Re: [Suit] Draft-ietf-suit-manifest encryption use

[Brendan Moran <[e-mail]>]

Hi Russ,

> I thought we talked about moving the whole COSE_Encrypt structure so that it was not covered by the signature.  That would allow a party in the distribution path to change the COSE_Recipients without damage to the signature.  Since we a re using detached payload, the implementation needs to remember the resulting CEK.  Which is still needed in your proposal, I believe.

I realise that I wasn’t explicit about this.

> 2. The manifest references encryption information by URI. The typical approach is to place the encryption info in the SUIT_Envelope, then reference it by a numeric reference.

This was meant to indicate that COSE_Encrypt is placed in the SUIT_Envelope, which is not covered by signature. I’ll work up an example ASAP.

Brendan


>
>> On Jun 2, 2021, at 5:59 AM, Brendan Moran <[e-mail]> wrote:
>>
>> During the virtual interim, we raised the point that the COSE_Recipients for a COSE_Encrypt should not be covered by a signature or digest. This prevents a management system from sending each recipient only the COSE_Recipient structure that pertains to it. This is not ideal for the structure of the manifest.
>>
>> I can see several ways forward:
>> 1. Key agreement is explicitly out-of-band. The manifest uses COSE_Encrypt0 exclusively. No changes are needed to the manifest. The kid header parameter is used to distinguish between keys for different payloads.
>>
>> 2. The manifest references encryption information by URI. The typical approach is to place the encryption info in the SUIT_Envelope, then reference it by a numeric reference. (e.g. 12 for key 12 in the current SUIT_Envelope). This approach permits the distributor to edit the COSE_Recipients, which allows a firmware author to include all recipients. The distributor can then remove all but the intended recipient. Federated distributors are also possible, where the COSE_Recipients is reduced at each level of distribution.
>>
>> 3. Break COSE’s existing conventions: set COSE_Recipients to nil in order to represent that COSE_Recipients is detached. This is problematic for two reasons: first, it means that we break compatibility with existing COSE libraries, since they will not expect a detached COSE_Recipients; second, it leaves no way to indicate where to find COSE_Recipients. Instead of ’nil’ we could use an int.
>>
>> I think we should probably discard Option 3. I worry that Option 2 exposes a number of options for tampering with the COSE_Encrypt. It also means that the parser has to advance past the manifest in order to locate the COSE_Encrypt blocks. The envelope should not contain an enormous number of elements, so it may be acceptable to simply hold a table in memory of the key, start, end of each element of the envelope.
>>
>> We could enable both 1 and 2 by changing the current SUIT Parameter:
>> ORIGINAL:
>>       SUIT_Encryption_Info = COSE_Encrypt_Tagged/COSE_Encrypt0_Tagged
>> PROPOSED:
>>       SUIT_Encryption_Info = int / COSE_Encrypt_Tagged/COSE_Encrypt0_Tagged
>>
>>
>> Alternatively, we could enable both 1 and 2 by adding a new parameter:
>>
>> SUIT_Parameters //= (suit-parameter-encryption-ref
>>   => int)
>>
>> Best Regards,
>> Brendan
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