DTLS 1.3 signature_algorithms_cert Extension #788

adrianosela · 2026-01-30T18:11:49Z

DTLS 1.3 signature_algorithms_cert Extension

Addresses part of #776: Adds support for the signature_algorithms_cert extension.

Note: Per https://www.rfc-editor.org/rfc/rfc8446#section-4.2.3 and https://www.rfc-editor.org/rfc/rfc8446#section-1.3 this extension may optionally apply to DTLS 1.2, so we can safely ship any net-new additions to the API e.g. CertificateSignatureSchemes in config.

codecov · 2026-01-30T18:15:07Z

Codecov Report

❌ Patch coverage is 82.26950% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.69%. Comparing base (9495bef) to head (7d91c46).

Files with missing lines	Patch %	Lines
conn.go	22.22%	6 Missing and 1 partial ⚠️
crypto.go	87.09%	2 Missing and 2 partials ⚠️
flight1handler.go	0.00%	3 Missing and 1 partial ⚠️
flight3handler.go	0.00%	3 Missing and 1 partial ⚠️
flight0handler.go	0.00%	2 Missing ⚠️
pkg/protocol/extension/extension.go	50.00%	2 Missing ⚠️
...ocol/extension/generic_signature_hash_extension.go	94.28%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #788      +/-   ##
==========================================
- Coverage   81.72%   81.69%   -0.04%     
==========================================
  Files         108      110       +2     
  Lines        6062     6161      +99     
==========================================
+ Hits         4954     5033      +79     
- Misses        712      728      +16     
- Partials      396      400       +4

Flag	Coverage Δ
go	`81.69% <82.26%> (-0.04%)`	⬇️

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adrianosela · 2026-01-30T18:37:12Z

This PR needs some clean-up and a lot more unit tests. But I figured I'd open it and ask for any early feedback cc:// @theodorsm @JoTurk

adrianosela · 2026-01-31T20:27:34Z

This PR is ready for formal review now.

adrianosela · 2026-02-01T02:50:31Z

If you want to play with the new CertificateSignatureSchemes config setting, you may take a look at the end-to-end listen/dial example in this commit, which I intend to PR after this goes out.

theodorsm

Tnx! I left some comment.

As this introduces a new config flag, perhaps we should wait until #786 is merged? What do you think @JoTurk?

pkg/protocol/extension/signature_algorithms_cert.go

handshaker.go

theodorsm · 2026-02-02T15:55:03Z

conn.go

+	// Parse certificate signature schemes (if provided)
+	// If not provided, certificate validation will fall back to signatureSchemes
+	var certSignatureSchemes []signaturehash.Algorithm
+	if len(config.CertificateSignatureSchemes) > 0 {
+		certSignatureSchemes, err = signaturehash.ParseSignatureSchemes(
+			config.CertificateSignatureSchemes,
+			config.InsecureHashes,
+		)
+		if err != nil {
+			return nil, err
+		}
+	}
+


Is signaturesSchemes set as fallback (I do support it tho, as that how they do it in /crypto/tls)? The way I read the code, it is not currently.

Shoot -- very good catch -- thank you!

Actually... I just removed the fallback comment... now there is no fallback.

Can you point me to where in /crypto/tls the code falls back? AFAICT, there is no config setting for the _cert option in crypto/tls.

I see you removed the comment, but I do think we should set the signaturesSchemes as fallback as they do in /crypto/tls. https://cs.opensource.google/go/go/+/master:src/crypto/tls/handshake_client.go;l=123;drc=acd65ebb13a11f1b070b63a66b35bb1b15934409

That is, we should start to send the signature_algorithms_cert with DTLS 1.2 with the default list provided by ParseSignatureSchemes. If we remove the length check it should do that, like we do with signatureSchemes above.

JoTurk · 2026-02-02T17:21:07Z

I'm going to review this tonight. Looks so cool thank you so much.

perhaps we should wait until #786 is merged? What do you think @JoTurk?

