Re: Last Call: <draft-herzog-static-ecdh-04.txt> (Use of static-static Elliptic-Curve Diffie-Hellman key agreement in Cryptographic Message Syntax) to Informational RFC

[Rene Struik <[e-mail]>]

Dear Jonathan:

Thanks for our phone call yesterday afternoon (Thu March 10th) and your
summary below.

Please see my further comments below on your feedback on my review
comments (T-h), (T-i), (T-k), and (T-l),
a) where you labelled (T-h) and (T-i) as "overtaken by events";
b) where (T-k) refers to CTR mode;
c) where (T-l) refers to speed-up support.

I have some security concerns re #a, a question re #b, and a remark re
#c. If lack of time, at least try and address the security concerns.

Please feel free to discuss.

Have a nice weekend (well, it probably already started).

Best regards, Rene


On 10/03/2011 5:50 PM, Herzog, Jonathan - 0668 - MITLL wrote:
> Just to keep everyone informed: Dr. Struik and I spoke by phone earlier today about his comments. My recollection of the conversation is that he accepted most of the comments as resolved, modulo the following additional details:
>
> (And Dr. Struik! One of our agreements has been overtaken by events! Please see below.)
>
>
> On Mar 7, 2011, at 1:51 PM, Herzog, Jonathan - 0668 - MITLL wrote:
>
>> On Mar 3, 2011, at 12:37 PM, Russ Housley wrote:
>>
>>>> (E-e) p. 4, l. -5: The motivation for specifying ECDH seems to be not so much that ECMQV is around (but having problems, as you stated), 
>>>> but that static-static-ECDH is *not* 
>>>> around. Thus, specifying static-static-ECDH adds more options to the solution space.
>> I'm not sure how to interpret this. Is this just a general comment, or are you suggesting some change to the draft? if the later, can you say a little more about the change you would like to see?
> This comment got folded into another comment about our motivation. The new version dispenses with motivations (1) and (3), focusing on the fact that ECMQV is no longer in Suite B. Therefore, some participants will not be able to use ECMQV for policy reasons, and may also be unable to sign their messages for lack of a certified signature key. This Draft presents static-static ECDH as an alternative mechanism for achieving source authentication if they should happen to have a certified ECDH key.
>
>
>
>
>>>> (T-g) p. 6, Clause 2.2, l. -5: At first, it is suggested that the sending agent obtains the recipient's public key somehow (e.g., from its 
>>>> certificate), thus suggesting that certificates are not the only option by which the public key may be obtained. However, later on it is stated 
>>>> that "it confirms that both certificates [...]", thus suggesting that each of the parties involved in this message flow have public keys that
>>>> are certified and that only those can be used. This is confusing.
>> But you are right: the use of 'e.g.' is a bit confusing. To resolve this confusion, I propose to change Section 2.2 from
>>
>>   When using static-static ECDH with EnvelopedData, the sending agent
>>   first obtains the recipient's EC public key and domain parameters
>>   (e.g. from the recipient's certificate). 
>>
>> to
>>
>>          When using static-static ECDH with EnvelopedData, the
>> 	  sending agent first obtains the EC public key(s) and domain
>> 	  parameters contained in the recipient's certificate.
>>
>> and Section 2.3 from 
>>
>>   The receiver then retrieves the static EC public key identified in
>>   the rid field.
>>
>> to
>>
>> 	  The receiver then retrieves the sender's certificate
>> 	  identified in the rid field.
>
> This last sentence now reads:
>
>    The receiver then retrieves the sender's certificate identified in
>    the rid field, and extracts the EC public key(s) and domain
>    parameters contained therein. 
>
>
>
>>>> (T-h) p. 6, Clause 2.2, l. -6 ff: Given the lack of shall/should/may language, it is unclear whether one stipulates that one
>>>> checks that public keys in the certificate are on a specific curve (i.e., one does public key validation) or something more relaxed (such as checking
>>>> that the claimed elliptic curve domain parameters are the same, without checking the public keys themselves. The para would benefit from some 
>>>> firmed-up language here. This should also clarify whether one, in fact, checks the validity of the certificate that included the public key
>>
>> Good points. The language of this draft was based on that in Section 3.1.2 of RFC 3278, but it could be firmed up. 
>>
>> With regard to parameter validation, SEC1 (Section 3.2.2) lists a few methods by which a public-key can be checked for valid parameters:
>>
>> * Full check,
>> * Partial check, and
>> * Trust the CA. 
>>
>> (I'm paraphrasing a bit.) Since RFC 5480 doesn't provide any way for the CA to mark the parameters as 'checked' or 'not checked', I'll have our Draft say that the sender and receiver:
>>
>> * SHOULD do a full parameter check for standard ECDH, and
>> * SHOULD do a full check for co-factor ECDH, or failing that, SHOULD do a partial check (as seems to be permitted in SEC1, Section 3.2.3).
>
> ***** Dr. Struik! This has been overtaken by events! ************
>
> Due to IPR concerns, I have removed these checks from the draft. The relevant sections now read:
>
> Section 2.2:
>
>    When using static-static ECDH with EnvelopedData, the sending agent
>    first obtains the EC public key(s) and domain parameters contained in
>    the recipient's certificate.  It MUST confirm the following at least
>    once per recipient-certificate:
>
>    o  That both certificates (the recipient's certificate and its own)
>       contain public-key values with the same curve parameters, and
>
>    o  That both of these public-key values are marked as appropriate for
>       ECDH (that is, marked with algorithm-identifiers id-ecPublicKey or
>       id-ecDH [RFC5480]).
>
RS>>
[First a disclaimer: I have no stake in the ground here, except
advocating good security practices]

Fair enough, as long as this does not make the security considerations
sections null and void.

