Expected behavior
When i first sign up for DeltaChat or any other time, i can initiate mutuals discovery. This is when I allow DeltaChat to check my contacts if they are already in DeltaChat
I will be told if my contact is finally in DeltaChat
I know there is a security & privacy concern about this so I will refer to Bluesky trying their best to solve that concern by hashing the contact:
The blog post: Find Your Friends on Bluesky - Bluesky
The RFC: Request For Comments: A secure contact import scheme for social networks | Bluesky
Of course Bluesky can do that because it is checking phone number and centralized service, so this proposal may be rejected for that reason.
Actual behavior
I only know if my contact in DeltaChat if they told me directly
Example Images
From Bluesky:
From Telegram and article trying to fix the noises: https://medium.com/@bedmutha.riddhi/ux-case-study-solving-contact-joined-telegram-chat-feature-of-telegram-b56effd599e8
ian
February 3, 2026, 4:40pm
2
I moved this comment to its separate issue:
A user would install DC
On first start, DC would scan for email addresses (along with fingerprints and public keys) in the the device address book, the IMAP folder of the DC accounts of the user and possible further “external” IMAP accounts (not otherwise added to DC). If any entry has an email address not known to DC, DC would send a probe email to it.
If DC on the other side accepts this contact and proceeds with encrypted communication, extend the device address book entry with the public ke…
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Probing email for all email address in address book not yet DeltaChat is actually different idea.
This topic is actually to discover if someone else already in DeltaChat with both me and my contacts registering our intent to discover mutuals to a service and that service save our telltale like phone numbers or email address.
ian
February 3, 2026, 4:56pm
4
In that case, this is a duplicate of the following:
Hello there,
please forgive me if this has already been widely discussed but I didn’t see it anywhere:
I’d like to propose a lookup server (or actually confirmation server).
Motivation
I want a “phone book” in DeltaChat that shows me who I can contact, who will not just get an email but also “speak DeltaChat”
When inspecting a contact on Android (surly on iOS too) I get hints what messengers that contact is using. That is how I usually decide how to contact another person. And I’d like t…
A really nice feature, wich would separate DC from other Messengers, could be the function to pull matching emails from the server address book on contact search.
I think this would be a huge benefit for companies and organizations and you can’t do on Whatsapp
AFAIK Imap don’t support this (only exchange) but some email Clients just using LDAP or AD instead: https://serverfault.com/questions/235447/getting-contact-list-from-imap-server
So this could be the way to go. Maybe allow it to be conf…
Introduction
Yep, you heard it right. I know it’s a wild idea.
But with Delta Chat becoming more and more normie-friendly (e.g. instant onboarding) I thought I’d shoot my shot.
Why it’s important
We know that practically every messenger including Signal uses phone numbers to look for contacts.
All family members usually have each other’s phone number, unlike, let’s say, Instagram or Snapchat or email address.
So when you aren’t, want to connect with you on a messenger, all you have to do is …
Some people seem to want address book matching, and Quicksy is a derivate of the Conversations XMPP App providing that feature.
They also published the server code:
As jabber IDs look the same as email addresses, there might even only be a patch needed when storing both on a shared server…
Hallo zusammen,
gibt es eine Einstellung, die anzeigt, welche meiner Kontakte bereits die Delta Chat App nutzt - also Delta Chat nicht als Email-Programm, sondern als Messenger?
Besten Dank
Gerhard
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Thanks, there is one that i miss, so i have replied there:
So i only know there is already this proposal even though i have created new one: Discover mutuals
The cost is something to consider. It is possible if the user actually want to pay, then maybe it is separate bot/app than DeltaChat default one, but then we won’t get matching numbers…
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ian
February 3, 2026, 5:16pm
6
Good catch. Note that if you write the link on its own line, a preview will be generated for it inline.
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Thanks, that’s neat trick!
If there is any other feedback, please feel free to reply.
I think my proof of concept will be a separate app for discovery of mutuals on other protocols/networks, not only DeltaChat, since end users will need to pay for the SMS or email verification, and keeping the service up, so generalizing this hopefully will allow adoption.
I have a feeling someone already does that kind of app though, but maybe not with DeltaChat as one of the “I am currently using this app”.
ian
February 4, 2026, 6:48am
8
There was a proprietary service for this in the past:
Keybase is a key directory that maps social media identities to encryption keys (including, but not limited to PGP keys) in a publicly auditable manner. Additionally it offers an end-to-end encrypted chat and cloud storage system, called Keybase Chat and the Keybase Filesystem respectively. Files placed in the public portion of the filesystem are served from a public endpoint, as well as locally from a filesystem union-mounted by the Keybase client.
Keybase supports publicly connecting Twitter...
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