engine for Your Life”. The a16z backed startup is gunning to produce a comprehensive log of every action you take on your computer, and then fortify it with OCR and transcription functionality that we’ve come to love in other tools.

How does it work?

Rewind records and archives your computer usage locally at 3750x compression. 3rd parties aren’t harvesting your data “into the cloud” somewhere, and users have the ability to blacklist certain apps to prevent private information being logged. In return, Rewind users are relieved from the burden of taking notes on everything they’ve consumed, or having to retrace their digital steps to retrieve any previous piece of information. Rewind’s founder and CEO, Dan Siroker, says that he “wants humans to have perfect memory” — and this certainly feels like the first step towards downloadable consciousness.

On the surface it may seen like a run-of-the-mill productivity app for Mac users. It feels almost certain that Apple will acquire (or copy) Rewind within the next couple of years, and then merge it with their Time Machine product not long after. New features will be added over time, and the app will become a staple part of a Mac user’s life.

But as soon as you start to think about the treasure trove of local data that is being collected and analysed by the app, the more that this transcends productivity. Apple is potentially sitting on an advertising goldmine, but only if it can protect users’ privacy.

Enter Zero Knowledge Proofs (ZKPs)

wtf is a ZKP?

Without going deep into the maths that I won’t pretend to understand, zero-knowledge proofs are a type of cryptography. They allow one party to prove to another party that they know a certain piece of information without revealing the information itself.

It’s super useful for things like verifying digital identity, and also all the rage in the crypto space as a scaling solution. But the use cases for digital advertising are potentially even bigger.

Brave has been doing this for a while using a similar model, and have racked up an impressive 20M daily active users on the way. But this is nowhere near the level of scale that can turn them into an advertising powerhouse.

How does this tie in with digital advertising?


  1. Something like Rewind could log your computer and phone use, aggressively compress the data, then summarise it via machine learning.
  2. Users could then be put into a number of different interest and behaviour categories, but not granular enough to be personally identifiable.
  3. Zero-knowledge proofs of those categories get generated locally on your device, and these are sent back to the ad platform completely anonymised. For example, is the user of X device interested in buying men’s shoes based on their computer usage; yes or no.
  4. When an advertiser wants to deliver an ad, they specify their targeting preferences.
  5. If the answer is yes to point #3, the shoe ad is delivered to the user but nothing is revealed to the ad platform about who the user is.
  6. The user is rewarded per-impression for sharing their data.

What we’re left with is a targeted ad that isn’t reliant on privacy-invasive cookies or the collection of first party data. Win-win!

What magical ad platform does this?

Just Brave Ads for the moment.

Who holds the power?

By being in control of both its hardware and software, Apple’s in a ridiculously solid position.

In the same way that the iPhone is increasingly becoming better at processing all things AR, the same trend will likely continue for privacy. End to end encryption of messages, Touch ID, Face ID, and T2 chips are just some examples. With the possibility of integrating a tool like Rewind, they may soon be able to create user profiles that are so privacy-centric that they only exist on the device and nowhere else.

Apple is no stranger to asserting its dominance in the name of privacy. Each new iOS update ruffles the digital marketing community’s feathers in a different way, and it’s only a matter of time before there’s a final blow.

While ZKP technology could allow for highly targeted, non-intrusive advertising, it also gives Apple a significant advantage over the “traditional” ad giants like Google and Facebook, who rely heavily on third-party data. And it’s not just about having the hardware to enable such a change — Apple has a loyal user base that trusts its commitment to privacy. It’s literally a selling point.

What about the users?

This new era could mean an absolute win for the users. First, their privacy would be better protected. No more worries about data breaches, identity theft, or unwanted targeted advertising.

Second, unlike “old school” advertising where the user is the product, users could be rewarded for their data. Not in the form of a “free app” or service, but in cold, hard, fungible value. Once the train starts moving, everyone will wanna get compensated for their data.

What next?

We’re sitting on the cusp of an era where user privacy is respected, data is secure, advertising is personalised but not intrusive, and users are rewarded fairly.

Only time will tell how Apple (or any of the other big dogs) makes this shift. But you do get the feeling that we’re edging closer to an inflection point where everything changes.

???? Bonus Prediction

For what it’s worth, here’s a potential timeline to think about:

  1. Apple acquires Rewind in near future
  2. It becomes a staple part of MacOS and iOS
  3. Apple harnesses user data using the feature, and builds a privacy-centric ad platform.
  4. Apple Silicon becomes more and more optimised for privacy and ZKP computation.
  5. Apple becomes the pseudo-gatekeeper of user data and the new head honcho of digital advertising.
  6. Apple adopts a similar model to Brave, where users are rewarded for sharing their data and being shown ads.
  7. Apple becomes a bank and releases a stablecoin (backed by their massive fiat reserves) that incentivises users to stick within the Apple ecosystem.
  8. TBC

Wanna subscribe?

More Reading