[Suit] Suit manifest with variable recipients

[Brendan Moran <[e-mail]>]

At the virtual interim, we discussed the possibility of altering the recipients (e.g. pruning some during distribution) of a COSE_Encrypt structure in a manifest. In the current structure, a signed digest is calculated over the COSE_Encrypt, which will means any change to the recipient list will cause a change in digest, meaning that the signature of the manifest will not match.

The result is that COSE_Recipients MUST be held outside of the manifest’s chain of trust. This complicates matters somewhat. It would be better if most of the COSE_Encrypt was held under the trust umbrella while the COSE_Recipients were held outside the umbrella. However, COSE does not support detached Recipient lists. This may be a failing of COSE, and perhaps it should be discussed there. In the interim, however, SUIT needs to move forward with some solution to deploying variable recipient lists.

I can see several options:


Option 1: Place the COSE_Encrypt outside the manifest, reference it by URI (remote) or Integer (envelope)
Modify suit-parameter-encryption-info to take a tstr (URI) or integer (Envelope Reference)

This means that all devices supporting the encryption-info parameter MUST support both integrated, detached local, and detached remote modes. I don’t think this is really in the spirit of SUIT.


Option 2: Place the COSE_Encrypt outside the manifest, reference it by Integer (envelope)
Add a new suit-parameter-encryption-info-detached that takes the integer (Envelope Reference)

This has the benefit of being unambiguous. However, it creates a new problem: what happens if both the suit-parameter-encryption-info-detached and the suit-parameter-encryption-info are set? Does there need to be a rule that one explicitly unsets the other? How does that work with permissions?


Option 3: Abuse COSE slightly: Place a COSE_Encrypt0 in the manifest. Place a list of recipients outside the manifest.

This has the benefit of being what we actually want, but it probably won’t play nice with COSE libraries.




On balance, I think we should probably go with option 2. I recommend that we add the following CDDL


SUIT_Parameters //= (
  suit-parameter-encryption-info-detached => suit-encryption-info-detached-key
)

$$SUIT_Envelope_Extensions //=  (
  suit-encryption-info-detached-key => bstr .cbor COSE_Encrypt
)

suit-encryption-info-detached-key = nint / uint .ge 24


This provides an explicit parameter to use when placing the COSE_Encrypt in the envelope. It’s unambiguous and should be reasonably easy to implement. This could be added to either draft-ietf-suit-manifest or draft-ietf-suit-firmware-encryption.

Best Regards,
Brendan
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