Re: [TLS] Is it possible for a client to offer TLS 1.3, but not be forced to support RSA PSS in TLS 1.2?

Benjamin Kaduk <bkaduk@akamai.com> Tue, 29 May 2018 19:13 UTC

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Date: Tue, 29 May 2018 14:13:19 -0500
From: Benjamin Kaduk <bkaduk@akamai.com>
To: Andrey Jivsov <crypto@brainhub.org>
Cc: "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [TLS] Is it possible for a client to offer TLS 1.3, but not be forced to support RSA PSS in TLS 1.2?
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On Tue, May 29, 2018 at 11:57:39AM -0700, Andrey Jivsov wrote:
> Greetings.
> 
> TLS 1.3 draft in sec 4.2.3.  Signature Algorithms tells that if a client
> wants to negotiate TLS 1.3, it must support an upgraded (and
> incompatible) version of TLS 1.2, the one that changes RFC 5246 to allow
> RSA-PSS in sec. 7.4.1.4.1. Signature Algorithms.
> 
> You might recall that the possibility to negotiate between PSS and
> RSASSA-PKCS1-v1_5 in TLS 1.3 handshake, just as it is allowed for X.509
> signatures, was discussed on the mailing list. The WG decision then was
> to hard-wire PSS in the TLS 1.3 handshake.
> 
> I don't recall any discussion on going further than this, all the way to
> changing the 10-year old TLS 1.2.
> 
> Unfortunately, our products have issues with PSS beyond our control. The
> only solution left to avoid receiving PSS with TLS 1.2 is to never
> negotiate TLS 1.3 as a client. Another solution is insecure fallback,
> but we presently don't do this.
> 
> Is my reading of the situation correct? Thank you.

Sounds like it:

   RSASSA-PKCS1-v1_5 algorithms  Indicates a signature algorithm using
      RSASSA-PKCS1-v1_5 [RFC8017] with the corresponding hash algorithm
      as defined in [SHS].  These values refer solely to signatures
      which appear in certificates (see Section 4.4.2.2) and are not
      defined for use in signed TLS handshake messages, although they
      MAY appear in "signature_algorithms" and
      "signature_algorithms_cert" for backward compatibility with TLS
      1.2,

-Ben