encoding/asn1: fix explicitly tagged Times.

https://golang.org/cl/153770043/ tried to fix the case where a
implicitly tagged Time, that happened to have the same tag as
GENERALIZEDTIME, shouldn't be parsed as a GENERALIZEDTIME.

It did so, mistakenly, by testing whether params.tag != nil. But
explicitly tagged values also have a non-nil tag and there the inner
tag actually does encode the type of the value.

This change instead tests whether the tag class is UNIVERSAL before
assuming that the tag contains type information.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/152380044

src/encoding/asn1/asn1.go

@@ -640,7 +640,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 	// when it sees a string, so if we see a different string type on the
 	// wire, we change the universal type to match.
 	if universalTag == tagPrintableString {
-		if params.tag == nil {
+		if t.class == classUniversal {
 			switch t.tag {
 			case tagIA5String, tagGeneralString, tagT61String, tagUTF8String:
 				universalTag = t.tag
@@ -652,7 +652,7 @@ func parseField(v reflect.Value, bytes []byte, initOffset int, params fieldParam
 
 	// Special case for time: UTCTime and GeneralizedTime both map to the
 	// Go type time.Time.
-	if universalTag == tagUTCTime && params.tag == nil && t.tag == tagGeneralizedTime {
+	if universalTag == tagUTCTime && t.tag == tagGeneralizedTime && t.class == classUniversal {
 		universalTag = tagGeneralizedTime
 	}
 

src/encoding/asn1/asn1_test.go

@@ -817,3 +817,51 @@ func TestStringSlice(t *testing.T) {
 		}
 	}
 }
+
+type explicitTaggedTimeTest struct {
+	Time time.Time `asn1:"explicit,tag:0"`
+}
+
+var explicitTaggedTimeTestData = []struct {
+	in  []byte
+	out explicitTaggedTimeTest
+}{
+	{[]byte{0x30, 0x11, 0xa0, 0xf, 0x17, 0xd, '9', '1', '0', '5', '0', '6', '1', '6', '4', '5', '4', '0', 'Z'},
+		explicitTaggedTimeTest{time.Date(1991, 05, 06, 16, 45, 40, 0, time.UTC)}},
+	{[]byte{0x30, 0x17, 0xa0, 0xf, 0x18, 0x13, '2', '0', '1', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '+', '0', '6', '0', '7'},
+		explicitTaggedTimeTest{time.Date(2010, 01, 02, 03, 04, 05, 0, time.FixedZone("", 6*60*60+7*60))}},
+}
+
+func TestExplicitTaggedTime(t *testing.T) {
+	// Test that a time.Time will match either tagUTCTime or
+	// tagGeneralizedTime.
+	for i, test := range explicitTaggedTimeTestData {
+		var got explicitTaggedTimeTest
+		_, err := Unmarshal(test.in, &got)
+		if err != nil {
+			t.Errorf("Unmarshal failed at index %d %v", i, err)
+		}
+		if !got.Time.Equal(test.out.Time) {
+			t.Errorf("#%d: got %v, want %v", i, got.Time, test.out.Time)
+		}
+	}
+}
+
+type implicitTaggedTimeTest struct {
+	Time time.Time `asn1:"tag:24"`
+}
+
+func TestImplicitTaggedTime(t *testing.T) {
+	// An implicitly tagged time value, that happens to have an implicit
+	// tag equal to a GENERALIZEDTIME, should still be parsed as a UTCTime.
+	// (There's no "timeType" in fieldParameters to determine what type of
+	// time should be expected when implicitly tagged.)
+	der := []byte{0x30, 0x0f, 0x80 | 24, 0xd, '9', '1', '0', '5', '0', '6', '1', '6', '4', '5', '4', '0', 'Z'}
+	var result implicitTaggedTimeTest
+	if _, err := Unmarshal(der, &result); err != nil {
+		t.Fatalf("Error while parsing: %s", err)
+	}
+	if expected := time.Date(1991, 05, 06, 16, 45, 40, 0, time.UTC); !result.Time.Equal(expected) {
+		t.Errorf("Wrong result. Got %v, want %v", result.Time, expected)
+	}
+}