I agree, sorry i got busy with life stuff. I'll get #786 ready for review tonight too.

adrianosela · 2026-02-02T17:48:04Z

Thanks for the review @theodorsm. Looking forward to your @JoTurk :D

I've made your requested changes in new commits @theodorsm , just to make review easier. Once you are happy I will squash into 1.

theodorsm

Wow, thanks for the fast fix @adrianosela. I like the generic signature extension.

I went over it once more and noticed two things.

flight4handler.go

theodorsm · 2026-02-02T18:33:53Z

conn.go

+	// Parse certificate signature schemes (if provided)
+	// If not provided, certificate validation will fall back to signatureSchemes
+	var certSignatureSchemes []signaturehash.Algorithm
+	if len(config.CertificateSignatureSchemes) > 0 {
+		certSignatureSchemes, err = signaturehash.ParseSignatureSchemes(
+			config.CertificateSignatureSchemes,
+			config.InsecureHashes,
+		)
+		if err != nil {
+			return nil, err
+		}
+	}
+


I see you removed the comment, but I do think we should set the signaturesSchemes as fallback as they do in /crypto/tls. https://cs.opensource.google/go/go/+/master:src/crypto/tls/handshake_client.go;l=123;drc=acd65ebb13a11f1b070b63a66b35bb1b15934409

That is, we should start to send the signature_algorithms_cert with DTLS 1.2 with the default list provided by ParseSignatureSchemes. If we remove the length check it should do that, like we do with signatureSchemes above.

adrianosela · 2026-02-02T19:03:51Z

@theodorsm

signature_algotihms_cert is no longer included in a ServerHello: nice catch
we now do set the signaturesSchemes as fallback ( as in crypto/tls), followed your suggestion of removing the len check like we do with signatureSchemes

theodorsm

I am so sorry about the back-and-forth, but I had to take a bit longer time to reason about the handshake state machine. I added more comments.

Also, we should do as you did previously, with checking for the length of the provided CertificateSignatureSchemes and not fallback to signaturesSchemes in config. I looked at some captures from firefox and chrome and they do not add the signature_algorithms_cert by default. We have the same policy for both extensions:

Implementations which have the same policy in both cases MAY omit the "signature_algorithms_cert" extension.

flight1handler.go

flight3handler.go

flight4handler.go

flight5handler.go

theodorsm

LGTM! Let's rebase and merge once the config options are merged.

adrianosela requested a review from theodorsm January 30, 2026 18:11

adrianosela force-pushed the sig-algs-update branch from 419ae92 to aa4aef6 Compare January 30, 2026 18:36

adrianosela force-pushed the sig-algs-update branch from aa4aef6 to f8ccea5 Compare January 31, 2026 20:14

DTLS 1.3 signature_algorithms_cert Extension

ba183cf

adrianosela force-pushed the sig-algs-update branch from f8ccea5 to ba183cf Compare January 31, 2026 20:24

theodorsm requested changes Feb 2, 2026

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Refactor Generic Signature Hash Extensions

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adrianosela force-pushed the sig-algs-update branch from 2614981 to e956596 Compare February 2, 2026 17:35

adrianosela force-pushed the sig-algs-update branch from c409294 to ddf8c35 Compare February 2, 2026 17:54

Remove Inaccurate Comment RE: Fallback to Sig Algs

5c1d633

adrianosela force-pushed the sig-algs-update branch from ddf8c35 to 5c1d633 Compare February 2, 2026 17:57

theodorsm requested changes Feb 2, 2026

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adrianosela force-pushed the sig-algs-update branch from d45deb6 to 211fed9 Compare February 2, 2026 18:57

Match crypto/tls's signature_algorithms_cert impl

8ec7ed9

adrianosela force-pushed the sig-algs-update branch from 211fed9 to 8ec7ed9 Compare February 2, 2026 18:59

adrianosela requested a review from theodorsm February 2, 2026 19:03

theodorsm requested changes Feb 2, 2026

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Align Behavior with RFC

7d91c46

adrianosela force-pushed the sig-algs-update branch from 298f8d8 to 7d91c46 Compare February 3, 2026 03:24

adrianosela requested a review from theodorsm February 3, 2026 03:33

theodorsm approved these changes Feb 3, 2026

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DTLS 1.3 signature_algorithms_cert Extension #788

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adrianosela commented Jan 30, 2026