encoding/asn1: handle application tag in Marshal

Fixes #20488

Change-Id: Iae963b612aea3d9e814b08f655e2eb019ece256e
Reviewed-on: https://go-review.googlesource.com/44110
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

src/encoding/asn1/marshal.go

@@ -560,7 +560,6 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 	if !ok {
 		return nil, StructuralError{fmt.Sprintf("unknown Go type: %v", v.Type())}
 	}
-	class := ClassUniversal
 
 	if params.timeType != 0 && tag != TagUTCTime {
 		return nil, StructuralError{"explicit time type given to non-time member"}
@@ -610,27 +609,33 @@ func makeField(v reflect.Value, params fieldParameters) (e encoder, err error) {
 
 	bodyLen := t.body.Len()
 
-	if params.explicit {
+	class := ClassUniversal
-		t.tag = bytesEncoder(appendTagAndLength(t.scratch[:0], tagAndLength{class, tag, bodyLen, isCompound}))
-
-		tt := new(taggedEncoder)
-
-		tt.body = t
-
-		tt.tag = bytesEncoder(appendTagAndLength(tt.scratch[:0], tagAndLength{
-			class:      ClassContextSpecific,
-			tag:        *params.tag,
-			length:     bodyLen + t.tag.Len(),
-			isCompound: true,
-		}))
-
-		return tt, nil
-	}
-
 	if params.tag != nil {
+		if params.application {
+			class = ClassApplication
+		} else {
+			class = ClassContextSpecific
+		}
+
+		if params.explicit {
+			t.tag = bytesEncoder(appendTagAndLength(t.scratch[:0], tagAndLength{ClassUniversal, tag, bodyLen, isCompound}))
+
+			tt := new(taggedEncoder)
+
+			tt.body = t
+
+			tt.tag = bytesEncoder(appendTagAndLength(tt.scratch[:0], tagAndLength{
+				class:      class,
+				tag:        *params.tag,
+				length:     bodyLen + t.tag.Len(),
+				isCompound: true,
+			}))
+
+			return tt, nil
+		}
+
 		// implicit tag.
 		tag = *params.tag
-		class = ClassContextSpecific
 	}
 
 	t.tag = bytesEncoder(appendTagAndLength(t.scratch[:0], tagAndLength{class, tag, bodyLen, isCompound}))

src/encoding/asn1/marshal_test.go

@@ -71,6 +71,11 @@ type defaultTest struct {
 	A int `asn1:"optional,default:1"`
 }
 
+type applicationTest struct {
+	A int `asn1:"application,tag:0"`
+	B int `asn1:"application,tag:1,explicit"`
+}
+
 type testSET []int
 
 var PST = time.FixedZone("PST", -8*60*60)
@@ -152,6 +157,7 @@ var marshalTests = []marshalTest{
 	{defaultTest{0}, "3003020100"},
 	{defaultTest{1}, "3000"},
 	{defaultTest{2}, "3003020102"},
+	{applicationTest{1, 2}, "30084001016103020102"},
 }
 
 func TestMarshal(t *testing.T) {