Re: HTTP Signature CG report

If clients have custody of keys, then `foo@example.com` could wait for `
bar@example.com` to make a post, and then sign an activity with the same ID
(e.g. "example.com/posts/102930") and send it to all of the instances that
bar has followers at. This would mean that nobody would be able to see
bar's posts (if foo wins the race condition for distributing it, which
in practice is often easy if foo has a local account on example.com). The
vulnerability is much worse if your implementation is using predictable
IDs, but it exists conceptually any time that an origin is shared between
multiple users. For this reason, actor keys in ActivityPub used for HTTP
Signatures or LDSigs cannot be given to the user. There is no way that a
local server could avoid this vulnerability through validation, since the
client (if it has access to the HTTP Signature private key) could always
bypass their server to talk to other federated servers directly. There is
no way to avoid this vulnerability without 1) making keys server-private or
2) throwing out HTTP Signatures entirely and resolving every incoming
activity.

Regarding whether "sharing private keys with a server" is a security fault,
that is a claim that has no basis in the ActivityPub trust model, which
considers your actor ID authoritative using the rules defined for its
resolution. There are other types of URIs, like DIDs, that could provide
the types of guarantees you're looking for, but if you're using a normal
HTTPs URL, then your server *by definition* is going to be the authority on
what your public key is, so there's no way to avoid trusting it without an
entirely different cryptographic approach than any deployed so far within
the fediverse.


On Wed, Feb 7, 2024 at 11:19 AM Dmitri Zagidulin <dzagidulin@gmail.com>
wrote:

> > The approach you present (Allowing the client application to sign
> requests) has well known security flaws
>
> Yeah, I agree with Cristiano, it's only a security flaw if implemented
> wrongly on the server side. Client-side signing is what we should try and
> move the whole ecosystem to.
>
> On Wed, Feb 7, 2024 at 10:11 AM Cristiano Longo <
> cristianolongo@opendatahacklab.org> wrote:
>
>> Hi nightpool, may you provide some reference about this security faults?
>> However I consider sharing my private key with a server a security fault as
>> well.
>> On 07/02/24 15:55, nightpool wrote:
>>
>> Hi Christine,
>>
>>
>> The approach you present (Allowing the client application to sign
>> requests) has well known security flaws and allows for any attacker to DOS
>> and prevent posts from being created for any other user on their server due
>> to ID exhaustion. Therefore I do not recommend this scheme be used for any
>> multiuser server (where multiple users share one domain name).
>>
>> (And obviously, for single user servers the client and the server are
>> probably in the same trust boundary)
>>
>> On Wed, Feb 7, 2024, 8:29 AM Cristiano Longo <
>> cristianolongo@opendatahacklab.org> wrote:
>>
>>>
>>> On 06/02/24 11:04, Marcus Rohrmoser wrote:
>>>
>>> Hi Ryan and nightpool,
>>>
>>> thanks for taking initiative to improve the specifications mandaratorily used in today's fediverse. The previous word of mouth culture is IMO a big obstacle to interoperability.
>>>
>>> On Mon, 5 Feb 2024 11:34:07 -0800
>>> Ryan Barrett <public@ryanb.org> <public@ryanb.org> wrote:
>>>
>>>
>>> We've started collecting candidate requirements inhttps://github.com/swicg/activitypub-webfinger/issues . Feel free to
>>> comment or add your own proposals!
>>>
>>> While I somehow feel the appeal of cryptography, I miss the hard facts about it's benefits here.
>>>
>>> We have the tls umbrella, so there should be a certain trust in that the other side is who they pretend to be. Like in "the same private key as last time", right?
>>>
>>> Introducing mechanisms (in application layer) where the communicating party (the user) has no hold of the private key, still requires full trust to the holder i.e. the server application and the underlying storage system and their operators.
>>>
>>> What do we gain? We still have to trust the application.
>>>
>>> Please writing this report take into consideration that the signature
>>> may be produced client-side. I produced a proof of concept of an
>>> activity pub server where signature is performed client side:
>>> https://www.opendatahacklab.org/lap_src, sources at
>>> https://github.com/cristianolongoodhl/LittleActivityPub.
>>>
>>> Given that, we just could block based on self-declared incoming actor id + ssl validity, save complexity and not have to resort on volatile standard drafts.
>>>
>>> What am I missing?
>>>
>>> Marcus
>>>
>>>
>>>