However, I am a little bit puzzled here, since this does not seem to
address any of the ambiguities noted in my comment (T-h). After all,
the only change to the text of Section 2.2 of
draft-herzog-static-ecdh-05 you suggest seems to be to replace the phrase
"It confirms that" by "It MUST confirm the following at least once per
recipient-certificate", without any further changes.

This does *not* address, since it is entirely unclear whether
a) one checks that public keys in the certificate are on a specific
curve (i.e., one does public key validation)
or
b) something more relaxed (such as checking that the claimed elliptic
curve domain parameters are the same, without checking the public keys
themselves;
c) whether one, in fact, checks the validity of the certificate that
included the public key

Ad #c:
Without checking the validity of certificates (item #c) {by performing a
cryptographic signature verification operation at least once}, one might
as well do away with certificates altogether, since no implicit key
authentication assurances can be obtained.

Ad #b:
By just checking that the certificate has a substring indicating the
purported domain parameters of the other entity's public key (item #b),
one does not seem to have any assurance that that public key is, in
fact, indeed of the proper form, i.e., on the curve and a generator of
the prime order cyclic subgroup of the curve indicated by the domain
parameters (unless the CA did those checks and by issuing the cert
attests to this and one indeed verifies the certificate itself
cryptographically).

Ad #a:
Without checking whether the public keys exchanged by both entities in
the protocol are on the same curve, one opens oneself up to a plethora
of potential attacks (small subgroup attack, invalid point attack, etc.
-- all well-documented in the cryptographic literature and also
referenced in the Security Consideration section).

It would help if you could comment on the lingering ambiguities I noted
in my original comment T-h (elaborated upon above with #a, #b, and #c)
and which, unless I misunderstand, are not taken away by your suggested
resolution. Further, it would help if you could indicate whether the
intention is to publish the draft with sufficient safeguards so as not
to succumb to well-documented vulnerabilities.

<<RS

> Section 2.3:
>
>
>    The receiver then retrieves the sender's certificate identified in
>    the rid field, and extracts the EC public key(s) and domain
>    parameters contained therein.  It MUST confirm the following at least
>    once per sender-certificate:
>
>    o  That both certificates (the sender's certificate and its own)
>       contain public-key values with the same curve parameters, and
>
>    o  That both of these public-key values are marked as appropriate for
>       ECDH (that is, marked with algorithm-identifiers id-ecPublicKey or
>       id-ecDH [RFC5480]).
>
>
>
RS>>
Same concern as with (T-h) above.
<<RS
>>>> (T-k) p. 11, Clause 6, l. 3 (also l. 15): Why not introduce the CTR encryption mode as an option, at least when authenticity is provided? 
>>>> After all, CTR mode allows implementation of block-ciphers with just the forward encryption mode and offers parallelization and precomputation 
>>>> prospects.
>> I left it out because I have serious reservations about the security of counter mode. But in looking in to your question, I see there's an even-more serious problem: I can't find an RFC for AES-in-counter-mode for CMS. Perhaps, though, my Google-foo is insufficient. Do you have a pointer to an appropriate RFC?
> Neither Dr. Struik nor I could find OIDs for AES in counter mode, and so they remain absent from the Draft.
>
>
RS>>
Okay - if OIDs are the only "stairway to heaven".

Question:
Shouldn't we put together an I-D that specifies the CTR mode and on OID
for this??? It would be a shame not to have this available (or could we
refer to NIST SP 800-38A and does NIST have an OID for this???).
Any thoughts?

<<RS
>
>
>>>> (T-l) General: When static-static ECDH, as specified here, stipulates checking of the certificate including the public key and that certificate is
>>>> an ECDSA certificate, significant speed-ups of the computations are possible by combining the key computation step and ECDSA signature verification
>>>> -- cf. 
>>>> http://www.ietf.org/proceedings/78/slides/saag-7.pdf.
>>>> or the SAC 2010 paper referenced in that IETF-78 presentation. These results also apply here
>>>> (and can obviously be ignored or embraced depending upon implementation). I would suggest adding a one-line statement that if ECDSA is used, one shall 
>>>> use the "friendly ECDSA" scheme as in the IETF-78 presentation (which has the same format as the ordinary one).
>
> I told Dr. Struik that I preferred to leave this out of the draft, and he (I believe) agreed.
>
>
RS>>
We can indeed deal with fostering speed-ups separately (as long as this
is not pushed in a cob-webbed corner!). The interesting thing is that
implementers of the draft could still move towards these "Friendly
ECDSA" techniques, without violating the current draft, so the door is
not completely closed on that one.
<<RS

> Thanks all.
>
>


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