[Sframe] Clarifying question about keying SFrame from MLS

["Mularczyk, Marta" <[e-mail]>]

Hi all,

I have a clarifying question about section 5.2 of the draft on keying SFrame from MLS.

The paragraph "To generate keys and nonces for SFrame[...]" explains how to export a base_key. But it does not explain how to compute a "sender_base_key" (like in section 5.1) for a specific KID (a KID includes the sender index), which is what we actually need. How is the sender_base_key for MLS computed?

The way I intuitively expected it to work was that a KID = (epoch_id, sender_index, context) corresponds to a sender_base_key equal to MLS-Exporter("SFrame 1.0 Base Key" + sender_index + context, "", AEAD.Nk) exported from the MLS epoch number epoch_id. Am I missing something?

Some more minor inconsistencies in the paragraph "Once an SFrame stack has been provisioned [...]":

  *   sframe_epoch_secret should probably be the base_key?
  *   sender_base_key is mentioned but it is not explained how it's computed

Thank you for your help!
Marta