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

Jonathan Lennox <jonathan@vidyo.com> Thu, 07 April 2016 13:45 UTC

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From: Jonathan Lennox <jonathan@vidyo.com>
To: Christer Holmberg <christer.holmberg@ericsson.com>
Thread-Topic: 4572 update: forbid weak hashes?
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Date: Thu, 07 Apr 2016 13:45:09 +0000
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Subject: Re: [MMUSIC] 4572 update: forbid weak hashes?
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> On Apr 7, 2016, at 10:35 AM, Christer Holmberg <christer.holmberg@ericsson.com> wrote:
> 
> Hi Jonathan,
> 
>> For the RFC 4572 update, I think we should add that receivers MUST NOT match on fingerprints >computed with weak hashes, and senders SHOULD NOT send them.
>> 
>> 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? 
> 
> Then, if you only receive a weak hash, isn't it still better to match than doing nothing?

Not if the hash is so weak it’s not secure, and a plausible implementation wouldn’t be using it.  Such an offer might be a bid-down attack of some sort.  You’re better off rejecting the stream.

>> Weak hashes definitely include MD2 and MD5.  I guess we still want SHA-1 to be supported, though?
> 
> I'll let others decide on whether we still want to allow SHA-1, but the suggestion is to make SHA-256 the preferred one.

Oh, absolutely, no question.  But since 4572 preferred SHA-1, it probably needs to stay supported for some time.

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