Re: [TLS] Iotdir last call review of draft-ietf-tls-md5-sha1-deprecate-04

Sean Turner <sean@sn3rd.com> Tue, 04 May 2021 17:23 UTC

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From: Sean Turner <sean@sn3rd.com>
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Date: Tue, 4 May 2021 13:23:10 -0400
Cc: Loganaden Velvindron <loganaden@gmail.com>, TLS List <tls@ietf.org>, iot-directorate@ietf.org, draft-ietf-tls-md5-sha1-deprecate.all@ietf.org, last-call@ietf.org
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To: Daniel Migault <mglt.ietf@gmail.com>
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Subject: Re: [TLS] Iotdir last call review of draft-ietf-tls-md5-sha1-deprecate-04
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Daniel,

Thanks for your (and the WG’s) patience on this. Responses in line.

spt

> On Apr 9, 2021, at 14:54, Sean Turner <sean@sn3rd.com> wrote:
>> On Jan 22, 2021, at 08:23, Daniel Migault <mglt.ietf@gmail.com> wrote:
>> On Fri, Jan 22, 2021 at 12:13 AM Loganaden Velvindron <loganaden@gmail.com> wrote:
>> On Fri, Jan 22, 2021 at 7:30 AM Daniel Migault <mglt.ietf@gmail.com> wrote:
>> > On Tue, Oct 27, 2020 at 11:34 PM Sean Turner <sean@sn3rd.com> wrote:
>> >>
>> >>
>> >> Please note the comment about Section 3 suggests changing server behavior from SHOULD NOT to a MUST NOT.
>> >>
>> >> > On Oct 27, 2020, at 10:19, Daniel Migault via Datatracker <noreply@ietf.org> wrote:
>> >> >
>> >> > Reviewer: Daniel Migault
>> >> > Review result: Ready with Nits
>> >> >
>> >> > Hi,
>> >> >
>> >> >
>> >> > I reviewed this document as part of the IoT Directorate's ongoing effort to
>> >> > review all IETF documents being processed by the IESG.  These comments were
>> >> > written primarily for the benefit of the Security Area Directors.  Document
>> >> > authors, document editors, and WG chairs should treat these comments just like
>> >> > any other IETF Last Call comments.
>> >> >
>> >> > Review Results: Ready with Nits
>> >> >
>> >> > Please find my comments below.
>> >> >
>> >> > Yours,
>> >> > Daniel
>> >> >
>> >> >
>> >> >         Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
>> >> >                  draft-ietf-tls-md5-sha1-deprecate-04
>> >> > [...]
>> >> >
>> >> > 1.  Introduction
>> >> >
>> >> >   The usage of MD5 and SHA-1 for signature hashing in TLS 1.2 is
>> >> >   specified in [RFC5246].  MD5 and SHA-1 have been proven to be
>> >> >   insecure, subject to collision attacks [Wang].  In 2011, [RFC6151]
>> >> >   detailed the security considerations, including collision attacks for
>> >> >   MD5.  NIST formally deprecated use of SHA-1 in 2011
>> >> >   [NISTSP800-131A-R2] and disallowed its use for digital signatures at
>> >> >   the end of 2013, based on both the Wang, et. al, attack and the
>> >> >   potential for brute-force attack.  In 2016, researchers from INRIA
>> >> >   identified a new class of transcript collision attacks on TLS (and
>> >> >   other protocols) that rely on efficient collision-finding algorithms
>> >> >   on the underlying hash constructions [Transcript-Collision].
>> >> >   Further, in 2017, researchers from Google and CWI Amsterdam
>> >> >   [SHA-1-Collision] proved SHA-1 collision attacks were practical.
>> >> >   This document updates [RFC5246] and [RFC7525] in such a way that MD5
>> >> >   and SHA-1 MUST NOT be used for digital signatures.  However, this
>> >> >   document does not deprecate SHA-1 in HMAC for record protection.
>> >> >
>> >> > <mglt>
>> >> > RFC6194 may be mentioned as a reference for
>> >> > not deprecating HMAC-SHA-1 as well as an
>> >> > additional reference to [NISTSP800-131A-R2].
>> >>
>> >> Are asking for something like this:
>> >>
>> >> OLD:
>> >>
>> >>   In 2011, [RFC6151] detailed the security considerations,
>> >>   including collision attacks for MD5.
>> >>
>> >> NEW:
>> >>
>> >>   In 2011, [RFC6151] [RFC6194] detailed the security considerations,
>> >>   including collision attacks for MD5 and SHA-1, respectively.
>> >>
>> >> > Reading the text the situation of HMAC with
>> >> > MD5 is unclear. Since we specify that SHA-1
>> >> > is not deprecated for HMAC we may specify
>> >> > the status for HMAC with MD5. Given RFC6151 I
>> >> > hope the reason is that MD5 and HMAC-MD5 has
>> >> > already been deprecated but I have not found
>> >> > this. Maybe that would worth mentioning it
>> >> > is deprecated already.
>> >> >
>> >> > </mglt>
>> >>
>> >> Are you asking for something like this:
>> >>
>> >> OLD:
>> >>
>> >>   However, this document does not deprecate SHA-1 in HMAC
>> >>   for record protection.
>> >>
>> >>   However, this  document does not deprecate MD-5 or SHA-1 HMAC
>> >>   for record protection.
>> >
>> > <mglt>
>> > maybe we could add these are still considered as secured at the time of writing.  It is also tempting to add that given we deprecate sha1 and md5 in the signature, it is tempting to suggest to use unless necessary hmac-sha256.  I have commented the PR12
>> > </mglt>

