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

["Herzog, Jonathan - 0668 - MITLL" <[e-mail]>]

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]).

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]).




>>> (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.





>>> (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.


Thanks all.


-- 
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