Re: [TLS] Delegated Credentials Question about PSS

Nick Sullivan <nick@cloudflare.com> Wed, 16 October 2019 00:13 UTC

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From: Nick Sullivan <nick@cloudflare.com>
Date: Tue, 15 Oct 2019 17:13:13 -0700
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To: Martin Thomson <mt@lowentropy.net>
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Subject: Re: [TLS] Delegated Credentials Question about PSS
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One may note that no matter what the choice is with respect to RSA, this
particular wrinkle also applies more broadly. For example, if a client
advertises support for ed25519 in "signature_algorithms" in order to
support ed25519 delegated credentials, it should also be prepared to
receive an ed25519 certificate.

Nick

On Tue, Oct 15, 2019 at 5:00 PM Martin Thomson <mt@lowentropy.net>; wrote:

> I think that what Nick proposes is consistent with this, and it also
> matches my preference.  Requiring a PSS OID is nice in that it creates a
> strong constraint.
>
> However, there is a wrinkle.  The way you signal support for the signature
> scheme is through the signature_algorithms extension.  If you advertise
> rsa_pss_pss_*, then that means that you are also indicating support for an
> end entity certificate with a PSS SPKI.  Some certificate validation
> software hasn't been updated to support that and might choke if they got a
> PSS SPKI in a certificate.
>
> I don't know if we could support RSA without double checking our code.
> Adding support shouldn't be too hard, and we don't current advertise PSS
> support, so it's not a complete deal breaker for us, but it's worth
> highlighting.
>
> On Tue, Oct 15, 2019, at 15:58, Russ Housley wrote:
> > When we were working on RFC 4055, we thought it would be important to
> > allow a certified RSA public key to be bound to a specific algorithm
> > (e.g., RSA-PSS or RSA-OAEP). However, in transition, we thought it
> > would be important for the certified RSA public key to be used with any
> > of the algorithms (constrained by key usage extension).
> >
> > Section 1.2 says:
> >
> >  The rsaEncryption object identifier continues to identify the subject
> >  public key when the RSA private key owner does not wish to limit the
> >  use of the public key exclusively to either RSASSA-PSS or RSAES-OAEP.
> >
> >  ...
> >
> >  When the RSA private key owner wishes to limit the use of the public
> >  key exclusively to RSASSA-PSS, then the id-RSASSA-PSS object
> >  identifier MUST be used in the algorithm field within the subject
> >  public key information ...
> >
> >  When the RSA private key owner wishes to limit the use of the public
> >  key exclusively to RSAES-OAEP, then the id-RSAES-OAEP object
> >  identifier MUST be used in the algorithm field within the subject
> >  public key information ...
> >
> > I would like to continue with the approach.
> >
> > Russ
> >
> >
> > > On Oct 15, 2019, at 6:34 PM, Nick Sullivan <nick=
> 40cloudflare.com@dmarc.ietf.org>; wrote:
> > >
> > > TLSWG,
> > >
> > > I'd like some feedback on a potential issue raised by Martin Thomson
> at the last IETF. The question is about the interaction between the SPKI
> and the signature scheme for RSA delegated credentials. The main concern is
> around the interaction between the rsaEncryption OID and the signature
> scheme, specifically PKCS#1v1.5, which we disallow in TLS 1.3 but not
> explicitly in DCs (yet). This issue is tracked on Github as issues/28 <
> https://github.com/tlswg/tls-subcerts/issues/28>;.
> > >
> > > Given the feedback on Github, I see two main choices to resolve this
> issue:
> > >
> > > 1) Allow RSA Credentials with the rsaEncryption OID in the SPKI to be
> used with the rsa_pss_rsae_* signature schemes, but disallow rsa_pkcs1_*
> signature schemes.
> > > 2) Forbid RSA Credentials with the rsaEncryption OID (and associated
> signature schemes) and require an RSA PSS OIDs for rsa_pss_pss_* signature
> schemes.
> > >
> > > Does anyone have a strong preference for one of these options?
> > >
> > > My take:
> > > The rsa_pss_rsae_* suites are a hack created to allow TLS 1.3 to be
> backward-compatible with existing rsaEncryption OID certs while enabling
> RSA-PSS. We don't have this legacy in delegated credentials, so I'm
> inclined to prefer 2).
> > >
> > > The only reason I see to go for 1) is the risk of implementation
> difficulties. The RSA PSS OIDs are hard to get right in code. However,
> considering that RSA DCs are unlikely to be widely used in favor of
> elliptic-curve DCs, the implementation risk seems low and restricted to
> implementations who choose to support RSA DCs as an optional feature.
> > >
> > > Nick
> > >
> > >  _______________________________________________
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> >
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> >
>
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