<spt>

I think we address or will address this one in this to-be updated PR:
https://github.com/tlswg/draft-ietf-tls-md5-sha1-deprecate/pull/12/files

In fact, I plan to submit updates to the PR for all of the changes so we can look at them in one place.

<spt>

>> >> <
>> >> > [...]
>> >> >
>> >> > 2.  Signature Algorithms
>> >> >
>> >> >   Clients MUST NOT include MD5 and SHA-1 in the signature_algorithms
>> >> >   extension.  If a client does not send a signature_algorithms
>> >> >   extension, then the server MUST abort the handshake and send a
>> >> >   handshake_failure alert, except when digital signatures are not used
>> >> >   (for example, when using PSK ciphers).
>> >> >
>> >> > <mglt>
>> >> > It seems to me that the server behavior might
>> >> > be defined as well. In our case this could be
>> >> > something around the lines the server MUST
>> >> > ignore MD5 and SHA1 values in the signature
>> >> > algorithm extension.
>> >> >
>> >> > </mglt>
>> >>
>> >> I guess that would be the way to absolutely stamp them out, but don’t we get the same result because the client is not sending the values in the signaure_algorithms extension?
>> >>
>> > <mglt>
>> > The reason I would maybe have preferred to have indications for the client and the server is that it is always easier for a server to say that it is following the specs than to indicate that the client is not.
>> > This is correct the result is similar, but client usually complement servers and we usually specify both. I believe that would be better to be updated.
>> > </mglt>

<spt>

So … personally, I tend to agree with you. However, we (the royal we) get what we want by deprecating it on the client side so to maintain the rough consensus we currently have I am proposing that we leave the server indications out.

<spt>

>> >> > 3.  Certificate Request
>> >> >
>> >> >   Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest
>> >> >   messages.
>> >> >
>> >> > <mglt>
>> >> > It seems to me that the same level of
>> >> > authentication should be provided for both
>> >> > peers and that server MUST NOT  include MD5
>> >> > or SHA-1.
>> >> >
>> >> > A SHOULD NOT status might be welcome for a
>> >> > smooth transition. At that time, collision
>> >> > for MD5 and SHA1 are known for years. It is likely
>> >> > that software that still need MD5 or SHA1 are
>> >> > likely to never upgrade, so I doubt a smooth
>> >> > path worth being taken.
>> >> > </mglt>
>> >>
>> >> This has been a SHOULD NOT since it was a first published as an individual draft and through the WGLC. I would not feel comfortable changing it now without further discussion.
>> >>
>> >> I tend to think it is okay to leave as SHOULD NOT because the server MUST use values from the now ever present signature_algorithms extension and MD5 and SHA1 are not going to be there. If the server is going to go nuts and include MD5 and SHA-1 in the CertifiateRequest even though it’s not been asked, well, you got bigger problems.
>> >>
>> > <mglt>
>> > Let's say that there are some softwares using SHA-1 / MD5 that will be upgraded. I would have probably incline to put MUST NOT... but SHOULD NOT carries the message, and I do not believe that is worth further discussion.
>> > </mglt>

<spt>

Again, I tend to agree with you on the MUST NOT, but also agree SHOULD NOT gets it done. In light of that, we should no change here.

