Re: End-to-end Encrypted Messaging in ActivityPub

Evan, I like the approach you illustrate using as2:mediaType: multipart/encryption + as2:content.
I’ll have to read a bit about existing multipart/encryption specs.

One thing that’s not quite clear to me from your proposal is what you have in mind for the encryption key that would be used? And how readers would get it?
I think a hybrid encryption scheme would probably make the most sense in 1-to-many broadcast, where the encryption is only done once with a symmetric key, but then the symmetric key may itself be asymmetrically encrypted once for each authorized audience member. Then if a reader encounters an encrypted as2:content, and they don’t already have a key identified by the encrypted blob envelope, maybe then the reader could request the key by key-id explicitly from the original author (as2:attributedTo).
I don’t these details are critical to decide now. It’s useful just to specify how encrypted blobs can be included in as2, but that alone seems insufficient to guarantee interoperability.

Another solution that might be worth considering is using JWE as the media type, and including JWE as the value of the as2:content property.
https://en.wikipedia.org/wiki/JSON_Web_Encryption

One of the benefits over multipart/encryption, I think, seems to be that JWE might be more straightforward to decrypt in the browser (e.g. using globalThis.atop and globalThis.JSON.parse) than the multipart/encrypted media type. IMO it’s worth drafting an extension for at least these two and seeing what people decide to adopt over time (i.e. specifying a convention that is already a valid use of the protocols).

Will keep thinking about this, especially if there might be a more serialization-independent way, or a good way of allowing for this use case with the encryption happening at another layer.
e.g. https://www.researchgate.net/publication/317177393_Self-Enforcing_Access_Control_for_Encrypted_RDF
or https://datatracker.ietf.org/wg/mls/about/

It’s also worth being familiar with DIDComm, which has been designed for e2ee from the start. https://identity.foundation/didcomm-messaging/spec/ (it uses JWE)
I’m still assessing didcomm, which is a big composition of things and under DIF not w3c.
But did-core is w3c, and I’d really like to cooperate with the DID ecosystem (and e.g. the w3c credentials community group) for anything about cryptography.



> On May 19, 2023, at 07:25, Evan Prodromou <evan@prodromou.name> wrote:
> 
> I published a blog post about an architecture for end-to-end encrypted messaging in ActivityPub:
> 
> https://evanp.me/2023/05/19/end-to-end-encrypted-messages-over-activitypub/
> 
> One option for this group is to publish Note documents. I think developing a standard mechanism for E2EE with multiple implementations could be a huge benefit for social web. I’d be happy to participate in such a subgroup!
> 
> Evan