crypto/x509: use truncated SHA-256 for SubjectKeyId

Fixes #71746

Change-Id: I6a6a46568b092933d8ac2039df99ee9f0edf6e56
Reviewed-on: https://go-review.googlesource.com/c/go/+/674477
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/godebug.md

@@ -183,6 +183,9 @@ limits. The default value `updatemaxprocs=1` will enable periodic updates.
 Go 1.25 disabled SHA-1 signature algorithms in TLS 1.2 according to RFC 9155.
 The default can be reverted using the `tlssha1=1` setting.
 
+Go 1.25 switched to SHA-256 to fill in missing SubjectKeyId in
+crypto/x509.CreateCertificate. The setting `x509sha256skid=0` reverts to SHA-1.
+
 Go 1.25 corrected the semantics of contention reports for runtime-internal locks,
 and so removed the [`runtimecontentionstacks` setting](/pkg/runtime#hdr-Environment_Variable).
 

doc/next/6-stdlib/99-minor/crypto/x509/71746.md

@@ -0,0 +1,2 @@
+[CreateCertificate] now uses truncated SHA-256 to populate the `SubjectKeyId` if
+it is missing. The GODEBUG setting `x509sha256skid=0` reverts to SHA-1.

src/crypto/x509/x509.go

@@ -29,6 +29,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rsa"
 	"crypto/sha1"
+	"crypto/sha256"
 	"crypto/x509/pkix"
 	"encoding/asn1"
 	"encoding/pem"
@@ -1728,12 +1729,22 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
 
 	subjectKeyId := template.SubjectKeyId
 	if len(subjectKeyId) == 0 && template.IsCA {
-		// SubjectKeyId generated using method 1 in RFC 5280, Section 4.2.1.2:
-		//   (1) The keyIdentifier is composed of the 160-bit SHA-1 hash of the
-		//   value of the BIT STRING subjectPublicKey (excluding the tag,
-		//   length, and number of unused bits).
-		h := sha1.Sum(publicKeyBytes)
-		subjectKeyId = h[:]
+		if x509sha256skid.Value() == "0" {
+			x509sha256skid.IncNonDefault()
+			// SubjectKeyId generated using method 1 in RFC 5280, Section 4.2.1.2:
+			//   (1) The keyIdentifier is composed of the 160-bit SHA-1 hash of the
+			//   value of the BIT STRING subjectPublicKey (excluding the tag,
+			//   length, and number of unused bits).
+			h := sha1.Sum(publicKeyBytes)
+			subjectKeyId = h[:]
+		} else {
+			// SubjectKeyId generated using method 1 in RFC 7093, Section 2:
+			//    1) The keyIdentifier is composed of the leftmost 160-bits of the
+			//    SHA-256 hash of the value of the BIT STRING subjectPublicKey
+			//    (excluding the tag, length, and number of unused bits).
+			h := sha256.Sum256(publicKeyBytes)
+			subjectKeyId = h[:20]
+		}
 	}
 
 	// Check that the signer's public key matches the private key, if available.
@@ -1781,6 +1792,8 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
 	})
 }
 
+var x509sha256skid = godebug.New("x509sha256skid")
+
 // pemCRLPrefix is the magic string that indicates that we have a PEM encoded
 // CRL.
 var pemCRLPrefix = []byte("-----BEGIN X509 CRL")

src/internal/godebugs/table.go

@@ -70,6 +70,7 @@ var All = []Info{
 	{Name: "x509keypairleaf", Package: "crypto/tls", Changed: 23, Old: "0"},
 	{Name: "x509negativeserial", Package: "crypto/x509", Changed: 23, Old: "1"},
 	{Name: "x509rsacrt", Package: "crypto/x509", Changed: 24, Old: "0"},
+	{Name: "x509sha256skid", Package: "crypto/x509", Changed: 25, Old: "0"},
 	{Name: "x509usefallbackroots", Package: "crypto/x509"},
 	{Name: "x509usepolicies", Package: "crypto/x509", Changed: 24, Old: "0"},
 	{Name: "zipinsecurepath", Package: "archive/zip"},

src/runtime/metrics/doc.go

@@ -384,6 +384,10 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the crypto/x509
 		package due to a non-default GODEBUG=x509rsacrt=... setting.
 
+	/godebug/non-default-behavior/x509sha256skid:events
+		The number of non-default behaviors executed by the crypto/x509
+		package due to a non-default GODEBUG=x509sha256skid=... setting.
+
 	/godebug/non-default-behavior/x509usefallbackroots:events
 		The number of non-default behaviors executed by the crypto/x509
 		package due to a non-default GODEBUG=x509usefallbackroots=...