<spt>

>> >> > 4.  Server Key Exchange
>> >> >
>> >> >   Servers MUST NOT include MD5 and SHA-1 in ServerKeyExchange messages.
>> >> >   If a client receives a MD5 or SHA-1 signature in a ServerKeyExchange
>> >> >   message it MUST abort the connection with the illegal_parameter
>> >> >   alert.
>> >> >
>> >> > <mglt>
>> >> > As per section 2, the client has clearly
>> >> > indicated it does not support signature with
>> >> > MD5/SHA1, so Server Key Exchange should not
>> >> > end up with signature with SHA1/MD5.
>> >> >
>> >> > """
>> >> > If the client has offered the "signature_algorithms" extension, the
>> >> >   signature algorithm and hash algorithm MUST be a pair listed in that
>> >> >   extension.
>> >> > """
>> >> >
>> >> > It also seems to me that the constraint of
>> >> > including a MD5 and SHA-1 signature is
>> >> > related to the Certificate. I suspect that
>> >> > some clarification are needed here.
>> >>
>> >> It’s about the digitally-signed struct for the dhe_dss and dhe_rsa cases in ServerKeyExchange.
>> >
>> > <mglt>
>> > sure. My point here was that Certificate MUST be signed by the signature, hash provided by the extension. This mandates the CAs deprecates SHA1 as well, and I am unclear if that is a correct assumption. I think this could be addressed in a section or note related to Certificate.
>> > </mglt>
>> >>
>> 
>> Daniel, do you mean this ?
>> 
>> https://cabforum.org/2014/10/16/ballot-118-sha-1-sunset/
>> 
>> <mglt>
>> I think this is a usefull reference to show no problems are expected from certificate validation. Thanks.
>> </mglt> 
>> 
>> >>
>> >> > Since the case where the extension becomes
>> >> > mandatory, the quoted text above of RFC 5246
>> >> > might be updated as well, though this does
>> >> > not appear that necessary.
>> >>
>> >> So we might do it, but the question is whether implementers are going to be confused if we don’t update it.  I tend to think that the changes in s2 are clear that the extension will be present (except when sigs are not used) if the handshake is to complete.
>> >>
>> >> > </mglt>
>> >>
>> >> Not sure anything needs to be changed in this section based on the above.
>> >>
>> > <mglt>
>> > I see your point and agree.
>> > </mglt>

<spt> 

We landed on adding a reference to the CAB Forum SHA1 sunset statement. I think we can do that in the introduction:

Note that the CABF has also deprecated use of SHA-1 [CABF].

Informative reference:
[CABF] https://cabforum.org/2014/10/16/ballot-118-sha-1-sunset/

I also looked for a similar statement for md5, but md5 was deprecated in the initial version of their baseline requirements, i.e., there is no statement because it was never allowed. In other words, there’s no statement to reference for MD5, but that should be okay.

<spt>

>> >> > 5.  Certificate Verify
>> >> >
>> >> >   Clients MUST NOT include MD5 and SHA-1 in CertificateVerify messages.
>> >> >   If a server receives a CertificateVerify message with MD5 or SHA-1 it
>> >> >   MUST abort the connection with handshake_failure or
>> >> >   insufficient_security alert.
>> >> >
>> >> >
>> >> > <mglt>
>> >> >
>> >> > 6. Certificate
>> >> >
>> >> > Unless I am missing something, it seems to me
>> >> > that signature may also be found in the
>> >> > Certificate messages for the chain as well in
>> >> > the restriction of the signature algorithm.
>> >> > The end certificate is associated to the peer
>> >> > while other certificate are related to a CA.
>> >> >
>> >> > It seems that client and server behavior may
>> >> > be specified. The quoted text below may be
>> >> > helpful to clarify.
>> >> >
>> >> > """
>> >> > If the client provided a "signature_algorithms" extension, then all
>> >> >   certificates provided by the server MUST be signed by a
>> >> >   hash/signature algorithm pair that appears in that extension.
>> >> > """
>> >> >
>> >> > </mglt>
>> >>
>> >> Are you suggesting that a new section be added to address the Certificate message?
>> >
>> >
>> > <mglt>
>> > yes. I have the impression that since SHA1/MD5 MUST NOT be mentioned in the "signature_algorithms", this assumes that CAs do not sign using these algorithms. I tend to believe that worth being mentioned. As mentioned before, I think that could be mentioned in the draft.
>> > </mglt>

