crypto/tls: enable signature algorithm BoGo tests (and fix two bugs)

The two bugs are very minor:

- We were trying to set the ConnectionState CurveID field even if the
  RSA key exchange was in use

- We were sending the wrong alert from TLS 1.2 clients if none of the
  certificate signature algorithms were supported

Change-Id: I6a6a46564f5a9f1a5d44e54fc59a650118ad67d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/675918
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/crypto/tls/auth.go

@@ -149,20 +149,18 @@ func legacyTypeAndHashFromPublicKey(pub crypto.PublicKey) (sigType uint8, hash c
 var rsaSignatureSchemes = []struct {
 	scheme          SignatureScheme
 	minModulusBytes int
-	maxVersion      uint16
 }{
 	// RSA-PSS is used with PSSSaltLengthEqualsHash, and requires
 	//    emLen >= hLen + sLen + 2
-	{PSSWithSHA256, crypto.SHA256.Size()*2 + 2, VersionTLS13},
-	{PSSWithSHA384, crypto.SHA384.Size()*2 + 2, VersionTLS13},
-	{PSSWithSHA512, crypto.SHA512.Size()*2 + 2, VersionTLS13},
+	{PSSWithSHA256, crypto.SHA256.Size()*2 + 2},
+	{PSSWithSHA384, crypto.SHA384.Size()*2 + 2},
+	{PSSWithSHA512, crypto.SHA512.Size()*2 + 2},
 	// PKCS #1 v1.5 uses prefixes from hashPrefixes in crypto/rsa, and requires
 	//    emLen >= len(prefix) + hLen + 11
-	// TLS 1.3 dropped support for PKCS #1 v1.5 in favor of RSA-PSS.
-	{PKCS1WithSHA256, 19 + crypto.SHA256.Size() + 11, VersionTLS12},
-	{PKCS1WithSHA384, 19 + crypto.SHA384.Size() + 11, VersionTLS12},
-	{PKCS1WithSHA512, 19 + crypto.SHA512.Size() + 11, VersionTLS12},
-	{PKCS1WithSHA1, 15 + crypto.SHA1.Size() + 11, VersionTLS12},
+	{PKCS1WithSHA256, 19 + crypto.SHA256.Size() + 11},
+	{PKCS1WithSHA384, 19 + crypto.SHA384.Size() + 11},
+	{PKCS1WithSHA512, 19 + crypto.SHA512.Size() + 11},
+	{PKCS1WithSHA1, 15 + crypto.SHA1.Size() + 11},
 }
 
 // signatureSchemesForCertificate returns the list of supported SignatureSchemes
@@ -202,7 +200,7 @@ func signatureSchemesForCertificate(version uint16, cert *Certificate) []Signatu
 		size := pub.Size()
 		sigAlgs = make([]SignatureScheme, 0, len(rsaSignatureSchemes))
 		for _, candidate := range rsaSignatureSchemes {
-			if size >= candidate.minModulusBytes && version <= candidate.maxVersion {
+			if size >= candidate.minModulusBytes {
 				sigAlgs = append(sigAlgs, candidate.scheme)
 			}
 		}
@@ -219,10 +217,9 @@ func signatureSchemesForCertificate(version uint16, cert *Certificate) []Signatu
 	}
 
 	// Filter out any unsupported signature algorithms, for example due to
-	// FIPS 140-3 policy, tlssha1=0, or any downstream changes to defaults.go.
-	supportedAlgs := supportedSignatureAlgorithms(version)
+	// FIPS 140-3 policy, tlssha1=0, or protocol version.
 	sigAlgs = slices.DeleteFunc(sigAlgs, func(sigAlg SignatureScheme) bool {
-		return !isSupportedSignatureAlgorithm(sigAlg, supportedAlgs)
+		return isDisabledSignatureAlgorithm(version, sigAlg, false)
 	})
 
 	return sigAlgs

src/crypto/tls/bogo_config.json

@@ -20,8 +20,6 @@
         "TLS-ECH-Client-Reject-NoChannelID-TLS13": "We don't support sending channel ID",
         "TLS-ECH-Client-Reject-NoChannelID-TLS12": "We don't support sending channel ID",
 
-        "ServerAuth-SHA1-Fallback*": "We don't ever support SHA-1 in TLS 1.2, so we fail if there are no signature_algorithms",
-
         "TLS-ECH-Client-GREASE-IgnoreHRRExtension": "We don't support ECH GREASE because we don't fallback to plaintext",
         "TLS-ECH-Client-NoSupportedConfigs-GREASE": "We don't support ECH GREASE because we don't fallback to plaintext",
         "TLS-ECH-Client-GREASEExtensions": "We don't support ECH GREASE because we don't fallback to plaintext",
@@ -40,7 +38,19 @@
         "PostQuantumNotEnabledByDefaultInClients": "We do enable it by default!",
         "*-Kyber-TLS13": "We don't support Kyber, only ML-KEM (BoGo bug ignoring AllCurves?)",
 
