Add spec for shared-history room keys (MSC3061)

[uhoreg]

spec PR for matrix-org/matrix-spec-proposals#3061

Preview: https://pr1655--matrix-spec-previews.netlify.app

[poljar]

Should we push this to the spec, considering the problems which were subsequently discovered with this MSC3061?

[richvdh]

Pending a decision about wtf we are doing here

[richvdh]

Given this completed FCP, it belongs in the spec. If we want to rip it out the spec again, that will require a new MSC.

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[richvdh]

Apparently there are security concerns, specifically around sharing keys for past messages. We'll disclose more details in a few weeks, and then we can decide how best to proceed here.

[turt2live]

clearing team review request while awaiting security impact

[murlock1000]

With the recent migration of element clients to the rust SDK this introduced a regression of the spec (Breaks historical room key sharing feature). Any updates on this @turt2live ?

[dkasak]

The reason this was blocked was disclosed in https://matrix.org/blog/2024/10/security-disclosure-matrix-js-sdk-and-matrix-react-sdk/ and the associated advisories. The plan forward is outlined in this section.

[turt2live]

Formalizing "proposed spec changes" at https://matrix.org/blog/2024/10/security-disclosure-matrix-js-sdk-and-matrix-react-sdk/#proposed-specification-changes as requiring action. That future MSC will have impact on this spec PR's text.

[richvdh]

MSC3061 has been un-merged, so we're going to abandon this for now.