63 lines
2.8 KiB
Markdown
63 lines
2.8 KiB
Markdown
# MSC2874: Single SSSS
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[Secure Secret Storage and
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Sharing](https://github.com/matrix-org/matrix-doc/pull/1946) (SSSS) was
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designed to allow the user to create multiple keys that would be able to
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decrypt different subsets of the secrets. However, the vast majority of users
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do not need this feature.
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This proposal defines how clients should behave if they only wish to support a
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single key, by defining which key clients should use if multiple keys are
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present. It also makes the `name` field in the `m.secret_storage.key.*` events
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optional, as this field was mainly added to allow a user to select between
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different keys.
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## Proposal
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If a client wants to present a simplified interface to users by not supporting
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multiple SSSS keys, then the client should use the default key (the key listed
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in the `m.secret_storage.default_key` account data event.) If there is no
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default key the client may behave as if there is no SSSS key at all. When such
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a client creates an SSSS key, it must mark that key as being the default key.
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The `name` field in the `m.secret_storage.key.*` account data events is
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optional, rather than required. If a client wishes to display multiple keys to
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a user and a given key does not have a `name` field, the client may use a
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default name as the key's name, such as "Unnamed key", or "Default key" if the
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key is marked as default.
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For example, when a client creates a key with ID `abcdefg`, it will create an
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`m.secret_storage.key.abcdefg` account data event to store information about
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the key. It will then mark it as the default key by setting the
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`m.secret_storage.default_key` account data to `{"key": "abcdefg"}`. When
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another client logs in after this, it will see that the default key has been
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set, and will know to use that key as the SSSS key.
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## Potential issues
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If secrets are encrypted using a key that is not marked as default, a client
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might not decrypt the secrets, even if it would otherwise be able to.
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## Alternatives
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Rather than solely relying on the key marked as default, a client could guess
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at what key to use. For example, it could look at the secrets that it needs,
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see what keys they are encrypted with, and if there is only one common key,
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then it could use that. (This is what Element currently does.) Or if there
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are multiple keys, it could use some sort of heuristic to pick a key. However,
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this approach can be error-prone, and it is better to rely on an explicit
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marking.
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## Security considerations
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None
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## Unstable prefix
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An unstable prefix is not needed for a behaviour change in choosing the key to
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use as there are no event/endpoint changes.
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Some clients already omit the `name` field (notably, matrix-js-sdk
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unintentionally does this -- mea culpa), and this does not seem to be causing
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issues, so an unstable prefix seems unnecessary for this.
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