<spt>

I think I get your point that we could tell the CAs: hey don’t issue these types of certs. But, TLS usually stays out of the PKI game ass best as possible and as we’ve noted the CAB Forum has basically said that for us.

<spt>

>> >> > 6.  Updates to RFC5246
>> >> >
>> >> >   [RFC5246], The Transport Layer Security (TLS) Protocol Version 1.2,
>> >> >   suggests that implementations can assume support for MD5 and SHA-1 by
>> >> >   their peer.  This update changes the suggestion to assume support for
>> >> >   SHA-256 instead, due to MD5 and SHA-1 being deprecated.
>> >> >
>> >> >   In Section 7.4.1.4.1: the text should be revised from:
>> >> >
>> >> >   OLD:
>> >> >
>> >> >   "Note: this is a change from TLS 1.1 where there are no explicit
>> >> >   rules, but as a practical matter one can assume that the peer
>> >> >   supports MD5 and SHA- 1."
>> >> >
>> >> >   NEW:
>> >> >
>> >> >   "Note: This is a change from TLS 1.1 where there are no explicit
>> >> >   rules, but as a practical matter one can assume that the peer
>> >> >   supports SHA-256."
>> >> >
>> >> >
>> >> > <mglt>
>> >> > I am reading the Note as an explanation on
>> >> > why sha was taken as the default hash
>> >> > function with the following rules.
>> >> >
>> >> > """
>> >> > If the client does not send the signature_algorithms extension, the
>> >> >   server MUST do the following:
>> >> >
>> >> >   -  If the negotiated key exchange algorithm is one of (RSA, DHE_RSA,
>> >> >      DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had
>> >> >      sent the value {sha1,rsa}.
>> >> >
>> >> >   -  If the negotiated key exchange algorithm is one of (DHE_DSS,
>> >> >      DH_DSS), behave as if the client had sent the value {sha1,dsa}.
>> >> >
>> >> >   -  If the negotiated key exchange algorithm is one of (ECDH_ECDSA,
>> >> >      ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}.
>> >> > """
>> >> >
>> >> > The current document does not update the
>> >> > default hash function from sha to sha256 to
>> >> > avoid interoperability issue where one peer
>> >> > takes sha while the other one takes sha-256.
>> >>
>> >> You are right that this section, which is updating TLS1.2 [RFC5246], is not changing the default to SHA-256, but the recommendations for configuring TLS 1.2, which are provided in RFC 7525/BCP 195, is being updated to recommend SHA-256 in the very next section.
>> >>
>> > <mglt>
>> > Updating the update works. It believe that saying a section should be remove is not too hard, and it seems to me that mentioning the default behaviour is important. I would say that could get implementers confused to implement part of the specifications that do not hold any more. I would prefer to have this being addressed.
>> >
>> > I am reading RFC7525 as recommending a non default parameter while this document removed the support of default parameters. So to me the updating the status of the default parameters seems more updating the 5246 then 7525.
>> > </mglt>
>> >
>> >> > As a results, these rules and the "Note" may
>> >> > eventually all together be replaced by your
>> >> > text of section 2.
>> >> >
>> >> > The following text may also be removed:
>> >> >
>> >> > """
>> >> > If the client supports only the default hash and signature algorithms
>> >> >   (listed in this section), it MAY omit the signature_algorithms
>> >> >   extension.
>> >> > """
>> >>
>> >> So we might do it, but the question is whether implementers are going to be confused if we don’t do it. I think because s1 already says that client MUST send a signature_algorithms extension that we don’t have to indicate that that particular sentence be dropped/removed from the draft. I will admit this document mixes the two ways to do a bis document, i.e., old/new and describing blanket changes, but I think the intent is pretty clear based on the brevity of the draft.
>> >>
>> >
>> > <mglt>
>> > I agree we may be able to find all the dots. I think this provides more insight to clarify the reading of 5246. I agree the intend is clearly stated in the title and section 2 addresses most of it and I believe that some flexibility is permitted, but that is unclear to me where to put the bar. I still believe that could be easily be addressed.
>> > </mglt>
>> >
>> >>
>> >> > Regarding the Note, it seems to be that the
>> >> > removal of support for MD5/SHA1 will result
>> >> > in interoperability issues. At this point,
>> >> > the issue is due to the obsolescence of the
>> >> > implementation as deprecation of SHA1/Md5 has
>> >> > started a long time ago.
>> >> >
>> >> > It is unclear to me how normative is
>> >> > interpreted "can assume". Was the support of
>> >> > MD5/SHA1 a SHOULD or a MUST? In both case, if
>> >> > we were willing to maintain interoperability
>> >> > between software that only implemented
>> >> > MD5/SHA1, we should take a slower path and
>> >> > introducing SHA-256 and having were MD5/SHA1
>> >> > kept for interoperability purpose before
>> >> > being deprecated. I do not think we should
>> >> > take that path as implementations that
>> >> > currently do not support SHA-256 are unlikely
>> >> > to be updated and that deprecation of
>> >> > SHA1/MD5 has started a long time ago.
>> >> >
>> >> > I would however mention the issue of
>> >> > interoperability in the  section but not in
>> >> > the text to update. In the text to update I
>> >> > would maybe suggest that the support of
>> >> > SHA-256 comes with a normative MUST
>> >> > statement.
>> >> >
>> >> >
>> >> > </mglt>
>> >>
>> >> I think we can accomplish migrating to SHA-256 by updating RFC 7525/BCP 195.
>> >
>> >
>> > <mglt>
>> > yes, but the current update only RECOMMENDs RFC7525.
>> > </mglt>

