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crypto/x509: always emit a critical SAN extension if the Subject is empty.

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The RFC is a little ambiguous here: “the subject field contains an empty
sequence” could mean that it's a non-empty sequence where one of the
sets contains an empty sequence. But, in context, I think it means “the
subject field is an empty sequence”.

Fixes #22249

Change-Id: Idfe1592411573f6e871b5fb997e7d545597a0937
Reviewed-on: https://go-review.googlesource.com/70852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Adam Langley 2017-10-13 15:36:01 -07:00
parent 2f1de1593e
commit 3a395e2283
2 changed files with 46 additions and 2 deletions
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12
src/crypto/x509/x509.go
View File

@ -1708,7 +1708,7 @@ func isIA5String(s string) error {
return nil
}
func buildExtensions(template *Certificate, authorityKeyId []byte) (ret []pkix.Extension, err error) {
func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId []byte) (ret []pkix.Extension, err error) {
ret = make([]pkix.Extension, 10 /* maximum number of elements. */)
n := 0
@ -1817,6 +1817,10 @@ func buildExtensions(template *Certificate, authorityKeyId []byte) (ret []pkix.E
if (len(template.DNSNames) > 0 || len(template.EmailAddresses) > 0 || len(template.IPAddresses) > 0 || len(template.URIs) > 0) &&
!oidInExtensions(oidExtensionSubjectAltName, template.ExtraExtensions) {
ret[n].Id = oidExtensionSubjectAltName
// https://tools.ietf.org/html/rfc5280#section-4.2.1.6
// “If the subject field contains an empty sequence ... then
// subjectAltName extension ... is marked as critical”
ret[n].Critical = subjectIsEmpty
ret[n].Value, err = marshalSANs(template.DNSNames, template.EmailAddresses, template.IPAddresses, template.URIs)
if err != nil {
return
@ -2042,6 +2046,10 @@ func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgori
return
}
// emptyASN1Subject is the ASN.1 DER encoding of an empty Subject, which is
// just an empty SEQUENCE.
var emptyASN1Subject = []byte{0x30, 0}
// CreateCertificate creates a new certificate based on a template.
// The following members of template are used: AuthorityKeyId,
// BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage,
@ -2096,7 +2104,7 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
authorityKeyId = parent.SubjectKeyId
}
extensions, err := buildExtensions(template, authorityKeyId)
extensions, err := buildExtensions(template, bytes.Equal(asn1Subject, emptyASN1Subject), authorityKeyId)
if err != nil {
return
}

36
src/crypto/x509/x509_test.go
View File

@ -524,6 +524,14 @@ func TestCreateSelfSignedCertificate(t *testing.T) {
t.Errorf("%s: ExtraNames didn't override Country", test.name)
}
for _, ext := range cert.Extensions {
if ext.Id.Equal(oidExtensionSubjectAltName) {
if ext.Critical {
t.Fatal("SAN extension is marked critical")
}
}
}
found := false
for _, atv := range cert.Subject.Names {
if atv.Type.Equal([]int{2, 5, 4, 42}) {
@ -1736,3 +1744,31 @@ func TestAdditionFieldsInGeneralSubtree(t *testing.T) {
t.Fatalf("failed to parse certificate: %s", err)
}
}
func TestEmptySubject(t *testing.T) {
template := Certificate{
SerialNumber: big.NewInt(1),
DNSNames: []string{"example.com"},
}
derBytes, err := CreateCertificate(rand.Reader, &template, &template, &testPrivateKey.PublicKey, testPrivateKey)
if err != nil {
t.Fatalf("failed to create certificate: %s", err)
}
cert, err := ParseCertificate(derBytes)
if err != nil {
t.Fatalf("failed to parse certificate: %s", err)
}
for _, ext := range cert.Extensions {
if ext.Id.Equal(oidExtensionSubjectAltName) {
if !ext.Critical {
t.Fatal("SAN extension is not critical")
}
return
}
}
t.Fatal("SAN extension is missing")
}