Zero-knowledge by design
A shared address book that stays current — without a server that can read it.
Crowdbook lets a family, friend group, club, class, team or any group keep one another's phone numbers, emails and addresses up to date automatically. Every detail is encrypted on your device before it leaves. The server only ever stores ciphertext it has no way to decrypt.
For iOS and Android · Syncs into your phone's own Contacts app.
Soccer Team · Parents
Synced- JR Jordan Rivera New number · updated
- SO Sam Okafor Email added
- PN Priya Nair Home address
End-to-end encrypted — stored on the server as ciphertext only.
Everyone's contacts go stale. Nobody wants to fix them.
People change numbers, move houses, switch emails. In our ever increasing reliance
on digital communications, we don't keep address books up-to-date, even within family
groups. But the moment we actually need, for instance, everyone's addresses (wedding invitations, graduation announcements,
greeting cards, postcards on a trip, condolence messages, etc.) we end up spamming
group chats, texting, and calling everyone to make sure our info is current.
The usual fix — a shared spreadsheet or a contacts app run by some company —
means handing your whole address book to a server that can read every word.
Crowdbook does the sharing without the surrender. And it does the sharing passively. Each
participant maintains their own contact information. A single source of truth from the most
reliable source.
Move? New phone number? Change your discord handle? Update your information in Crowdbook and all your
Crowdbook contacts will get the changes updated on their devices automatically within a day.
Shared, current, and genuinely private
Four ideas do the heavy lifting.
Zero-knowledge encryption
Contact details are encrypted on your device with AES-256-GCM before they're uploaded. The server stores only ciphertext and never holds the keys to read it.
Share by group, field by field
Join a group and choose exactly which attributes you share into it — your cell but not your address, say. Different groups see different subsets.
Syncs into your real contacts
Updates flow straight into your phone's native address book, so the right number is there when you actually dial — no separate app to open.
Data minimization built in
Only the attributes you explicitly share are ever synced. Delete your account and your keys, blobs, and shared attributes are purged and tombstoned for your peers.
How it works, in three steps
The clever cryptography stays out of your way. Here's the whole idea.
- 1 Create or join a group Start a group for your family or club, or accept an invite link. A group key is generated on your device and wrapped individually for every member.
- 2 Choose what you share Pick the attributes you want that group to have. Your device encrypts them and uploads only ciphertext.
- 3 Everyone stays current, automatically Each member's device pulls the encrypted updates, decrypts them locally, and merges them into the phone's contacts in the background.
Made for the groups you already have
Any circle of people who need each other's details — and would rather not hand them to a company.
"Zero-knowledge" isn't a slogan here. It's the architecture.
Your password never crosses the wire — login is a signed challenge. Your keys are derived on-device and stored only in wrapped, unreadable form. Even we cannot decrypt your contacts. That's not a promise; it's what the server is technically incapable of doing.
Crowdbook is in early development
We're building the iOS and Android apps now. Tell us about your group and we'll let you know when it's ready to try.