<spt>

I think we are in agreement that by updating RFC 7525, which this I-D does, then we are recommending clients move to SHA-256. Also, RFC 7525 is a BCP so it’s a wee-bit more.

<spt>

>> >> > Velvindron, et al.       Expires April 12, 2021                 [Page 3]
>> >> >
>> >> > Internet-Draft      draft-ietf-tls-md5-sha1-deprecate       October 2020
>> >> >
>> >> >
>> >> > 7.  Updates to RFC7525
>> >> >
>> >> >   [RFC7525], Recommendations for Secure Use of Transport Layer Security
>> >> >   (TLS) and Datagram Transport Layer Security (DTLS) recommends use of
>> >> >   SHA-256 as a minimum requirement.  This update moves the minimum
>> >> >   recommendation to use stronger language deprecating use of both SHA-1
>> >> >   and MD5.  The prior text did not explicitly include MD5 or SHA-1; and
>> >> >   this text adds guidance to ensure that these algorithms have been
>> >> >   deprecated..
>> >> >
>> >> >   Section 4.3:
>> >> >
>> >> >   OLD:
>> >> >
>> >> >   When using RSA, servers SHOULD authenticate using certificates with
>> >> >   at least a 2048-bit modulus for the public key.  In addition, the use
>> >> >   of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
>> >> >   more details).  Clients SHOULD indicate to servers that they request
>> >> >   SHA-256, by using the "Signature Algorithms" extension defined in TLS
>> >> >   1.2.
>> >> >
>> >> >   NEW:
>> >> >
>> >> >   Servers SHOULD authenticate using certificates with at least a
>> >> >   2048-bit modulus for the public key.
>> >> >
>> >> >   In addition, the use of the SHA-256 hash algorithm is RECOMMENDED;
>> >> >   and SHA-1 or MD5 MUST NOT be used (see [CAB-Baseline] for more
>> >> >   details).  Clients MUST indicate to servers that they request SHA-
>> >> >   256, by using the "Signature Algorithms" extension defined in TLS
>> >> >   1.2.
>> >> >
>> >> > <mglt>
>> >> > I understand the reason we do specify that
>> >> > hash algorithms that MUST NOT been used. This
>> >> > is fine in the context of this document, but
>> >> > it seems to me that if we were writing the
>> >> > updated specification we may have rather
>> >> > mentioned a minimum level of security hash
>> >> > function needs to be met - in our case
>> >> > SHA-256. I leave the co-authors make the
>> >> > appropriated choice.
>> >> >
>> >> > </mglt>
>> >>
>> >> Can you clarify what you would like changed? I am just confused because SHA-256 is RECOMMENDED in the proposed new text.
>> >>
>> > <mglt>
>> > I suppose I proposed to move RECOMMENDED to MUST to accomplish the transition as I do not see RECOMMENDED sufficient to guarantee interoperability. At that point, I am inclined to say the MUST status is achieve as there are quite few hash functions deployed and available and that the life time of TLS 1.2 is expected to be limited. This could be made RECOMMENDED acceptable, but MUST would be preferred if possible. Is there anything I am missing or any reason to favour RECOMMENDED over MUST ?
>> > </mglt>

