Re: [Suit] COSE_Sign vs. COSE_Mac

[Brendan Moran <[e-mail]>]

Hi Russ,
It depends on the size of the payload. When we’re talking about whole-image updates, it’s completely reasonable, however that won’t always be the case. If we have diff images, they can be quite small, depending on the scope of the change. A 2KiB signature on a 2KiB compressed diff may be a deal-breaker on low-bandwidth networks.

Brendan

> On 4 Jun 2018, at 17:52, Russ Housley <[e-mail]> wrote:
>
> Brendan:
>
> That is a significant key management burden.  I would like to explore the assertion that hash-based signatures have too much data overhead.  Are you saying that they can buffer the firmware load, but not the signature value?
>
> Russ
>
>
>> On Jun 4, 2018, at 11:24 AM, Brendan Moran <[e-mail]> wrote:
>>
>> This is a solution that is intended to address a segment of devices that cannot use public-key due to code size or processing overhead, and cannot use hash-based signatures due to data overhead. Provided that every device has a unique PSK, COSE_Mac provides adequate security. However, it introduces significant key management overhead.
>>
>> Thanks,
>> Brendan
>>
>>> On 4 Jun 2018, at 16:16, Russ Housley <[e-mail]> wrote:
>>>
>>> I am struggling to see a case where COSE_Mac provides acceptable protection.  With a MAC, the sender and the recipient must use the same key.  This means that any recipient can make a modification and produce a valid MAC for the modified object.
>>>
>>> With a digital signature, the sender uses a private key, and the recipients use a public key, so the recipients cannot produce modified objects.
>>>
>>> Why accept this possibility?
>>>
>>> Russ
>>>
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>>
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>

IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.