-        "*-SignDefault-*": "TODO, partially it encodes BoringSSL defaults, partially we might be missing some implicit behavior of a missing flag",
+        "*-RSA_PKCS1_SHA256_LEGACY-TLS13": "We don't support the legacy PKCS#1 v1.5 codepoint for TLS 1.3",
+        "*-Verify-RSA_PKCS1_SHA256_LEGACY-TLS12": "Likewise, we don't know how to handle it in TLS 1.2, so we send the wrong alert",
+        "*-VerifyDefault-*": "Our signature algorithms are not configurable, so there is no difference between default and supported",
+        "Ed25519DefaultDisable-*": "We support Ed25519 by default",
+        "NoCommonSignatureAlgorithms-TLS12-Fallback": "We don't support the legacy RSA exchange (without tlsrsakex=1)",
+
+        "*_SHA1-TLS12": "We don't support SHA-1 in TLS 1.2 (without tlssha1=1)",
+        "Agree-Digest-SHA1": "We don't support SHA-1 in TLS 1.2 (without tlssha1=1)",
+        "ServerAuth-SHA1-Fallback*": "We don't support SHA-1 in TLS 1.2 (without tlssha1=1), so we fail if there are no signature_algorithms",
+
+        "Agree-Digest-SHA256": "We select signature algorithms in peer preference order. We should consider changing this.",
+        "ECDSACurveMismatch-Verify-TLS13": "We don't enforce the curve when verifying. This is a bug. We need to fix this.",
+        "*-Verify-ECDSA_P224_SHA256-TLS13": "Side effect of the bug above. BoGo sends a P-256 sigAlg with a P-224 key, and we allow it.",
 
         "V2ClientHello-*": "We don't support SSLv2",
         "SendV2ClientHello*": "We don't support SSLv2",
@@ -62,8 +72,10 @@
         "CurveID-Resume*": "unexposed curveID is not stored in the ticket yet",
         "BadRSAClientKeyExchange-4": "crypto/tls doesn't check the version number in the premaster secret - see processClientKeyExchange comment",
         "BadRSAClientKeyExchange-5": "crypto/tls doesn't check the version number in the premaster secret - see processClientKeyExchange comment",
-        "CheckLeafCurve": "TODO: first pass, this should be fixed",
         "SupportTicketsWithSessionID": "We don't support session ID resumption",
+        "ResumeTLS12SessionID-TLS13": "We don't support session ID resumption",
+
+        "CheckLeafCurve": "TODO: first pass, this should be fixed",
         "KeyUpdate-RequestACK": "TODO: first pass, this should be fixed",
         "SupportedVersionSelection-TLS12": "TODO: first pass, this should be fixed",
         "UnsolicitedServerNameAck-TLS-TLS1": "TODO: first pass, this should be fixed",
@@ -88,19 +100,6 @@
         "Resume-Server-OmitPSKsOnSecondClientHello": "TODO: first pass, this should be fixed",
         "Renegotiate-Server-Forbidden": "TODO: first pass, this should be fixed",
         "Renegotiate-Client-Forbidden-1": "TODO: first pass, this should be fixed",
-        "Client-Sign-RSA_PKCS1_SHA1-TLS13": "TODO: first pass, this should be fixed",
-        "Client-Sign-RSA_PKCS1_SHA256-TLS13": "TODO: first pass, this should be fixed",
-        "Client-Sign-RSA_PKCS1_SHA384-TLS13": "TODO: first pass, this should be fixed",
-        "Client-Sign-RSA_PKCS1_SHA512-TLS13": "TODO: first pass, this should be fixed",
-        "Client-Sign-ECDSA_SHA1-TLS13": "TODO: first pass, this should be fixed",
-        "Client-Sign-ECDSA_P224_SHA256-TLS13": "TODO: first pass, this should be fixed",
-        "ClientAuth-NoFallback-TLS13": "TODO: first pass, this should be fixed",
-        "ClientAuth-NoFallback-ECDSA": "TODO: first pass, this should be fixed",
-        "ClientAuth-NoFallback-RSA": "TODO: first pass, this should be fixed",
-        "ECDSACurveMismatch-Verify-TLS13": "TODO: first pass, this should be fixed",
-        "Ed25519DefaultDisable-NoAdvertise": "TODO: first pass, this should be fixed",
-        "Ed25519DefaultDisable-NoAccept": "TODO: first pass, this should be fixed",
-        "NoCommonSignatureAlgorithms-TLS12-Fallback": "TODO: first pass, this should be fixed",
         "UnknownExtension-Client": "TODO: first pass, this should be fixed",
         "UnknownUnencryptedExtension-Client-TLS13": "TODO: first pass, this should be fixed",
         "UnofferedExtension-Client-TLS13": "TODO: first pass, this should be fixed",
@@ -153,7 +152,6 @@
         "TrailingMessageData-TLS13-ClientCertificate-TLS": "TODO: first pass, this should be fixed",
         "TrailingMessageData-TLS13-ClientCertificateVerify-TLS": "TODO: first pass, this should be fixed",
         "TrailingMessageData-TLS13-ServerCertificate-TLS": "TODO: first pass, this should be fixed",
-        "ResumeTLS12SessionID-TLS13": "We don't support session ID resumption",
         "SkipEarlyData-TLS13": "TODO: first pass, this should be fixed",
         "DuplicateKeyShares-TLS13": "TODO: first pass, this should be fixed",
         "Server-TooLongSessionID-TLS13": "TODO: first pass, this should be fixed",

src/crypto/tls/bogo_shim_test.go

@@ -35,8 +35,10 @@ var (
 
 	isHandshakerSupported = flag.Bool("is-handshaker-supported", false, "")
 