<spt>

As noted earlier, RECOMMENDED gets us what we want and keeps the rough consensus so I would like to propose no change.

I see that -06 made this change, and it needs to get changed back. 1st the OLD text block needs to remain as-is; the OLD text is just quoted. 2nd MUST use causes problems down the road because we would need to update this to SHA-512 (or whatever) later to change the MUST use.

<spt>

>> >> > 8.  IANA Considerations
>> >> >
>> >> >   The document updates the "TLS SignatureScheme" registry to change the
>> >> >   recommended status of SHA-1 based signature schemes to N (not
>> >> >   recommended) as defined by [RFC8447].  The following entries are to
>> >> >   be updated:
>> >> >
>> >> >       +--------+----------------+-------------+-------------------+
>> >> >       | Value  |  Description   | Recommended |     Reference     |
>> >> >       +--------+----------------+-------------+-------------------+
>> >> >       | 0x0201 | rsa_pkcs1_sha1 |      N      | [RFC8446][RFCTBD] |
>> >> >       | 0x0203 |   ecdsa_sha1   |      N      | [RFC8446][RFCTBD] |
>> >> >       +--------+----------------+-------------+-------------------+
>> >> >
>> >> >   Other entries of the resgistry remain the same.
>> >> >
>> >> >
>> >> > <mglt>
>> >> > It seems to me that TLS 1.2 is using the TLS
>> >> > hash and TLS signature registry TLS signature
>> >> > registry and TLS 1.3 is using Signature
>> >> > Scheme.
>> >> >
>> >> > I suspect that TLS hash values for sha1 and
>> >> > md5 should be deprecated. And RFCTBD should
>> >> > be added for sha1 and md5. Note that the
>> >> > SHOULD NOT status for CertificateRequest
>> >> > may have prevented such deprecation.
>> >>
>> >> The TLS HashAlgorithm and TLS SignatureAlgorithm registries do not have a Recommended column. Likewise, there’s not a notes column. What I think we could do is replace the reference to [RFC5246] with [RFCTBD] (so it’s points to this document when it is published).
>> >>
>> > <mglt>
>> > yes. My understanding so far is that the document deprecate SHA-1 and MD5 for TLS 1.3 not for TLS 1.2 for the IANA section.
>> > </mglt>
>> >
>> >> > A side effect is these code points for
>> >> > signature scheme that were assigned for
>> >> > compatibility with legacy (TLS 1.2)
>> >> > signatures must not be used anymore -  if
>> >> > there are no more valid with TLS 1.2.
>> >> > </mglt>
>> >>
>> >> This is what changing the Recommended to “N” is above so I think we’re good here?
>> >>
>> > <mglt>
>> > yes, my point was to indicate that currently "N" deprecates the TLS 1.2 legacy protocol for TLS 1.3 as opposed to the the protocols for TLS 1.2. Unless I misinterpreted the IANA registries I did not have the impression that the signature scheme replaced the registries of TLS 1.2. It is possible I am missing something with the IANA registries, but otherwise, I think the draft should be updated.
>> > </mglt>

AH, now i get it. To address this, what we need to do is add the IANA considerations:

IANA is also requested to update the Reference for the TLS SignatureAlgorithm and TLS HashAlgorithm registries to refer to this RFC:

OLD:

Reference
[RFC5246][RFC8447]

NEW:

Reference
[RFC5246][RFC8447][RFC-to-be]