Re: [IPsec] NUDGE: WG Last Call for draft-ietf-ipsecme-dh-checks

Andrey Jivsov <openpgp@brainhub.org> Tue, 09 April 2013 00:20 UTC

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Date: Mon, 08 Apr 2013 17:17:43 -0700
From: Andrey Jivsov <openpgp@brainhub.org>
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To: "Scott Fluhrer (sfluhrer)" <sfluhrer@cisco.com>
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Cc: "ipsec@ietf.org" <ipsec@ietf.org>
Subject: Re: [IPsec] NUDGE: WG Last Call for draft-ietf-ipsecme-dh-checks
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Sorry, I was scanning too quickly.

I meant section 2.1 primes, the primes in the form (renaming my earlier 
definition) p = k*2+1.

Wouldn't receiving g^k be as much of a problem as receiving 1 for the 
public value? No matter what my private exponent is, the shared secret 
will be either of the two values.

On 04/08/2013 04:15 PM, Scott Fluhrer (sfluhrer) wrote:
>
>
>> -----Original Message-----
>> From: ipsec-bounces@ietf.org [mailto:ipsec-bounces@ietf.org] On Behalf
>> Of Andrey Jivsov
>> Sent: Monday, April 08, 2013 6:46 PM
>> To: ipsec@ietf.org
>> Subject: Re: [IPsec] NUDGE: WG Last Call for draft-ietf-ipsecme-dh-checks
>>
>> Sec 2.2:
>>> It MUST check both that the peer's public value is in range (1 < r
>>>        < p-1) and that r**q = 1 mod p (where q is the size of the
>>>        subgroup, as listed in the RFC).
>>
>> Would it make sense to specify a more economical test for strong prime
>> groups?
>
> "Strong groups", that is, groups with (p-1)/2 prime, are listed in section 2.1; and yes, the test there is considerably cheaper.
>
>>
>> If q is meant to be p = q*2+1, there are only two possibilities for the value <
>> p-1 received from the peer to be in the wrong subgroup. One of them is 1,
>> which is ruled out by the check above. Another one is g^q.
>> It's a fixed quantity for the given modp group. Seems like a memcmp with a
>> fixed quantity g^q is the best way to address the problem.
>
> Actually, g^q == 1; I don't think that's what you mean.
>
> Now, there is certainly the possibility of the value being in the wrong subgroup; but there are far more than two possibilities.  Here's the entire list for strong groups:
>
> 1; that's rejected by the test in section 2.1
> p-1; that's also rejected by the test in section 2.1
> primitive elements; those are elements r which have order p-1.  These are not rejected by the test.
>
> (in addition, there are KE values that don't correspond to actual group elements; 0 and values >= p; those are rejected too).
>
> Now, there are q-1 different primitive elements; that's more than we could reasonably list.  We could specify a test to reject primitive elements; however, that test isn't cheap (it can be done cheaper than the full r**q==1 test, nevertheless, not cheaply.  In addition, an attacker injecting a primitive element could use it to deduce the lsbit of the private exponent; however that cannot deduce any more than that.  I don't believe that the expense of the full test is worth protecting one bit of the exponent.
>
>>
>> On 04/08/2013 02:46 PM, Paul Hoffman wrote:
>>> [[ So far, we have received only *one* review of this document, from
>>> Tero. If we don't receive more reviews, the document might not
>>> progress due to lack of interest. Please review this document within
>>> the next week and contribute your review to the list. ]]
>>>
>>> Greetings. This is the start of the WG Last Call for
>>> draft-ietf-ipsecme-dh-checks; the WG period will end in two weeks, on
>>> April 15. The current draft is available at
>>> http://tools.ietf.org/html/draft-ietf-ipsecme-dh-checks-01
>>>
>>> Given that this will be a Standards Track document, it is important for it to
>> be reviewed by as many people as possible. Possible results of individual
>> reviewing the document are:
>>>
>>> - "Looks fine, please publish"
>>>
>>> - "Looks fine, here are some comments"
>>>
>>> - "Has some problems, here they are"
>>>
>>> - Other things of that sort
>>>
>>> Many people on this mailing list are IPsec implementers but are mostly or
>> completely silent on the mailing list. If you are one of those people, doing a
>> WG Last Call review is a good way to participate usefully in the WG. Please
>> strongly consider (a) reading the current draft and (b) sending a message to
>> the list with your short or long review. If there are too few reviews on this
>> document, we could get pushback from the IESG about the document.
>>>
>>> --Paul Hoffman
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