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

I just wanted to re-iterate the question. We have two options:

1. If m= line has multiple fingerprints with hash functions X, Y, Z, at
least one fingerprint with each function must match DTLS association for
the media to be accepted. For example, if m= line has two fingerprints with
hash function SHA-1, and two with SHA-512, DTLS association certificate
must match at least one SHA-1 and one SHA-512 fingerprint.

2. If m= line has multiple fingerprints with hash functions X, Y, Z, at
least one fingerprint with any hash function which is considered secure
enough must match DTLS association for the media to be accepted. For
example, if m= line has two fingerprints with hash function SHA-1, and two
with SHA-512, DTLS association certificate must match one of either SHA-1
or SHA-512 fingerprints.

Option 1 has a benefit of being more secure since it provides the combined
secure of individual hash functions. It does put additional restrictions
how equipment can be combined into producing a single m= line. For instance
if fingerprints from two servers are combined into a single m= line and it
is not known in advance which one will be used, both servers will need to
always support the same hash functions and be updated at the same time.

Option 2 is less secure since it is only as secure as the least secure
hash. On the other hand option 2 allows to be more flexible in how
equipment is combined to form the single m= line. Two servers in above
example, for instance, can support different hash functions.

I would ask the group to state their preferences.

I prefer option 2 but can live with option 1.

Regards,

_____________
Roman Shpount

On Fri, Apr 8, 2016 at 11:08 AM, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:

> I don't have a horse in this race. I think those of you who do need to
> finish deciding what the rules are for multiple fingerprints, and then
> write the result into the draft update. Clearly it is ambiguous at the
> moment.
>
>         Thanks,
>         Paul
>
> On 4/7/16 5:37 PM, Cullen Jennings wrote:
>
>>
>> On Apr 7, 2016, at 11:20 AM, Roman Shpount <roman@telurix.com
>>> <mailto:roman@telurix.com>> wrote:
>>>
>>> You should check all the hashes you consider secure and if any one of
>>> them matches the DTLS association certificate, accept the media.
>>>
>>
>> +1
>>
>> This allows us to do things like include hash for two servers even when
>> you re not sure which server will actually be used.
>>
>>
>>
>>
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