Re: [COSE] Warren Kumari's No Objection on draft-ietf-cose-hash-algs-04: (with COMMENT)

[Jim Schaad <[e-mail]>]

-----Original Message-----
From: Warren Kumari via Datatracker <[e-mail]> 
Sent: Monday, June 1, 2020 6:26 AM
To: The IESG <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; Ivaylo Petrov <[e-mail]>; [e-mail]
Subject: Warren Kumari's No Objection on draft-ietf-cose-hash-algs-04: (with COMMENT)

Warren Kumari has entered the following ballot position for
draft-ietf-cose-hash-algs-04: No Objection

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COMMENT:
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Thank you for writing this - it was an interesting read.

I do have one issue, which I cannot tell if it is simply me being dumb, or if the text needs more work.

“ Doing indirect signing allows for a signature to be validated without first downloading all of the  content associated with the signature.  This capability
can be of   even greater importance in a constrained environment as not all of 
 the content signed may be needed by the device.”

I’m unclear how this works —  itseems clear enough that I can verify that the signature matches the hash, but doesn’t the device need to still download and compute the hash over all of the content?

Otherwise I could take a hash and signature from content A, and claim that it is for content B. Sure, if the signature **doens’t** match the hash I know not to bother downloading the content at all, but if the sig does match the hash I still need to download the content to check that the hash is for this content....

Please help educate me!

[JLS] I am more than willing to help educate.  You are going to be seeing some of this from the SUIT people as well.  For a firmware type update you might defined a structure that looks like:
FirmwareBlock  = Array of (
	Text description of block
	Hardware target
	Version number
	URL to the bytes of the update
	Hash algorithm identifier
	Hash of the bytes of the update

A message containing an array of these blocks is created, a COSE signed message is then created containing this array of blocks as the content of the message and sent to the piece of hardware.

The hardware can then validate the signed message, walk the array of blocks, if the block applies download the bytes pointed to by the URL, hash the bytes downloaded and compare the hash values.  If the hash values match then the bytes can be applied to the device.  This allows a device to only download the bytes that are needed, ignoring those which have either already been applied or which are not relevant to the device.

This is what I call an indirect signature, the block of data pointed to by the URL is not part of the message is implicitly signed since the hash of the data block is signed rather than the actual data.  It is interesting to note that this is the way that CMS has always worked, the hash of the message is placed in an array of attributes and that array of attributes is what is signed by the signature algorithm. 

Nit: “ A pointer to the value that was hashed.  this could” — s/this/This/

[JLS] Fixed.