-	keyfile  = flag.String("key-file", "", "")
-	certfile = flag.String("cert-file", "", "")
+	keyfile      = flag.String("key-file", "", "")
+	certfile     = flag.String("cert-file", "", "")
+	ocspResponse = flagBase64("ocsp-response", "")
+	signingPrefs = flagIntSlice("signing-prefs", "")
 
 	trustCert = flag.String("trust-cert", "", "")
 
@@ -55,13 +57,17 @@ var (
 
 	resumeCount = flag.Int("resume-count", 0, "")
 
-	curves        = flagStringSlice("curves", "")
+	curves        = flagIntSlice("curves", "")
 	expectedCurve = flag.String("expect-curve-id", "", "")
 
+	verifyPrefs        = flagIntSlice("verify-prefs", "")
+	expectedSigAlg     = flag.String("expect-peer-signature-algorithm", "", "")
+	expectedPeerSigAlg = flagIntSlice("expect-peer-verify-pref", "")
+
 	shimID = flag.Uint64("shim-id", 0, "")
 	_      = flag.Bool("ipv6", false, "")
 
-	echConfigListB64           = flag.String("ech-config-list", "", "")
+	echConfigList              = flagBase64("ech-config-list", "")
 	expectECHAccepted          = flag.Bool("expect-ech-accept", false, "")
 	expectHRR                  = flag.Bool("expect-hrr", false, "")
 	expectNoHRR                = flag.Bool("expect-no-hrr", false, "")
@@ -71,7 +77,7 @@ var (
 	_                          = flag.Bool("expect-no-ech-name-override", false, "")
 	_                          = flag.String("expect-ech-name-override", "", "")
 	_                          = flag.Bool("reverify-on-resume", false, "")
-	onResumeECHConfigListB64   = flag.String("on-resume-ech-config-list", "", "")
+	onResumeECHConfigList      = flagBase64("on-resume-ech-config-list", "")
 	_                          = flag.Bool("on-resume-expect-reject-early-data", false, "")
 	onResumeExpectECHAccepted  = flag.Bool("on-resume-expect-ech-accept", false, "")
 	_                          = flag.Bool("on-resume-expect-no-ech-name-override", false, "")
@@ -105,7 +111,7 @@ var (
 type stringSlice []string
 
 func flagStringSlice(name, usage string) *stringSlice {
-	f := &stringSlice{}
+	f := new(stringSlice)
 	flag.Var(f, name, usage)
 	return f
 }
@@ -119,12 +125,59 @@ func (saf *stringSlice) Set(s string) error {
 	return nil
 }
 
+type intSlice []int64
+
+func flagIntSlice(name, usage string) *intSlice {
+	f := new(intSlice)
+	flag.Var(f, name, usage)
+	return f
+}
+
+func (sf *intSlice) String() string {
+	return strings.Join(strings.Split(fmt.Sprint(*sf), " "), ",")
+}
+
+func (sf *intSlice) Set(s string) error {
+	i, err := strconv.ParseInt(s, 10, 64)
+	if err != nil {
+		return err
+	}
+	*sf = append(*sf, i)
+	return nil
+}
+
+type base64Flag []byte
+
+func flagBase64(name, usage string) *base64Flag {
+	f := new(base64Flag)
+	flag.Var(f, name, usage)
+	return f
+}
+
+func (f *base64Flag) String() string {
+	return base64.StdEncoding.EncodeToString(*f)
+}
+
+func (f *base64Flag) Set(s string) error {
+	if *f != nil {
+		return fmt.Errorf("multiple base64 values not supported")
+	}
+	b, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		return err
+	}
+	*f = b
+	return nil
+}
+
 func bogoShim() {
 	if *isHandshakerSupported {
 		fmt.Println("No")
 		return
 	}
 
