Re: [jose] Call for Adoption: draft-jones-jose-fully-specified-algorithms

[Orie Steele <[e-mail]>]

Inline with snips (which I don't often use, so apologies if I get one
wrong), and thank you again for these excellent comments:

[snip]

Right, and the point of associating the "alg" with the key is to ensure
> that it is only used for one thing.
>

That's right, but if you need to know the key type and the alg to verify,
you have a "not fully specified alg".

https://datatracker.ietf.org/doc/html/rfc9053#section-2.1
https://datatracker.ietf.org/doc/html/rfc7518#section-3.1

ES256 is P-256 ECDSA with SHA-256 in JOSE.
ES256 is any curve ECDSA with SHA-256 in COSE.

ES256 in COSE can easily lead to cross curve scenarios, where the hash
function is correct (the same), but the curve is different...

You need to negotiate the triple: [kty, crv, alg] in COSE and the
singleton "alg" in JOSE.

[snip]

I'm not familiar with this issue or what "lower S" refers to here. Do you
> mean some implementations spell the algorithm "Es256K" with a lowercase s?
>

https://github.com/Legrandin/pycryptodome/issues/759

https://bitcoin.stackexchange.com/questions/85946/low-s-value-in-bitcoin-signature

(you replied before I hit send, but for others who might be curious, I'll
leave the reference)...

I agree it's a "bug" except those are called expected behavior when they
happen frequently enough... it was an analogy at any rate.


> I agree with your comment, but I don't see anything better to do than
> enable more precision, so that implementations that are aware of it, or
> that come after it's available, can take advantage of it.
>
>
> Given that an EC/OKP JWK already specifies the curve, this is a case where
> this draft doesn't make things more precise and just causes compatibility
> issues for no gain.
>
>
Your argument is that fully specified algorithms in JOSE & COSE are
redundant to the key type requirements (kty, crv, alg = fully specified)?

This is true, that's the nature of fully specifying things, you can rely on
1 value to uniquely identify a security relevant capability.

[snip]

What I mean is that this draft is motivated by saying that algorithms like
> "EdDSA" don't give enough information to be useful in negotiating an
> algorithm because one party might only support the curve Ed25519 while
> another supports only Ed448. Well, exactly the same thing happens with
> content encryption algorithms. Both parties might support "RSA-OAEP" key
> algorithm, but one supports only "A128GCM" and the other only supports
> "A128CBC-HS256". Following the logic of this draft, the issue is that
> "RSA-OAEP" is not fully specified,
>

Indeed, I agree.


> so we should really add the following additional algorithm identifiers:
>
> RSA-OAEP-A128GCM
> RSA-OAEP-A192GCM
> RSA-OAEP-A256GCM
> RSA-OAEP-A128CBC-HS256
> RSA-OAEP-A192CBC-HS384
> RSA-OAEP-A256CBC-HS512
>
> Not to mention doing the same for RSA-OAEP-256 and RSA1_5 and ECDH-ES and
> ECDH-ES+A128KW and ...
>
> This is what I mean when I say that "fully specifying" the algorithm
> results in a combinatorial explosion of cipher-suite like identifiers.
>

Except it's 2024, and as you said, systems already understand these
algorithms : )

We don't have to backport full specification for things that we think
people should not use, won't use in the future or that are not being used.

Creating a limited number of new code points does not break old code points.


> It doesn't seem like a sensible way to do things, which is why TLS 1.3
> doesn't do that any more. And this is also why OIDC, which is cited in the
> draft as a motivating example, has both metadata fields:
> id_token_encryption_alg_values_supported
> id_token_encryption_enc_values_supported
>
Indeed.


> Probably OIDC should add similar id_token_signing_crv_values_supported and
> so on metadata fields to address this properly, rather than trying to cram
> everything into a single algorithm identifier.
>

The intention is to give a "name" to a "reasonably safe thing that is
supported".

If changing the curve does not affect the security properties, then it
seems reasonable to omit it... is that true?

https://datatracker.ietf.org/doc/html/rfc8446#section-9.1

>  A TLS-compliant application MUST support digital signatures with
   rsa_pkcs1_sha256 (for certificates), rsa_pss_rsae_sha256 (for
   CertificateVerify and certificates), and ecdsa_secp256r1_sha256.

ecdsa_secp256r1_sha256 = ES256 in JOSE but != ES256 in COSE. (addressed in
this draft)

... later:

> The following values are marked as "Recommended":
      ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
      rsa_pss_rsae_sha256, rsa_pss_rsae_sha384, rsa_pss_rsae_sha512,
      rsa_pss_pss_sha256, rsa_pss_pss_sha384, rsa_pss_pss_sha512, and
      *ed25519*.

Here "ed25519" is defined as...
https://datatracker.ietf.org/doc/html/rfc8446#appendix-B.3.1.3

/* EdDSA algorithms */
ed25519(0x0807),

If TLS mirrored COSE and JOSE, the algorithm would be called "EdDSA" and
the curve would be unknown. (addressed in this draft)

TLS 1.3 algorithm name is better... "0x0807" refers to EdDSA with Ed25519.

Recent algorithms registered for use in COSE follow the same approach:

kty: HSS-LMS, alg: HSS-LMS
kty: WalnutDSA, alg: WalnutDSA

Naming the algorithm after the key (and all parameters necessary to use
it) is fine.

Naming the algorithm after the process that you used the key with is not.

Because that process breaks when:
a) multiple keys support the same process, and
b) we don't know which key is supposed to be used.

You argued that b is a negotiation problem, and I agree... but that problem
is compounded by a few poor names, adding a few more precise ones
eliminates it, and establishes a safer convention moving forward.

[snip]

My point is that "fully specified" is a vague term that depends on what you
> consider important and is likely to evolve over time.
>
>
Fully specified is not a vague term, it means that when I tell you
EdDSAEd25519 or "0x0807", you know its EdDSA (which uses SHA-512
internally) with Ed25519 and that it won't work with an Ed448 key...

Today, if I tell you EdDSA, you don't know if it's from an Ed25519 or Ed448
key.


> [snip the rest]
>
> In short, I don't think this draft improves anything and it makes some
> things worse so should be rejected on that basis.
>

There are at least 3 parts of your arguments (thank you again), that I
don't feel can be dismissed:

1. What cost for more precision is worth bearing in implementations and
registry maintenance... If the current draft is "too costly" it should be
rejected.
2. What should "fully specified" mean, for a series of processes, such as
hash algorithm, then sign algorithm or key establishment, and then
symmetric encryption.
3. Is the guidance we give on how to create new registries likely to cause
problems in the future, including repeating very long debates, regarding
how things were done, vs how they should be done.

ECDSA with P-521 can be used with lots of different hash first
approaches.... Are all those worth registering?... No! ECDSA P-521 with
SHA-1 makes no sense... arguing that it MUST be registered and that it
would not be used is a straw man fallacy.

Fully specifying things does not imply registering things that don't make
sense to fully specify or to use in 2024.

Maintaining the registries is not about "what is possible". It is about
"what is good for interoperability", "what lowers implementation burden",
and "what improves security", and most importantly in this draft, providing
guidance for new entries, that eliminates optionality, ambiguity and
hopefully makes future discussions on this topic shorter and more
satisfactory.

In your earlier reply you said "I’m fairly receptive to it in general, but
I think it might be closing the stable door after the horse has already
bolted."

It seems like part of standards is watching a different horse escape over
and over again, until consensus is documented to not leave doors open : )


> -- Neil
>


-- 


ORIE STEELE
Chief Technology Officer
www.transmute.industries

<https://transmute.industries>