Change:
The "tree_hash" is updated to represent the current tree and credentials <del> The "transcript_hash" is updated by a GroupOperation message "operation" in the following way: </del> <ins> The "transcript_hash" is updated with the data for an MLSPlaintext message encoding a group operation in two parts: This structure incorporates everything in an MLSPlaintext up to the confirmation field in the transcript that is included in that confirmation field (via the GroupState).  The confirmation and signature fields are then included in the transcript for the next operation.  The intermediate hash enables implementations to in corporate a plaintext into the transcript without having to store the whole MLSPlaintextOpAuthData structure. </ins> When a new one-member group is created (which requires no GroupOperation), the "transcript_hash" field is set to an all-zero

Discussion:
This PR addresses two issues that were noted in draft-05, one that is definitely a bug, and one that should improve security analysis. In draft-05, a circular dependency was introduced, wherein the confirmation MAC was calculated from the transcript, which includes the GroupOperation, which includes the confirmation MAC. Here we move the confirmation MAC up to the MLSPlaintext object to avoid this circularity. In all prior versions, the transcript covered only the GroupOperation objects, not the sender metadata or signatures. Here we cover the whole MLSPlaintext object, but in "staggered" form -- the confirmation MAC and signature aren't entered into the transcript until the next epoch.
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