[felsokning]

> ...the idea here is that every entity gets their own guid designating you, so the same guid is not known by more than one entity

Ok, now you're sending a list of guids that _can_ be emailed to, per negotiation? Otherwise, how are they sending to that specific guid? A guid is not a hash of an object but an identifier object (a 16-byte array, if I recall correctly) - it has to map to the recipient _somehow_.

In other words, in each SMTP exchange, that information would have to be stored in some form of look-up table, _somewhere_, on both the sending and receiving servers.

How do you enforce the senders destroying that table, so that many versions of it don't expose your half of the signature? Do you generate a new key per session? If so, where are you storing that key, in memory? How would you prevent the heap from exposing those keys in a process crash (say, where a dump is automatically generated - like in Windows)? How do you prevent a nefarious actor using A, B, or C from generating a flood of SMTP sessions and creating a tonne (yes, the metric kind) of these look-up tables in memory? What happens when back-pressure is hit? Do you force everything else to paging but keep the tables in memory?

[naasking]

> it has to map to the recipient _somehow_.

I think you're overthinking it. For simplicity, instead of [human-readable]@mydomain.com, let's use [guid]@mydomain.com, a dynamic set of unguessable aliases for your account. Your guids that have been handed out are completely under your control and stored on your server.

There are no cryptographic keys to manage here, just cryptographically secure identifiers that are stored on a server.

If you and I had been introduced, you would have a [e-mail] designating me in your address book, and I would have a [e-mail] for your address in my address book.

So revoking the guid you have for me is an operation that happens on my server and simply invalidates the only address that you have. This part is simple and why spam is easily stopped in its tracks.

The introduction protocol is the tricky part, because C and B would have different guids for A, so if B CC's A when messaging C, then there should be a way for C to resolve their guid for A. This is done via a petname system.

If C does not already have a mapping for A (and so doesn't know A), then it can request an introduction from B. C sends B an "introduce-me as C[guid-intro]" message with a new guid for C, then B then sends to A "here's who I call C [guid-intro]". Guid-intro is a use-once guid for introduction purposes.

A then sends to C[guid-intro], "hi, I'm A[new-guid]". C replies, "hi, I'm C[new-guid]". C then revokes guid-intro since it was used, and we're done. A, B and C each have their own guid addresses for each other. You can keep the audit trail of where you got a guid introduction in a database, but that's not strictly necessary for this to work.

This introduction protocol happens transparently to the user just by exchanging specific message types the server recognizes. It's a protocol that can be built atop SMTP just to manage the database of addresses that the SMTP server accepts.

[felsokning]

> If you and I had been introduced, you would have a [e-mail] designating me in your address book, and I would have a [e-mail] for your address in my address book.

The description, here, is no different than S/MIME encryption exchange - in that the guid exchange has to be done before it could be used.

You still have the issue of [A]Guid and [C]Guid correlating between themselves _and_ it being "unique" per SMTP session (after all, you said before that each guid has to be unique per session). This is where my earlier reference to generated guids being exchanged during the SMTP session comes into play. However, leaving that aside...

A single-use guid is no different than an SMTP address, if we're going based on the single guid inferred from that line -- and that guid has to be stored elsewhere in your system for it to be resolved to a recipient. So, you need something like a guid history array on the object for the forest to be able to resolve that guid (on recipient resolve) to a mail object inside your forest.

You have no mechanism (from your description) for B sending to [A]Guid or [C]Guid junk mail (assuming they've been able to discover the guids) using those guids. You say you would invalidate [A]Guid or [C]Guid -- but this doesn't resolve the issue of [A] and [C] now having to re-exchange Guids, for something that B has done.

So, now, all valid email between [A]Guid and [C]Guid is invalidated (per your description) and they're calling into your helpdesk, trying to understand why valid email isn't being delivered.

Do you tell them to re-exchange guids? How do they re-exchange guids when the mail system is dependent (directly) on those guids already being established on both sides? How do they "re-introduce" themselves, in other words, in that scenario?

[naasking]

> The description, here, is no different than S/MIME encryption exchange - in that the guid exchange has to be done before it could be used.

There are formal connections between some encryption protocols and what I'm describing here (effectively a system based on capability security, ie. this is modelling spam as an access control problem for an unbounded set of actors). Basically encryption let's you do away with extra storage requirements for the guids, but the cost is additional complexity around key management and revocation, and more compute cost. I haven't thought about it enough to see if there's a formal correspondence with S/MIME, but my proposal is very simple so I don't think you need to try to understand it through that lens.

> You still have the issue of [A]Guid and [C]Guid correlating between themselves _and_ it being "unique" per SMTP session

No, these guids are not per-session, they are persisted in a user's address book.

> and that guid has to be stored elsewhere in your system for it to be resolved to a recipient

Yes, each user's address book contains the guid address for a contact just like right now it contains an email address. Just take the existing address book and make the emails cryptographically unguessable guids. If you and I have the exact same set of contacts, none of our guid addresses will match. That's literally it.

> you need something like a guid history array on the object for the forest to be able to

No such history is needed. I really think you're overcomplicating this.

> You have no mechanism (from your description) for B sending to [A]Guid or [C]Guid junk mail (assuming they've been able to discover the guids) using those guids.

I don't understand what you're trying to describe here.

> You say you would invalidate [A]Guid or [C]Guid -- but this doesn't resolve the issue of [A] and [C] now having to re-exchange Guids, for something that B has done.

If A and C have been introduced per the protocol I described, then anything B does has no impact on the relationship between A and C. If B sends them junk mail, the user (A or C) could decide to revoke B's access to them, or may opt to not revoke if they think it was an accident.

You could opt to track who introduced you and make revocation decisions based on that extra info too, but it's not strictly necessary.

> How do they "re-introduce" themselves, in other words, in that scenario?

In the case that a guid address has to be revoked but you want to keep the connection, (perhaps the guid address leaked somehow), then the mail agent would have renew their connection by re-running the introduction protocol before revoking the previous guid, or they would have to request another introduction through someone they both know.

This is as simple as having a "mark as spam" button, and when the user clicks it, it asks if they want to block the user entirely or if this was accidental (or something). If the former, the system revokes immediately, if the latter the system re-runs the introduction protocol using the existing guids to get new ones, then revokes the old ones.