How Fenzly finds your friends without seeing your address book
There’s a moment in almost every social app’s onboarding that should give you pause: “Allow access to your contacts?” You tap yes, and in that instant your entire address book, hundreds of names, numbers, emails, often birthdays and home addresses is uploaded to a company’s servers. Not just your friends who use the app. Everyone. Your doctor, your ex, your landlord, people who never agreed to anything.
That’s how most apps solve the discovery problem: take everything, sort it out server-side. Your contacts become their data. And notice who else got no say: everyone in your address book. Your contacts didn’t choose your apps but most of them have been uploaded to dozens of companies’ servers anyway, by other people’s phones.
Fenzly needs to solve the same problem, you want to find which of your friends are already here. We just refuse to solve it that way.
Your address book never leaves your phone
The starting point is blunt: Fenzly never uploads your contact list. Not encrypted, not “anonymised”, not temporarily. The contacts you see in the app live in the local database on your device and nowhere else. Our server doesn’t have a copy, which as with everything behind the blind server means there’s nothing to leak, sell, or be compelled to produce.
But then how do you find your friends? That’s the interesting part.
Matching without revealing: the fingerprint trick
When you ask Fenzly “which of my contacts already use the app?”, your phone doesn’t send the contacts. It computes a fingerprint of each email or phone number, a hash: a short scrambled code derived from the address, from which the address itself can’t be directly read back.
The same trick happens on the other side. People who’ve chosen to be discoverable have the fingerprint of their email registered, never the raw list of who knows them. The server compares your fingerprints against those, and answers with matches: “these three fingerprints correspond to Fenzly users.” Your phone then looks up which contacts those were locally.
The result: you find your friends, and the server has processed a handful of opaque codes instead of ingesting your social circle. No match, no trace. And there’s deliberately no user search in Fenzly, you can’t type a stranger’s name or number and go fishing. Discovery works only between people who already know each other, and consent controls the rest.
Being findable is a choice you make, not one made for you
Being findable is separate from finding. Whether your own email or phone number can be matched by others is under your control, and Fenzly asks rather than assumes: you decide, and you can change your mind in the settings at any time.
It’s worth being honest about why the two channels aren’t equal, because most apps skip this part. An email address is long and personal enough that, hashed, it can’t realistically be guessed in bulk. Phone numbers make weaker fingerprints: there are only so many plausible numbers in a country, few enough that someone determined could hash all of them and compare. Hashing a phone number obscures it; it doesn’t make it unguessable.
For most people that residual risk amounts to little, the only thing a match reveals is that a number has a Fenzly account, never a position or a contact list. But for someone who doesn’t want a particular person to know they use the app at all, it matters. That’s why it’s a question we put to you instead of a switch we flip for you: you know your situation; we don’t.
That’s a pattern you’ll keep seeing in Fenzly: when a mechanism has a real limitation, we tell you what it is and hand you the decision.
Your number opens the door, it isn’t who you are
One more difference runs deeper than a setting. To be clear about what your phone number does do in Fenzly: it’s the default way to sign in, you receive a code by SMS and you’re in. But signing in is all it does. Your identity, the thing your family and friends are actually connected to, is an internal identifier that never changes.
In most messaging apps the two are fused: your number is your account, which is why changing numbers there means losing conversations and why breaches are so damaging. In Fenzly they’re separate. Change your number and nothing breaks, your circles, your history and your connections carry over, because none of them were ever attached to the number. You can add an email as a second way in, or even replace the phone entirely and live email-only. And there’s no username to squat, no public directory: the number is a door key you can swap, not the house.
Add it up and the picture is consistent: the people you know stay on your phone, the people who find you do so because both of you agreed, and no one, including us, gets to browse who knows whom.
Next: we step back from the mechanisms and ask the question that motivates all of them, what does a location history actually reveal about a person? More than most people would ever guess.