Re: Group 15 needed in draft-baushke-ssh-dh-group-sha2

If that is the default, it would ease to deprecate what we want to deprecate. And I agree its status should be indicated in the draft.

BR, 
Daniel


-----Original Message-----
From: Watson Ladd [mailto:[email protected]] 
Sent: Wednesday, August 17, 2016 10:29 AM
To: Daniel Migault <[email protected]>
Cc: denis bider (Bitvise) <[email protected]>; Curdle <[email protected]>; [email protected]; [email protected]; Mark D. Baushke <[email protected]>
Subject: Re: [Curdle] Group 15 needed in draft-baushke-ssh-dh-group-sha2

On Wed, Aug 17, 2016 at 7:15 AM, Daniel Migault <[email protected]> wrote:
> Hi,
>
>
>
>
>
> Going briefly through the draft, it seems redundant to have 
> MAY/OPTIONAL, SHOULD/RECOMMENDED, MAY/OPTIONAL.  I am not sure this 
> does not result in a combination of recommendation for the users as 
> well as a recommendation on algorithms to implement. I would recommend 
> we only focus on requirements for algorithm implementation. We should 
> also specify that all non specified algorithms in this document are “MAY”.

There must be a single algorithm that everyone is required to implement. Make it P256 SHA2. Do not make it P384. There are no efficient constant time implementations. (I have written a constant time P384 this summer at Mozilla: it is not that fast). By contrast OpenSSL has a constant time P256 implementation (possibly not used by
default: the rumors are unclear. Can you guys go ahead and ensure that it is used by default?)

That's on top of everyone uses P256+SHA256.

>
>
>
> I have not found any recommendations for these algorithms at the IANA 
> web page [1], nor in another document. If that is the case, maybe this 
> document should clarify this so a status can be assigned for each kex. 
> The IANA page also does not mention all algorithm and I have not found 
> documentation for all suites found in the manual.
>
>
>
> Updating the different algorithm should consider two aspects: security 
> and interoperability, which means a SHOULD NOT status is expected to 
> be done for a cipher suite with a SHOULD status. In other words going 
> from MUST to SHOULD NOT should be avoided unless there are strong 
> reasons to do so. So maybe we should do a little bit more cleanup.
>
>
>
> I do not know the current status for SSH but it would be good to end 
> up in
> group1 and SHA1 set to MUST NOT – eventually SHOULD NOT later being 
> updated to MUST NOT.
>
>
>
> SHA256 set to MUST, SHA386 to MAY and SHA512 set to SHOULD to have it 
> ready when SHA256 will be replaced.
>
>
>
> Maybe we should also consider the current use of the various group, 
> and those not widely used may be set to MAY.
>
>
>
> BR,
>
> Daniel
>
>
>
> [1]
> http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xhtml#ss
> h-parameters-16
>
>
>
>
>
> From: Curdle [mailto:[email protected]] On Behalf Of denis bider
> (Bitvise)
> Sent: Tuesday, August 16, 2016 5:19 PM
> To: Curdle <[email protected]>
> Cc: [email protected]; [email protected]; Mark D. Baushke 
> <[email protected]>
> Subject: [Curdle] Group 15 needed in draft-baushke-ssh-dh-group-sha2
>
>
>
> Hello everyone,
>
>
>
> this comment is with respect to the following draft specifying new 
> Diffie-Hellman groups for SSH key exchange:
>
>
>
> https://tools.ietf.org/html/draft-ietf-curdle-ssh-kex-sha2-03
>
>
>
> The current version of the draft specifies the following:
>
>
>
>   diffie-hellman-group14-sha256     MAY/OPTIONAL
>
>   diffie-hellman-group16-sha512     SHOULD/RECOMMENDED
>
>   diffie-hellman-group18-sha512     MAY/OPTIONAL
>
> A previous version of this draft specified the following methods:
>
>
>
> https://tools.ietf.org/html/draft-baushke-ssh-dh-group-sha2-03
>
>
>
>   diffie-hellman-group14-sha256     MAY/OPTIONAL
>
>   diffie-hellman-group15-sha512     MUST/REQUIRED/SHALL
>
>   diffie-hellman-group16-sha512     SHOULD/RECOMMENDED
>
>   diffie-hellman-group17-sha512     MAY/OPTIONAL
>
>   diffie-hellman-group18-sha512     MAY/OPTIONAL
>
> Note the presence of additional groups 15 and 17 which were removed in 
> version 4 of the original Baushke draft.
>
>
>
> Groups 15 and 17 were removed based on feedback from one implementer.
> Basically, this feedback was one line:
>
>
>
>> +1 to dropping the odd-numbered groups and onlist listing 
>> +group14/16/18
>
>
>
> I would like to counter this, and move to restore the previous table 
> including groups 15 and 17 - or failing that, at least group 15 - with 
> the same parameters as above, in version 3 of the original Baushke draft.
>
>
>
> My reasons for proposing this are as follows:
>
>
>
> - According to NSA recommendations, the 3072-bit strength would be the 
> current sweet spot between performance and acceptable security. Group 
> 15 is 3072-bit, whereas groups 14 and 16 are 2048- and 4096-bit.
>
>
>
> - The additional security of group 16 in comparison to group 15 is 
> estimated to be small. Symmetric security estimates I've seen are 80 
> bits for group 1 (1024-bit), 112 bits for group 14 (2048-bit), and 128 
> bits for group 15 (3072-bit). Based on this, I expect the security of 
> group 16 (4096-bit) to be between 136 - 144 symmetric bits.
>
>
>
> - Based on practical measurements, it appears that group 16 is about a 
> factor of 2 slower than group 15. With group 15, I'm getting about 20 
> full DH key exchanges per second; with group 16, I am getting around 
> 10. I think this difference is significant, and can affect real world 
> usage scenarios on heavily loaded servers.
>
>
>
> At this time, I do not have a particular need for group 17 (or 18), 
> but I find it peculiar that this draft would not specify a group that 
> matches the exact recommended DH group size suggested by the NSA. It 
> is weird that we have to choose either between group 14, which does 
> not meet the requirements; or group 16, which is significantly slower.
>
>
>
> For our next Bitvise SSH Server and Client versions, I have 
> implemented support for groups 15 as well as 16, where group 15 is 
> implemented with SHA-512, as specified above. When using DH key 
> exchange, our SSH Server will favor group 15, whereas group 16 will be 
> disabled by default for performance (but it will be enabled and preferred in the SSH Client).
>
>
>
> denis
>
>
>
>
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>



--
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--Rousseau.
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