Code change:
The "signature" field in an MLSPlaintext object is computed using the signing private key corresponding to the public key, which was authenticated by the credential at the leaf of the tree indicated by <del> the sender field.  The signature covers the plaintext metadata and message content, which is all of MLSPlaintext except for the "signature", the "confirmation_tag" and "membership_tag" fields.  If the sender is a member of the group, the signature also covers the GroupContext for the current epoch, so that signatures are specific to a given group and epoch. </del> <ins> the sender field.  The signature is computed using "SignWithLabel" with label ""MLSPlaintextTBS"" and with a content that covers the plaintext metadata and message content, which is all of MLSPlaintext except for the "signature", the "confirmation_tag" and "membership_tag" fields.  If the sender is a member of the group, the content also covers the GroupContext for the current epoch, so that signatures are specific to a given group and epoch. </ins> The "membership_tag" field in the MLSPlaintext object authenticates the sender's membership in the group.  For an MLSPlaintext with a

Related discussion:
text: s currently have two usages in the spec: Pre-published key packages that you can download and put in a leaf when adding a new member leaf node content that you update when doing a full Currently, things have been designed to use only one structure for both uses, leading to some weirdness such as: and are useful in the usage 1, but are parameters of the group in usage 2. is a good name in usage 1, but would better be named in usage 2. some data is important in usage 2 and have no meaning in usage 1, hence they were designed as "mandatory-when-updating extensions" (such as , and we were also discussing about adding the in to help prevent cross-group attacks) How about using different structures for these? // Essentially identical to KeyPackage struct { ProtocolVersion version; CipherSuite ciphersuite; HPKEPublicKey hpkeinitkey; opaque endpointid; Credential credential; Extension extensions; opaque signature; } PrePublishedKeyPackage; struct { HPKEPublicKey hpkekey; opaque endpointid; Credential credential; opaque parenthash; opaque groupid; Extension extensions; opaque signature; } LeafNodeContent; enum { reserved(0), prepublishedkeypackage(1), leafnodecontent(2), (255) } LeafNodeType; struct LeafNode { // This is not included in the signature, but with the "unambiguous signature" PR that's not a problem LeafNodeType type; select(URL) { case prepublishedkeypackage: PrePublishedKeyPackage data; case leafnode_content: LeafNodeContent data; } } What would you think about refactoring like this?
If we're doing this, I'd suggest including a dedicated public key to encrypt Welcome messages to for the . NAME suggested this at some point. It gives us better FS guarantees, because the receiver of the can delete the corresponding private key immediately after decryption, while they might have to keep the other key around for a bit until they can actually issue an update. This might also help clarify .
Ties into as well because is an example of an extension that is required and should be a field instead
NAME one concern I have is how is that field in created?
When sending a full , the participant includes a new in the (it replaces the field).