crypto/x509: consider parents by Subject if AKID has no match

If a certificate somehow has an AKID, it should still chain successfully
to a parent without a SKID, even if the latter is invalid according to
RFC 5280, because only the Subject is authoritative.

This reverts to the behavior before #29233 was fixed in 77013065. Roots
with the right subject will still be shadowed by roots with the right
SKID and the wrong subject, but that's been the case for a long time, and
is left for a more complete fix in Go 1.13.

Updates #30079

Change-Id: If8ab0179aca86cb74caa926d1ef93fb5e416b4bb
Reviewed-on: https://go-review.googlesource.com/c/161097Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/x509/cert_pool.go

@@ -71,10 +71,15 @@ func (s *CertPool) findPotentialParents(cert *Certificate) []int {
 	if s == nil {
 		return nil
 	}
+
+	var candidates []int
 	if len(cert.AuthorityKeyId) > 0 {
-		return s.bySubjectKeyId[string(cert.AuthorityKeyId)]
+		candidates = s.bySubjectKeyId[string(cert.AuthorityKeyId)]
+	}
+	if len(candidates) == 0 {
+		candidates = s.byName[string(cert.RawIssuer)]
 	}
-	return s.byName[string(cert.RawIssuer)]
+	return candidates
 }
 
 func (s *CertPool) contains(cert *Certificate) bool {

src/crypto/x509/verify_test.go

@@ -386,6 +386,19 @@ var verifyTests = []verifyTest{
 
 		errorCallback: expectHostnameError("not valid for any names"),
 	},
+	{
+		// A certificate with an AKID should still chain to a parent without SKID.
+		// See Issue 30079.
+		leaf:        leafWithAKID,
+		roots:       []string{rootWithoutSKID},
+		currentTime: 1550000000,
+		dnsName:     "example",
+		systemSkip:  true,
+
+		expectedChains: [][]string{
+			{"Acme LLC", "Acme Co"},
+		},
+	},
 }
 
 func expectHostnameError(msg string) func(*testing.T, int, error) bool {
@@ -1679,6 +1692,109 @@ h7olHCpY9yMRiz0=
 -----END CERTIFICATE-----
 `
 
