Re: [Cfrg] request for review of IPsec ESP and AH Usage Guidance

"Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu> Tue, 02 July 2013 15:58 UTC

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From: "Blumenthal, Uri - 0558 - MITLL" <uri@ll.mit.edu>
To: Paul Hoffman <paul.hoffman@vpnc.org>, Yoav Nir <ynir@checkpoint.com>
Date: Tue, 2 Jul 2013 11:57:46 -0400
Thread-Topic: [Cfrg] request for review of IPsec ESP and AH Usage Guidance
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Subject: Re: [Cfrg] request for review of IPsec ESP and AH Usage Guidance
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On 7/2/13 11:33 , "Paul Hoffman" <paul.hoffman@vpnc.org>; wrote:


>On Jul 2, 2013, at 8:18 AM, Yoav Nir <ynir@checkpoint.com>; wrote:
>
>> - I'm concerned that the only encryption algorithm is AES. Yes, I see
>>that TripleDES-CBC as a MAY, but that is by now the past. AES-128-CBC is
>>9 times the speed of 3DES (on an Intel platform without AES-NI based on
>>"openssl speed"), and with AES-NI the ratio is likely to jump to 20.
>>With GCM it's even more pronounced. So 3DES cannot be a reasonable
>>alternative to AES. I think we should have some alternative that is at
>>least at the SHOULD level.
>
>...and yet no alternative seemed reasonable enough for you to suggest.
>:-) Should we either (a) delay this document until there is a
>widely-agreed-on alternative that is better than 3DES or (b) pick
>something now that is not widely-agreed-on and try to promote it? Neither
>seems like a good option to me.

IMHO, at this point there's no alternative that is either necessary or
useful. Therefore I propose (c): leave this part alone and go on. If AES
does get broken - we'll have a bigger problem on our hands.

If patent issues aren't of concern - I would consider adding AES-*-OCB
mode as either SHOULD or at least MAY.


>>- I'm not sure what the point is of the MAY level. We MAY implement
>>anything: SEED, Camellia, GOST. That doesn't help with interoperability
>
>Documenting at least one MAY-level algorithm shows that an implementation
>must not assume that there is only one code point that it will need to
>ever care about.

As you pointed out, we MAY implement anything. If you absolutely insist
that an algorithm or two MUST be mentioned in the MAY clause (:-), why not
one or two of the AES competition finalists (those that did not have any
attacks published)?


> 
>>- I'm not sure about AES-GMAC for ESP authentication. Is there a reason
>>why someone would prefer to use AES-CBC or AES-CTR with AES-GMAC rather
>>than AES-GCM? Also, the HMAC-SHA256 algorithm has gained popularity
>>recently (meaning that a lot of customers are asking for it). It runs
>>significantly slower than HMAC-SHA1, but people have stopped reading at
>>"SHA-1 is no longer secure". Still, they're not asking for GMAC, they're
>>asking for SHA-256. So I think a document where the goal is
>>interoperability should focus on what is becoming the de-facto standard
>>as long as it's secure enough.
>
>Having the document list the rationale for using GMAC instead of an HMAC
>would indeed be good.

One reason to use AES-GMAC for ESP authentication is that ESP supports all
of the following: (a) don't encrypt but authenticate, (b) authenticate but
don't encrypt, (c) encrypt and authenticate. GMAC fits the (a) case
perfectly. In the past, people unfamiliar with how ESP and AH evolved, had
a misconception that when you need  authentication you use AH, when you
need encryption you use ESP, and when you need both you combine the two
(ESP+AH). IMHO it was mainly because of their ignorance of the main reason
behind creation of AH - the EXPORT CONTROL (of that time!) requirement for
an authentication-only mode that could not have encryption retrofitted in
in.

I see no reason to use GMAC (or any other MAC) over encrypted (e.g., with
AES-CBC) data: if you encrypt - there's no reason not to use AE/AEAD mode.
If you don't encrypt - use GMAC. When given a choice between GMAC and
HMAC-SHA256, the decision should be obvious (and not in favor of SHA :).



More detailed review to follow. :-)

TNX!