+	fmt.Printf("BoGo shim flags: %q", os.Args[1:])
+
 	// Test with both the default and insecure cipher suites.
 	var ciphersuites []uint16
 	for _, s := range append(CipherSuites(), InsecureCipherSuites()...) {
@@ -218,7 +271,39 @@ func bogoShim() {
 		if err != nil {
 			log.Fatalf("load key-file err: %s", err)
 		}
-		cfg.Certificates = []Certificate{pair}
+		for _, id := range *signingPrefs {
+			pair.SupportedSignatureAlgorithms = append(pair.SupportedSignatureAlgorithms, SignatureScheme(id))
+		}
+		pair.OCSPStaple = *ocspResponse
+		// Use Get[Client]Certificate to force the use of the certificate, which
+		// more closely matches the BoGo expectations (e.g. handshake failure if
+		// no client certificates are compatible).
+		cfg.GetCertificate = func(chi *ClientHelloInfo) (*Certificate, error) {
+			if *expectedPeerSigAlg != nil {
+				if len(chi.SignatureSchemes) != len(*expectedPeerSigAlg) {
+					return nil, fmt.Errorf("unexpected signature algorithms: got %s, want %v", chi.SignatureSchemes, *expectedPeerSigAlg)
+				}
+				for i := range *expectedPeerSigAlg {
+					if chi.SignatureSchemes[i] != SignatureScheme((*expectedPeerSigAlg)[i]) {
+						return nil, fmt.Errorf("unexpected signature algorithms: got %s, want %v", chi.SignatureSchemes, *expectedPeerSigAlg)
+					}
+				}
+			}
+			return &pair, nil
+		}
+		cfg.GetClientCertificate = func(cri *CertificateRequestInfo) (*Certificate, error) {
+			if *expectedPeerSigAlg != nil {
+				if len(cri.SignatureSchemes) != len(*expectedPeerSigAlg) {
+					return nil, fmt.Errorf("unexpected signature algorithms: got %s, want %v", cri.SignatureSchemes, *expectedPeerSigAlg)
+				}
+				for i := range *expectedPeerSigAlg {
+					if cri.SignatureSchemes[i] != SignatureScheme((*expectedPeerSigAlg)[i]) {
+						return nil, fmt.Errorf("unexpected signature algorithms: got %s, want %v", cri.SignatureSchemes, *expectedPeerSigAlg)
+					}
+				}
+			}
+			return &pair, nil
+		}
 	}
 	if *trustCert != "" {
 		pool := x509.NewCertPool()
@@ -242,26 +327,24 @@ func bogoShim() {
 		cfg.ClientAuth = VerifyClientCertIfGiven
 	}
 
-	if *echConfigListB64 != "" {
-		echConfigList, err := base64.StdEncoding.DecodeString(*echConfigListB64)
-		if err != nil {
-			log.Fatalf("parse ech-config-list err: %s", err)
-		}
-		cfg.EncryptedClientHelloConfigList = echConfigList
+	if *echConfigList != nil {
+		cfg.EncryptedClientHelloConfigList = *echConfigList
 		cfg.MinVersion = VersionTLS13
 	}
 
-	if len(*curves) != 0 {
-		for _, curveStr := range *curves {
-			id, err := strconv.Atoi(curveStr)
-			if err != nil {
-				log.Fatalf("failed to parse curve id %q: %s", curveStr, err)
-			}
+	if *curves != nil {
+		for _, id := range *curves {
 			cfg.CurvePreferences = append(cfg.CurvePreferences, CurveID(id))
 		}
 	}
 
-	if len(*echServerConfig) != 0 {
+	if *verifyPrefs != nil {
+		for _, id := range *verifyPrefs {
+			testingOnlySupportedSignatureAlgorithms = append(testingOnlySupportedSignatureAlgorithms, SignatureScheme(id))
+		}
+	}
+
+	if *echServerConfig != nil {
 		if len(*echServerConfig) != len(*echServerKey) || len(*echServerConfig) != len(*echServerRetryConfig) {
 			log.Fatal("-ech-server-config, -ech-server-key, and -ech-is-retry-config mismatch")
 		}
@@ -285,12 +368,8 @@ func bogoShim() {
 	}
 
 	for i := 0; i < *resumeCount+1; i++ {
-		if i > 0 && (*onResumeECHConfigListB64 != "") {
-			echConfigList, err := base64.StdEncoding.DecodeString(*onResumeECHConfigListB64)
-			if err != nil {
-				log.Fatalf("parse ech-config-list err: %s", err)
-			}
-			cfg.EncryptedClientHelloConfigList = echConfigList
+		if i > 0 && *onResumeECHConfigList != nil {
+			cfg.EncryptedClientHelloConfigList = *onResumeECHConfigList
 		}
 
 		conn, err := net.Dial("tcp", net.JoinHostPort("localhost", *port))
@@ -343,7 +422,7 @@ func bogoShim() {
 		if err != io.EOF {
 			retryErr, ok := err.(*ECHRejectionError)
 			if !ok {
-				log.Fatalf("unexpected error type returned: %v", err)
+				log.Fatal(err)
 			}
 			if *expectNoECHRetryConfigs && len(retryErr.RetryConfigList) > 0 {
 				log.Fatalf("expected no ECH retry configs, got some")
@@ -408,10 +487,21 @@ func bogoShim() {
 			if err != nil {
 				log.Fatalf("failed to parse -expect-curve-id: %s", err)
 			}
-			if tlsConn.curveID != CurveID(expectedCurveID) {
+			if cs.CurveID != CurveID(expectedCurveID) {
 				log.Fatalf("unexpected curve id: want %d, got %d", expectedCurveID, tlsConn.curveID)
 			}
 		}
+
+		// TODO: implement testingOnlyPeerSignatureAlgorithm on resumption.
+		if *expectedSigAlg != "" && !cs.DidResume {
+			expectedSigAlgID, err := strconv.Atoi(*expectedSigAlg)
+			if err != nil {
+				log.Fatalf("failed to parse -expect-peer-signature-algorithm: %s", err)
+			}
+			if cs.testingOnlyPeerSignatureAlgorithm != SignatureScheme(expectedSigAlgID) {
+				log.Fatalf("unexpected peer signature algorithm: want %s, got %s", SignatureScheme(expectedSigAlgID), cs.testingOnlyPeerSignatureAlgorithm)
+			}
+		}
 	}
 }
 
