Re: [Cfrg] Review of draft-irtf-cfrg-hpke-02

[Stephen Farrell <[e-mail]>]

Hiya,

On 18/12/2019 15:50, Ilari Liusvaara wrote:
> On Wed, Dec 18, 2019 at 01:39:56PM +0000, Stephen Farrell wrote:
>>
>> On 18/12/2019 13:24, John Mattsson wrote:
>>
>> Since I didn't get a substantive response on the list,
>> I'll repeat:
>>
>> "Urgh. Why do we always need to define every possible
>> option even though there's no real demand? There are
>> already far too many combinations of modes and suites
>> in hpke."
>>
>> Having more or less finished my implementation now, I
>> feel more strongly that additional combinatoric mess
>> is a bad plan. Removing options/simplifying would be
>> far better.
> 
> I do not see combinatoric mess. In fact, trying to "simplify" by
> eliminating combinations could very well _increase_ complexity due
> to some actual combinatorial mess.

I'm fine with the encoding of the suites/modes in the
HPKE context, so I'm not arguing for a single 16-bit
ciphersuite number.

But for implementation/test the cobminatorics still
kick in - with the current spec there are 96 combinations
of mode/kem/kdf/aead. The test vectors have one for
each (with one messed up I think and with confusion
as to compressed/uncompressed encoding for the NIST
curves in others).

96 options is approx 95 (or maybe 94) too many.

> 
> What can not be done is taking various axes, multiplying the
> number of points and taking the resulting product as some kind of
> metric of "complexity". In many cases, the sum is better metric than
> the product.
> 
> Remember the SSL 3.0/TLS 1.0-1.2 ciphersuite mess? That was result
> of trying to enumerate combinations. If you think it looks bad, it
> is even worse to implement... Even restricted only to recommended
> ones.
> 
> That said, I would drop NIST P-521 (as seems pretty much everybody
> is using NIST P-384 instead).
> 
> Then SHA-512 is on-the-fence: SHA-384 is likely more than strong
> enough, but SHA-384 is not strength-matched with Curve448 (assume
> P-521 gets dropped), while SHA-512 is. And having SHA-256, SHA-384
> and SHA-512 is very little additional complexity on top of SHA-256
> and SHA-384.
> 
> If one wanted combined curve/hash pairs, I would have:
> 
> NIST P-256 with SHA-256
> NIST P-384 with SHA-384
> X25519 with SHA-256
> X448 with SHA-512
> 
> However, this may be bad idea from complexity standpoint (it
> does not reduce complexity very much if at all).
> 
> Combining groups with ciphers is definitely a bad idea from
> complexity standpoint.
> 
>> We (CFRG) could punt on this by using existing IETF
>> IANA registries instead of creating new ones. There is
>> an almost fine match between what we need and some
>> existing registries (if one squints just a little;-)
>> that'd remove all this wrangling from our plate and
>> that could result in the hpke RFC having only one MUST
>> implement kem, kdf and aead. Additional options could
>> then be haggled over in the IETF each time someone
>> claims a need before subsequently being ignored by
>> almost all implementers and an even higher proportion
>> of deployments;-)
> 
> What registeries?

Ok, from here on, you'll probably want to hold your
noses;-)

HPKE currently wants to create IANA registries for the
KEM, KDF and AEAD.

I think in two of those cases we could fairly easily reuse
TLS registries for HPKE - the TLS supported groups [1] is
a nice match for the KEM, and the TLS hash algorithms [2]
would be fine for the HKDF variant.

I'm not sure if it'd be a good or bad idea to use the TLS
ciphersuites registry [3] for the AEAD - while one can
easily map those I guess the ambiguity could cause interop
issues, so maybe that'd be a bad plan. But maybe the
AEAD parameters registry [4] would be fine.

Or there may also be other IANA registries that could
be re-used for HPKE.

Cheers,
S.

[1]
https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-8
[2]
https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-18
[3]
https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4
[4] https://www.iana.org/assignments/aead-parameters/aead-parameters.xhtml



> 
> For groups there is the TLS groups, which actually has only four
> recommended entries (P256/384 and X25519/448). Unfortunately it
> has variety of less-than-cute stuff (not all of that banned in TLS
> 1.3) marked not recommended. Bonus points for having 3 entries that
> are duplicates except one is banned in TLS 1.3 and one is not.
> 
> Then for ciphers, there is TLS ciphersuite registry for TLS 1.3
> ciphers. That one combines hashes, sometimes in bit cryptographically
> "interesting" ways. It also recommends AES-128-CCM, which is not great
> (as it is associated with IoT stuff), plus has already got some less-
> than-cute stuff (including NULL ciphers and 64-bit tags) marked not
> recommended (with more less-than-cute stuff pending).
> 
> And both of these registeries are expert review, so they tend
> to accumulate variety of stuff.
> 
> 
> And future problem point is if someone lobs in some post-quantum key
> exchange stuff. Which will work somewhere between poorly and not at
> all. And post NISTPQC, that can very well have Recommended set.
> 
> 
> 
> -Ilari
>