Re: [MMUSIC] 4572 update: forbid weak hashes?

On 4/7/16 10:06 AM, Martin Thomson wrote:
> On 7 April 2016 at 10:35, Christer Holmberg
> <christer.holmberg@ericsson.com> wrote:
>>> If we do this, it removes some of the questions about “do you need to verify a fingerprint for every >hash algorithm, or only one.”
>>
>> What about saying that one MUST match on the strongest hash received?
>
> Maybe three pieces of advice are right.
>
> 1. Implement the strongest hash you can
> 2. Check all the hashes you can

If you check the strongest hash you can and it succeeds, then what is 
the point of checking any others? The one check should be sufficient if 
there is only one stream. It is a bit more complex if there are multiple 
streams (e.g., RTP and RTCP).

How about:

1) implement as many as you can of hashes you consider strong enough.

2) for all the streams you are originating:
    include fingerprints using all the hashes you support.
    (Or is it ok to include fewer?)

3) for streams you are receiving:

3.1) discard any fingerprints using hashes you don't support,
      and any that you consider too weak to be trusted.

3.2) sort the remaining fingerprints, strongest hashes first.

3.3) for each stream, check fingerprints from the list in order
      until you find one that matches.
      Reject the session if none match.

	Thanks,
	Paul

> 3. Don't accept a session if you can only check weak hashes
>
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