+const (
+	rootWithoutSKID = `
+Certificate:
+    Data:
+        Version: 3 (0x2)
+        Serial Number:
+            78:29:2a:dc:2f:12:39:7f:c9:33:93:ea:61:39:7d:70
+        Signature Algorithm: ecdsa-with-SHA256
+        Issuer: O = Acme Co
+        Validity
+            Not Before: Feb  4 22:56:34 2019 GMT
+            Not After : Feb  1 22:56:34 2029 GMT
+        Subject: O = Acme Co
+        Subject Public Key Info:
+            Public Key Algorithm: id-ecPublicKey
+                Public-Key: (256 bit)
+                pub:
+                    04:84:a6:8c:69:53:af:87:4b:39:64:fe:04:24:e6:
+                    d8:fc:d6:46:39:35:0e:92:dc:48:08:7e:02:5f:1e:
+                    07:53:5c:d9:e0:56:c5:82:07:f6:a3:e2:ad:f6:ad:
+                    be:a0:4e:03:87:39:67:0c:9c:46:91:68:6b:0e:8e:
+                    f8:49:97:9d:5b
+                ASN1 OID: prime256v1
+                NIST CURVE: P-256
+        X509v3 extensions:
+            X509v3 Key Usage: critical
+                Digital Signature, Key Encipherment, Certificate Sign
+            X509v3 Extended Key Usage:
+                TLS Web Server Authentication
+            X509v3 Basic Constraints: critical
+                CA:TRUE
+            X509v3 Subject Alternative Name:
+                DNS:example
+    Signature Algorithm: ecdsa-with-SHA256
+         30:46:02:21:00:c6:81:61:61:42:8d:37:e7:d0:c3:72:43:44:
+         17:bd:84:ff:88:81:68:9a:99:08:ab:3c:3a:c0:1e:ea:8c:ba:
+         c0:02:21:00:de:c9:fa:e5:5e:c6:e2:db:23:64:43:a9:37:42:
+         72:92:7f:6e:89:38:ea:9e:2a:a7:fd:2f:ea:9a:ff:20:21:e7
+-----BEGIN CERTIFICATE-----
+MIIBbzCCARSgAwIBAgIQeCkq3C8SOX/JM5PqYTl9cDAKBggqhkjOPQQDAjASMRAw
+DgYDVQQKEwdBY21lIENvMB4XDTE5MDIwNDIyNTYzNFoXDTI5MDIwMTIyNTYzNFow
+EjEQMA4GA1UEChMHQWNtZSBDbzBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABISm
+jGlTr4dLOWT+BCTm2PzWRjk1DpLcSAh+Al8eB1Nc2eBWxYIH9qPirfatvqBOA4c5
+ZwycRpFoaw6O+EmXnVujTDBKMA4GA1UdDwEB/wQEAwICpDATBgNVHSUEDDAKBggr
+BgEFBQcDATAPBgNVHRMBAf8EBTADAQH/MBIGA1UdEQQLMAmCB2V4YW1wbGUwCgYI
+KoZIzj0EAwIDSQAwRgIhAMaBYWFCjTfn0MNyQ0QXvYT/iIFompkIqzw6wB7qjLrA
+AiEA3sn65V7G4tsjZEOpN0Jykn9uiTjqniqn/S/qmv8gIec=
+-----END CERTIFICATE-----
+`
+	leafWithAKID = `
+	Certificate:
+    Data:
+        Version: 3 (0x2)
+        Serial Number:
+            f0:8a:62:f0:03:84:a2:cf:69:63:ad:71:3b:b6:5d:8c
+        Signature Algorithm: ecdsa-with-SHA256
+        Issuer: O = Acme Co
+        Validity
+            Not Before: Feb  4 23:06:52 2019 GMT
+            Not After : Feb  1 23:06:52 2029 GMT
+        Subject: O = Acme LLC
+        Subject Public Key Info:
+            Public Key Algorithm: id-ecPublicKey
+                Public-Key: (256 bit)
+                pub:
+                    04:5a:4e:4d:fb:ff:17:f7:b6:13:e8:29:45:34:81:
+                    39:ff:8c:9c:d9:8c:0a:9f:dd:b5:97:4c:2b:20:91:
+                    1c:4f:6b:be:53:27:66:ec:4a:ad:08:93:6d:66:36:
+                    0c:02:70:5d:01:ca:7f:c3:29:e9:4f:00:ba:b4:14:
+                    ec:c5:c3:34:b3
+                ASN1 OID: prime256v1
+                NIST CURVE: P-256
+        X509v3 extensions:
+            X509v3 Key Usage: critical
+                Digital Signature, Key Encipherment
+            X509v3 Extended Key Usage:
+                TLS Web Server Authentication
+            X509v3 Basic Constraints: critical
+                CA:FALSE
+            X509v3 Authority Key Identifier:
+                keyid:C2:2B:5F:91:78:34:26:09:42:8D:6F:51:B2:C5:AF:4C:0B:DE:6A:42
+
+            X509v3 Subject Alternative Name:
+                DNS:example
+    Signature Algorithm: ecdsa-with-SHA256
+         30:44:02:20:64:e0:ba:56:89:63:ce:22:5e:4f:22:15:fd:3c:
+         35:64:9a:3a:6b:7b:9a:32:a0:7f:f7:69:8c:06:f0:00:58:b8:
+         02:20:09:e4:9f:6d:8b:9e:38:e1:b6:01:d5:ee:32:a4:94:65:
+         93:2a:78:94:bb:26:57:4b:c7:dd:6c:3d:40:2b:63:90
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+`
+)
+
 var unknownAuthorityErrorTests = []struct {
 	cert     string
 	expected string