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Digests, signatures and chunk extensions

From: Samuel Hurst <samuelh@rd.bbc.co.uk>
Date: Thu, 1 Dec 2022 16:04:24 +0000
Message-ID: <cfacd6ca-4a9d-5df7-4d48-1925b503bc8c@rd.bbc.co.uk>
To: HTTP Working Group <ietf-http-wg@w3.org>
Hello HTTPWG,

I have a somewhat prickly question pertaining to something that I found 
in HTTP/1.1, around chunk extensions [1]. Specifically, where it 
mentions supplying per-chunk metadata "/such as a signature or hash/". 
However, upon reading the Digest [2] and Message Signatures [3] draft, 
they don't seem to cover specifying a chunk extension to add the hashes 
and signatures on a per-chunk basis. I've been doing some digging, but 
I've not been able to find anywhere that a chunk extension for 
presenting hashes and signatures for each chunk is specified, so is this 
somewhere else that I haven't been able to find yet?

The specific use-case which I've been tasked with figuring out is 
related to low-latency MPEG-DASH streaming, where you have media 
segments containing several CMAF chunks that can be decoded without 
receiving the full media segment. Each CMAF chunk gets sent the moment 
it is complete, possibly shaving a good few seconds off your 
distribution latency. Therefore, you don't have the whole object to 
perform a digest on when you start it, and therefore no digest or 
signature in the headers. Fine, you can just put it in a trailer, or so 
I thought.

However, if an intermediary or decoding client has to wait for the 
trailer section to come in to verify the integrity and authenticity of 
the representation that it has received, it rather defeats the goals of 
low latency streaming as you have to wait for the whole representation, 
so you may as well just stick it in the regular header and not push 
per-CMAF chunks. Therefore, being able to present a digest or signature 
on each of those chunks would be highly beneficial.

Doing this in HTTP/1.1 seems easier if it can be done with the 
aforementioned chunk extensions. I'm at a total loss of how you'd do 
this in H2 or H3, so I welcome any advice there.

Best Regards,
-Sam

[1]: https://www.rfc-editor.org/rfc/rfc9112.html#name-chunk-extensions
[2]: https://datatracker.ietf.org/doc/draft-ietf-httpbis-digest-headers/
[3]: https://datatracker.ietf.org/doc/draft-ietf-httpbis-message-signatures/
Received on Thursday, 1 December 2022 16:04:40 UTC

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