@@ -491,20 +581,36 @@ func TestBogoSuite(t *testing.T) {
 	assertResults := map[string]string{
 		"CurveTest-Client-MLKEM-TLS13": "PASS",
 		"CurveTest-Server-MLKEM-TLS13": "PASS",
+
+		// Various signature algorithm tests checking that we enforce our
+		// preferences on the peer.
+		"ClientAuth-Enforced":                    "PASS",
+		"ServerAuth-Enforced":                    "PASS",
+		"ClientAuth-Enforced-TLS13":              "PASS",
+		"ServerAuth-Enforced-TLS13":              "PASS",
+		"VerifyPreferences-Advertised":           "PASS",
+		"VerifyPreferences-Enforced":             "PASS",
+		"Client-TLS12-NoSign-RSA_PKCS1_MD5_SHA1": "PASS",
+		"Server-TLS12-NoSign-RSA_PKCS1_MD5_SHA1": "PASS",
+		"Client-TLS13-NoSign-RSA_PKCS1_MD5_SHA1": "PASS",
+		"Server-TLS13-NoSign-RSA_PKCS1_MD5_SHA1": "PASS",
 	}
 
 	for name, result := range results.Tests {
 		// This is not really the intended way to do this... but... it works?
 		t.Run(name, func(t *testing.T) {
 			if result.Actual == "FAIL" && result.IsUnexpected {
-				t.Fatal(result.Error)
+				t.Fail()
 			}
-			if expectedResult, ok := assertResults[name]; ok && expectedResult != result.Actual {
-				t.Fatalf("unexpected result: got %s, want %s", result.Actual, assertResults[name])
+			if result.Error != "" {
+				t.Log(result.Error)
+			}
+			if exp, ok := assertResults[name]; ok && exp != result.Actual {
+				t.Errorf("unexpected result: got %s, want %s", result.Actual, exp)
 			}
 			delete(assertResults, name)
 			if result.Actual == "SKIP" {
-				t.Skip()
+				t.SkipNow()
 			}
 		})
 	}

src/crypto/tls/common.go

@@ -309,6 +309,10 @@ type ConnectionState struct {
 
 	// testingOnlyDidHRR is true if a HelloRetryRequest was sent/received.
 	testingOnlyDidHRR bool
+
+	// testingOnlyPeerSignatureAlgorithm is the signature algorithm used by the
+	// peer to sign the handshake. It is not set for resumed connections.
+	testingOnlyPeerSignatureAlgorithm SignatureScheme
 }
 
 // ExportKeyingMaterial returns length bytes of exported key material in a new
@@ -1684,35 +1688,62 @@ func unexpectedMessageError(wanted, got any) error {
 	return fmt.Errorf("tls: received unexpected handshake message of type %T when waiting for %T", got, wanted)
 }
 
+var testingOnlySupportedSignatureAlgorithms []SignatureScheme
+
 // supportedSignatureAlgorithms returns the supported signature algorithms for
 // the given minimum TLS version, to advertise in ClientHello and
 // CertificateRequest messages.
 func supportedSignatureAlgorithms(minVers uint16) []SignatureScheme {
 	sigAlgs := defaultSupportedSignatureAlgorithms()
-	if fips140tls.Required() {
-		sigAlgs = slices.DeleteFunc(sigAlgs, func(s SignatureScheme) bool {
-			return !slices.Contains(allowedSignatureAlgorithmsFIPS, s)
-		})
+	if testingOnlySupportedSignatureAlgorithms != nil {
+		sigAlgs = slices.Clone(testingOnlySupportedSignatureAlgorithms)
 	}
-	if minVers > VersionTLS12 {
-		sigAlgs = slices.DeleteFunc(sigAlgs, func(s SignatureScheme) bool {
-			sigType, sigHash, _ := typeAndHashFromSignatureScheme(s)
-			return sigType == signaturePKCS1v15 || sigHash == crypto.SHA1
-		})
+	return slices.DeleteFunc(sigAlgs, func(s SignatureScheme) bool {
+		return isDisabledSignatureAlgorithm(minVers, s, false)
+	})
+}
+
+var tlssha1 = godebug.New("tlssha1")
+
+func isDisabledSignatureAlgorithm(version uint16, s SignatureScheme, isCert bool) bool {
+	if fips140tls.Required() && !slices.Contains(allowedSignatureAlgorithmsFIPS, s) {
+		return true
 	}
-	return sigAlgs
+
+	// For the _cert extension we include all algorithms, including SHA-1 and
+	// PKCS#1 v1.5, because it's more likely that something on our side will be
+	// willing to accept a *-with-SHA1 certificate (e.g. with a custom
+	// VerifyConnection or by a direct match with the CertPool), than that the
+	// peer would have a better certificate but is just choosing not to send it.
+	// crypto/x509 will refuse to verify important SHA-1 signatures anyway.
+	if isCert {
+		return false
+	}
+
+	// TLS 1.3 removed support for PKCS#1 v1.5 and SHA-1 signatures,
+	// and Go 1.25 removed support for SHA-1 signatures in TLS 1.2.
+	if version > VersionTLS12 {
+		sigType, sigHash, _ := typeAndHashFromSignatureScheme(s)
+		if sigType == signaturePKCS1v15 || sigHash == crypto.SHA1 {
+			return true
+		}
+	} else if tlssha1.Value() != "1" {
+		_, sigHash, _ := typeAndHashFromSignatureScheme(s)
+		if sigHash == crypto.SHA1 {
+			return true
+		}
+	}
+
+	return false
 }
 
