Quitting non-free services and search for blood donor: idea for an MxN contact discovery system

Background

I use a few non-free services, mostly instant messaging services to talk to my friends and acquaintances, but had plans to 'sunset' them by the end of the year. I was going to make a public post or the equivalent for $service, without spamming my whole network, explain my motivation and advertise libre means of contacts that they can use to reach me. I had a plan for every $service, and I was eagerly waiting to execute them. But recently I had to search for blood donors for a friend and my search exclusively used a popular non-free service. I'm not sure if the search would have been as effective if I had quite though.

The service is popular among us Indians and is run by a not-so-nice $BigCorp but since it is available free of cost, most people are willing to overlook the fact that it is harmful in the long run. It is one of the services that I have been trying to get rid of for the longest time; I've been running it in a sandboxed environment, using [Shelter] (TODO: get shelter's link), without exposing my contacts database and main file system to minimize damage. And I was all set to gradually phaseout and get off of the platform but the recent experience of contacting multiple people for an urgent cause suggests it will be more difficult than I had anticipated.

Problem

If I had to mount a similar search without the non-free instant messaging $service, I would have to ring each of the people that I messaged, which is more time confusing and requires more effort than simply forwarding and coordinating with multiple parties par rally via text messages. Also, the phone call process would require both parties, my friend and I, to be available at the same time. My friends are more busier than I am, so the search effort would have been bogged down and significantly less effective than the instant message approach.

In my opinion, this is problem is much more fundamental than quitting a platform. How do I contact someone who is on a platform that I don't have access to or am unwilling to use? Why do platforms, especially the social kind, impose these barriers implicitly? And how to break them without screwing up security or ruining user experience for all involved platforms?

For the web, we have RSS: anyone and everyone can subscribe to websites that implement RSS feeds, like my blog here, and receive updates whenever a new article is published. The subscriber isn't required to negotiate with the publisher, all they have to do is periodically check the feed for new entries, which will automatically be done for them by [their feed reader of choice] ( TODO: get archlinux feed reader list)

The real problem

We have emails and other free, federating alternatives like Matrix and the array of Fedi apps but they aren't just as widely adopted as the popular non-free service is. But that isn't the problem. How does one discover alternate ways to contact a person?

MxN contact discovery system

I propose a system where a person is able to search for alternate contacts on alternate networks using any prior contact that they might have. So if I have a person's Fedi address, I could use it to search for the person's, say, phone number or even their Matrix ID if they have one. This way, I get to choose an alternate means of contact that we both share or agree to use without directly negotiating with them. I mean, it's first contact, how does one negotiate even before establishing a means of communication?

The system would store two hashes of every contact identifier: one a strong, untraversable hash and the other a weak hash using a proved colliding hashing algorithm like MD5 or SHA-1. The strong hash will be used to uniquely search for a related contact identifiers' weak hashes, which will then have to be cracked by the searching party to retrieve the underlying contact identifier. The contact identifier can carry metadata like which network it belongs.

Hashed contacts or how to safely advertise contacts publicly

The hash serves multiple purposes:

1. Combat Spam

Publicly accessible, freely distributed contacts databases are a spammer's wet dream. A system like this shouldn't hold monopoly over who/what a person uses to establish communication. But to securely share contacts publicly, we need a method to restrict spammer/whole database access without compromising free distribution.

The weak hash generating using collision-prone algorithms like MD5 and SHA-1 will serve as an unconventional Proof-of-Work mechanism: the user will have to crack the hash to retrieve a contact. A spammer would have to do a lot of work cracking the hashes to mount a successful spam operation but a person that is interested in only single person's contact will have to spend a only comparatively reasonable amount of time to retrieve the contact.

2. No trusted entity:

In centralized systems, the service provider will act as a trusted entity and ensure secrecy of the information they host. This has the unfortunate effect of monopolizing the data hosted by them. A competing service would have to start from scratch and aggregate data, which if done with consent(as it should be), would require convincing people to start using the service.

The hash-based system doesn't require secrecy and data aggregated can be freely shared with other providers to create a sustainable, federated/peer-to-peer network.

My schedule at the moment is completely packed but if someone is interested in implementing this system, they are most welcome to do it. My contacts are available here if someone wants to brainstorm ideas :)