Re: [lamps] WGLC comments draft-ietf-lamps-cms-shakes-01

[Russ Housley <[e-mail]>]

> On Sep 5, 2018, at 12:47 PM, Jim Schaad <[e-mail]> wrote:
> 
> * Message Authentication Codes - Do we need to address a potential request
> from some of the IOT worlds such as SUIT where they want to use KMAC, but
> would be happy with a smaller authentication value?
>  
> Comment 6: Shorter tags are generally ok depending on each application: what are the potential attacks/issues and how much data are MACed under a random key. 
>  
> How small would you like for SUIT ? What is the maximum amount of data that will ever be MACed under ONE random key ?
>  
> This is for CMS, I don't think there is a need to reduce the size. But I could be wrong about this.  
> [JLS] One of the message formats that SUIT was using the last time I checked was CMS.  This means that if SUIT wants it then yes it would be for CMS.  Russ – do you think that they are going to want a “truncated” tag for KMAC?

SUIT will focus on COSE structures, but the charter allows a rfc4108bis.

So far, size in SUIT has not been a sticking point.  When the signature value or authentication tag is compared to a software image, a few extra octets is not a show stopper.

Russ