 // supportedSignatureAlgorithmsCert returns the supported algorithms for
 // signatures in certificates.
 func supportedSignatureAlgorithmsCert() []SignatureScheme {
-	sigAlgs := defaultSupportedSignatureAlgorithmsCert()
-	if fips140tls.Required() {
-		sigAlgs = slices.DeleteFunc(sigAlgs, func(s SignatureScheme) bool {
-			return !slices.Contains(allowedSignatureAlgorithmsFIPS, s)
-		})
-	}
-	return sigAlgs
+	sigAlgs := defaultSupportedSignatureAlgorithms()
+	return slices.DeleteFunc(sigAlgs, func(s SignatureScheme) bool {
+		return isDisabledSignatureAlgorithm(0, s, true)
+	})
 }
 
 func isSupportedSignatureAlgorithm(sigAlg SignatureScheme, supportedSignatureAlgorithms []SignatureScheme) bool {

src/crypto/tls/conn.go

@@ -51,6 +51,7 @@ type Conn struct {
 	didHRR           bool // whether a HelloRetryRequest was sent/received
 	cipherSuite      uint16
 	curveID          CurveID
+	peerSigAlg       SignatureScheme
 	ocspResponse     []byte   // stapled OCSP response
 	scts             [][]byte // signed certificate timestamps from server
 	peerCertificates []*x509.Certificate
@@ -1630,6 +1631,7 @@ func (c *Conn) connectionStateLocked() ConnectionState {
 	state.NegotiatedProtocol = c.clientProtocol
 	state.DidResume = c.didResume
 	state.testingOnlyDidHRR = c.didHRR
+	state.testingOnlyPeerSignatureAlgorithm = c.peerSigAlg
 	state.CurveID = c.curveID
 	state.NegotiatedProtocolIsMutual = true
 	state.ServerName = c.serverName

src/crypto/tls/defaults.go

@@ -24,53 +24,11 @@ func defaultCurvePreferences() []CurveID {
 	return []CurveID{X25519MLKEM768, X25519, CurveP256, CurveP384, CurveP521}
 }
 
-var tlssha1 = godebug.New("tlssha1")
-
 // defaultSupportedSignatureAlgorithms returns the signature and hash algorithms that
 // the code advertises and supports in a TLS 1.2+ ClientHello and in a TLS 1.2+
 // CertificateRequest. The two fields are merged to match with TLS 1.3.
 // Note that in TLS 1.2, the ECDSA algorithms are not constrained to P-256, etc.
 func defaultSupportedSignatureAlgorithms() []SignatureScheme {
-	if tlssha1.Value() == "1" {
-		return []SignatureScheme{
-			PSSWithSHA256,
-			ECDSAWithP256AndSHA256,
-			Ed25519,
-			PSSWithSHA384,
-			PSSWithSHA512,
-			PKCS1WithSHA256,
-			PKCS1WithSHA384,
-			PKCS1WithSHA512,
-			ECDSAWithP384AndSHA384,
-			ECDSAWithP521AndSHA512,
-			PKCS1WithSHA1,
-			ECDSAWithSHA1,
-		}
-	}
-	return []SignatureScheme{
-		PSSWithSHA256,
-		ECDSAWithP256AndSHA256,
-		Ed25519,
-		PSSWithSHA384,
-		PSSWithSHA512,
-		PKCS1WithSHA256,
-		PKCS1WithSHA384,
-		PKCS1WithSHA512,
-		ECDSAWithP384AndSHA384,
-		ECDSAWithP521AndSHA512,
-	}
-}
-
-// defaultSupportedSignatureAlgorithmsCert returns the signature algorithms that
-// the code advertises as supported for signatures in certificates.
-//
-// We include all algorithms, including SHA-1 and PKCS#1 v1.5, because it's more
-// likely that something on our side will be willing to accept a *-with-SHA1
-// certificate (e.g. with a custom VerifyConnection or by a direct match with
-// the CertPool), than that the peer would have a better certificate but is just
-// choosing not to send it. crypto/x509 will refuse to verify important SHA-1
-// signatures anyway.
-func defaultSupportedSignatureAlgorithmsCert() []SignatureScheme {
 	return []SignatureScheme{
 		PSSWithSHA256,
 		ECDSAWithP256AndSHA256,

