crypto/x509: generate SubjectKeyId for CAs

Fixes #26676

Change-Id: I5bc91d4a8161bc6ff25effcf93f551f735fef115
Reviewed-on: https://go-review.googlesource.com/c/go/+/227098
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

diff --git a/src/crypto/x509/x509.go b/src/crypto/x509/x509.go
index 2bf6d379eb322558485c15fc9c6ca6740e14f1f4..6776608d7585d5dfd3c017688fd503adf1417d10 100644 (file)
--- a/src/crypto/x509/x509.go
+++ b/src/crypto/x509/x509.go
@@ -17,6 +17,7 @@ import (
        "crypto/ed25519"
        "crypto/elliptic"
        "crypto/rsa"
+       "crypto/sha1"
        _ "crypto/sha1"
        _ "crypto/sha256"
        _ "crypto/sha512"
@@ -1686,7 +1687,7 @@ func isIA5String(s string) error {
        return nil
 }
 
-func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId []byte) (ret []pkix.Extension, err error) {
+func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId []byte, subjectKeyId []byte) (ret []pkix.Extension, err error) {
        ret = make([]pkix.Extension, 10 /* maximum number of elements. */)
        n := 0
 
@@ -1751,9 +1752,9 @@ func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId
                n++
        }
 
-       if len(template.SubjectKeyId) > 0 && !oidInExtensions(oidExtensionSubjectKeyId, template.ExtraExtensions) {
+       if len(subjectKeyId) > 0 && !oidInExtensions(oidExtensionSubjectKeyId, template.ExtraExtensions) {
                ret[n].Id = oidExtensionSubjectKeyId
-               ret[n].Value, err = asn1.Marshal(template.SubjectKeyId)
+               ret[n].Value, err = asn1.Marshal(subjectKeyId)
                if err != nil {
                        return
                }
@@ -2082,6 +2083,9 @@ var emptyASN1Subject = []byte{0x30, 0}
 // The AuthorityKeyId will be taken from the SubjectKeyId of parent, if any,
 // unless the resulting certificate is self-signed. Otherwise the value from
 // template will be used.
+//
+// If SubjectKeyId from template is empty and the template is a CA, SubjectKeyId
+// will be generated from the hash of the public key.
 func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv interface{}) (cert []byte, err error) {
        key, ok := priv.(crypto.Signer)
        if !ok {
@@ -2117,12 +2121,24 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
                authorityKeyId = parent.SubjectKeyId
        }
 
-       extensions, err := buildExtensions(template, bytes.Equal(asn1Subject, emptyASN1Subject), authorityKeyId)
+       encodedPublicKey := asn1.BitString{BitLength: len(publicKeyBytes) * 8, Bytes: publicKeyBytes}
+       pki := publicKeyInfo{nil, publicKeyAlgorithm, encodedPublicKey}
+       subjectKeyId := template.SubjectKeyId
+       if len(subjectKeyId) == 0 && template.IsCA {
+               // SubjectKeyId generated using method 1 in RFC 5280, Section 4.2.1.2
+               b, err := asn1.Marshal(pki)
+               if err != nil {
+                       return nil, err
+               }
+               h := sha1.Sum(b)
+               subjectKeyId = h[:]
+       }
+
+       extensions, err := buildExtensions(template, bytes.Equal(asn1Subject, emptyASN1Subject), authorityKeyId, subjectKeyId)
        if err != nil {
                return
        }
 
-       encodedPublicKey := asn1.BitString{BitLength: len(publicKeyBytes) * 8, Bytes: publicKeyBytes}
        c := tbsCertificate{
                Version:            2,
                SerialNumber:       template.SerialNumber,
@@ -2130,7 +2146,7 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
                Issuer:             asn1.RawValue{FullBytes: asn1Issuer},
                Validity:           validity{template.NotBefore.UTC(), template.NotAfter.UTC()},
                Subject:            asn1.RawValue{FullBytes: asn1Subject},
-               PublicKey:          publicKeyInfo{nil, publicKeyAlgorithm, encodedPublicKey},
+               PublicKey:          pki,
                Extensions:         extensions,
        }
 

diff --git a/src/crypto/x509/x509_test.go b/src/crypto/x509/x509_test.go
index c2f110e87b39d7a9644d5907afb642861ce9b057..b7c5679e7a67275705d4ee5e73c9473202d7ccd7 100644 (file)
--- a/src/crypto/x509/x509_test.go
+++ b/src/crypto/x509/x509_test.go
@@ -1710,6 +1710,33 @@ func TestNoAuthorityKeyIdInSelfSignedCert(t *testing.T) {
        }
 }
 
+func TestNoSubjectKeyIdInCert(t *testing.T) {
+       template := &Certificate{
+               SerialNumber: big.NewInt(1),
+               Subject: pkix.Name{
+                       CommonName: "Σ Acme Co",
+               },
+               NotBefore: time.Unix(1000, 0),
+               NotAfter:  time.Unix(100000, 0),
+
+               BasicConstraintsValid: true,
+               IsCA:                  true,
+       }
+       if cert := serialiseAndParse(t, template); len(cert.SubjectKeyId) == 0 {
+               t.Fatalf("self-signed certificate did not generate subject key id using the public key")
+       }
+
+       template.IsCA = false
+       if cert := serialiseAndParse(t, template); len(cert.SubjectKeyId) != 0 {
+               t.Fatalf("self-signed certificate generated subject key id when it isn't a CA")
+       }
+
+       template.SubjectKeyId = []byte{1, 2, 3, 4}
+       if cert := serialiseAndParse(t, template); len(cert.SubjectKeyId) == 0 {
+               t.Fatalf("self-signed certificate erased explicit subject key id")
+       }
+}
+
 func TestASN1BitLength(t *testing.T) {
        tests := []struct {
                bytes  []byte