src/crypto/tls/fips140_test.go

@@ -18,6 +18,7 @@ import (
 	"internal/testenv"
 	"math/big"
 	"net"
+	"os"
 	"runtime"
 	"strings"
 	"testing"
@@ -262,15 +263,19 @@ func fipsHandshake(t *testing.T, clientConfig, serverConfig *Config) (clientErr,
 
 func TestFIPSServerSignatureAndHash(t *testing.T) {
 	defer func() {
-		testingOnlyForceClientHelloSignatureAlgorithms = nil
+		testingOnlySupportedSignatureAlgorithms = nil
 	}()
+	defer func(godebug string) {
+		os.Setenv("GODEBUG", godebug)
+	}(os.Getenv("GODEBUG"))
+	os.Setenv("GODEBUG", "tlssha1=1")
 
 	for _, sigHash := range defaultSupportedSignatureAlgorithms() {
 		t.Run(fmt.Sprintf("%v", sigHash), func(t *testing.T) {
 			serverConfig := testConfig.Clone()
 			serverConfig.Certificates = make([]Certificate, 1)
 
-			testingOnlyForceClientHelloSignatureAlgorithms = []SignatureScheme{sigHash}
+			testingOnlySupportedSignatureAlgorithms = []SignatureScheme{sigHash}
 
 			sigType, _, _ := typeAndHashFromSignatureScheme(sigHash)
 			switch sigType {

src/crypto/tls/handshake_client.go

@@ -20,7 +20,6 @@ import (
 	"errors"
 	"fmt"
 	"hash"
-	"internal/byteorder"
 	"internal/godebug"
 	"io"
 	"net"
@@ -42,8 +41,6 @@ type clientHandshakeState struct {
 	ticket       []byte        // a fresh ticket received during this handshake
 }
 
-var testingOnlyForceClientHelloSignatureAlgorithms []SignatureScheme
-
 func (c *Conn) makeClientHello() (*clientHelloMsg, *keySharePrivateKeys, *echClientContext, error) {
 	config := c.config
 	if len(config.ServerName) == 0 && !config.InsecureSkipVerify {
@@ -126,9 +123,6 @@ func (c *Conn) makeClientHello() (*clientHelloMsg, *keySharePrivateKeys, *echCli
 		hello.supportedSignatureAlgorithms = supportedSignatureAlgorithms(minVersion)
 		hello.supportedSignatureAlgorithmsCert = supportedSignatureAlgorithmsCert()
 	}
-	if testingOnlyForceClientHelloSignatureAlgorithms != nil {
-		hello.supportedSignatureAlgorithms = testingOnlyForceClientHelloSignatureAlgorithms
-	}
 
 	var keyShareKeys *keySharePrivateKeys
 	if maxVersion >= VersionTLS13 {
@@ -732,8 +726,9 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 			c.sendAlert(alertIllegalParameter)
 			return err
 		}
-		if len(skx.key) >= 3 && skx.key[0] == 3 /* named curve */ {
-			c.curveID = CurveID(byteorder.BEUint16(skx.key[1:]))
+		if keyAgreement, ok := keyAgreement.(*ecdheKeyAgreement); ok {
+			c.curveID = keyAgreement.curveID
+			c.peerSigAlg = keyAgreement.signatureAlgorithm
 		}
 
 		msg, err = c.readHandshake(&hs.finishedHash)
@@ -819,7 +814,7 @@ func (hs *clientHandshakeState) doFullHandshake() error {
 		if c.vers >= VersionTLS12 {
 			signatureAlgorithm, err := selectSignatureScheme(c.vers, chainToSend, certReq.supportedSignatureAlgorithms)
 			if err != nil {
-				c.sendAlert(alertIllegalParameter)
+				c.sendAlert(alertHandshakeFailure)
 				return err
 			}
 			sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)

src/crypto/tls/handshake_client_tls13.go

@@ -694,6 +694,7 @@ func (hs *clientHandshakeStateTLS13) readServerCertificate() error {
 		c.sendAlert(alertDecryptError)
 		return errors.New("tls: invalid signature by the server certificate: " + err.Error())
 	}
+	c.peerSigAlg = certVerify.signatureAlgorithm
 
 	if err := transcriptMsg(certVerify, hs.transcript); err != nil {
 		return err

src/crypto/tls/handshake_server.go

@@ -16,7 +16,6 @@ import (
 	"errors"
 	"fmt"
 	"hash"
-	"internal/byteorder"
 	"io"
 	"time"
 )
@@ -632,8 +631,9 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 		return err
 	}
 	if skx != nil {
-		if len(skx.key) >= 3 && skx.key[0] == 3 /* named curve */ {
-			c.curveID = CurveID(byteorder.BEUint16(skx.key[1:]))
+		if keyAgreement, ok := keyAgreement.(*ecdheKeyAgreement); ok {
+			c.curveID = keyAgreement.curveID
+			c.peerSigAlg = keyAgreement.signatureAlgorithm
 		}
 		if _, err := hs.c.writeHandshakeRecord(skx, &hs.finishedHash); err != nil {
 			return err
@@ -789,6 +789,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 			c.sendAlert(alertDecryptError)
 			return errors.New("tls: invalid signature by the client certificate: " + err.Error())
 		}
+		c.peerSigAlg = certVerify.signatureAlgorithm
 
 		if err := transcriptMsg(certVerify, &hs.finishedHash); err != nil {
 			return err

src/crypto/tls/handshake_server_tls13.go

@@ -1115,6 +1115,7 @@ func (hs *serverHandshakeStateTLS13) readClientCertificate() error {
 			c.sendAlert(alertDecryptError)
 			return errors.New("tls: invalid signature by the client certificate: " + err.Error())
 		}
+		c.peerSigAlg = certVerify.signatureAlgorithm
 
 		if err := transcriptMsg(certVerify, hs.transcript); err != nil {
 			return err

src/crypto/tls/key_agreement.go

@@ -165,25 +165,29 @@ type ecdheKeyAgreement struct {
 	// and returned in generateClientKeyExchange.
 	ckx             *clientKeyExchangeMsg
 	preMasterSecret []byte
+
+	// curveID and signatureAlgorithm are set by processServerKeyExchange and
+	// generateServerKeyExchange.
+	curveID            CurveID
+	signatureAlgorithm SignatureScheme
 }
 
 func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
-	var curveID CurveID
 	for _, c := range clientHello.supportedCurves {
 		if config.supportsCurve(ka.version, c) {
-			curveID = c
+			ka.curveID = c
 			break
 		}
 	}
 
-	if curveID == 0 {
+	if ka.curveID == 0 {
 		return nil, errors.New("tls: no supported elliptic curves offered")
 	}
-	if _, ok := curveForCurveID(curveID); !ok {
+	if _, ok := curveForCurveID(ka.curveID); !ok {
 		return nil, errors.New("tls: CurvePreferences includes unsupported curve")
 	}
 
-	key, err := generateECDHEKey(config.rand(), curveID)
+	key, err := generateECDHEKey(config.rand(), ka.curveID)
 	if err != nil {
 		return nil, err
 	}
@@ -193,8 +197,8 @@ func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Cer
 	ecdhePublic := key.PublicKey().Bytes()
 	serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
 	serverECDHEParams[0] = 3 // named curve
-	serverECDHEParams[1] = byte(curveID >> 8)
-	serverECDHEParams[2] = byte(curveID)
+	serverECDHEParams[1] = byte(ka.curveID >> 8)
+	serverECDHEParams[2] = byte(ka.curveID)
 	serverECDHEParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHEParams[4:], ecdhePublic)
 
@@ -203,15 +207,14 @@ func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Cer
 		return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
 	}
 
-	var signatureAlgorithm SignatureScheme
 	var sigType uint8
 	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
-		signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
+		ka.signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
 		if err != nil {
 			return nil, err
 		}
-		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
+		sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
 		if err != nil {
 			return nil, err
 		}
@@ -249,8 +252,8 @@ func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Cer
 	copy(skx.key, serverECDHEParams)
 	k := skx.key[len(serverECDHEParams):]
 	if ka.version >= VersionTLS12 {
-		k[0] = byte(signatureAlgorithm >> 8)
-		k[1] = byte(signatureAlgorithm)
+		k[0] = byte(ka.signatureAlgorithm >> 8)
+		k[1] = byte(ka.signatureAlgorithm)
 		k = k[2:]
 	}
 	k[0] = byte(len(sig) >> 8)
@@ -284,7 +287,7 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	if skx.key[0] != 3 { // named curve
 		return errors.New("tls: server selected unsupported curve")
 	}
-	curveID := CurveID(skx.key[1])<<8 | CurveID(skx.key[2])
+	ka.curveID = CurveID(skx.key[1])<<8 | CurveID(skx.key[2])
 
 	publicLen := int(skx.key[3])
 	if publicLen+4 > len(skx.key) {
@@ -298,15 +301,15 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 		return errServerKeyExchange
 	}
 
-	if !slices.Contains(clientHello.supportedCurves, curveID) {
+	if !slices.Contains(clientHello.supportedCurves, ka.curveID) {
 		return errors.New("tls: server selected unoffered curve")
 	}
 
-	if _, ok := curveForCurveID(curveID); !ok {
+	if _, ok := curveForCurveID(ka.curveID); !ok {
 		return errors.New("tls: server selected unsupported curve")
 	}
 
-	key, err := generateECDHEKey(config.rand(), curveID)
+	key, err := generateECDHEKey(config.rand(), ka.curveID)
 	if err != nil {
 		return err
 	}
@@ -330,16 +333,16 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 	var sigType uint8
 	var sigHash crypto.Hash
 	if ka.version >= VersionTLS12 {
-		signatureAlgorithm := SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
+		ka.signatureAlgorithm = SignatureScheme(sig[0])<<8 | SignatureScheme(sig[1])
 		sig = sig[2:]
 		if len(sig) < 2 {
 			return errServerKeyExchange
 		}
 
-		if !isSupportedSignatureAlgorithm(signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
+		if !isSupportedSignatureAlgorithm(ka.signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
 			return errors.New("tls: certificate used with invalid signature algorithm")
 		}
-		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
+		sigType, sigHash, err = typeAndHashFromSignatureScheme(ka.signatureAlgorithm)
 		if err != nil {
 			return err
 		}