add FromObjectIdentifer(

src/encoding/asn1/asn1.go

@@ -293,12 +293,25 @@ func parseObjectIdentifier(bytes []byte) (s ObjectIdentifier, err error) {
 	return
 }
 
-var errInvalidDEROid = errors.New("invalid DER Object Identifier encoding")
+var (
+	errInvalidDEROid = errors.New("invalid DER Object Identifier encoding")
+	errInvalidOID    = errors.New("invalid oid")
+)
 
 type OID struct {
 	der []byte
 }
 
+func FromObjectIdentifer(oid ObjectIdentifier) (OID, error) {
+	enc, err := makeObjectIdentifier(oid)
+	if err != nil {
+		return OID{}, errInvalidOID
+	}
+	der := make([]byte, enc.Len())
+	enc.Encode(der)
+	return OID{der}, nil
+}
+
 // ParseOID parsed DER-encoded Object Identifier.
 // On success, der is referenced in the OID struct.
 // der must not be modified after parsing.

src/encoding/asn1/asn1_test.go

@@ -1239,5 +1239,15 @@ func TestOID(t *testing.T) {
 		if ok && !o.Equal(v.oid) {
 			t.Errorf("%v: after ToObjectIdentifer, is not equal to %v", v.raw, v.oid)
 		}
+
+		if v.oid != nil {
+			oid2, err := FromObjectIdentifer(v.oid)
+			if err != nil {
+				t.Errorf("%v: failed while creating OID from ObjectIdentifier: %v", v.raw, err)
+			}
+			if !oid2.Equal(oid) {
+				t.Errorf("%v: OID from ObjectIdentifier is not equal to oid", v.raw)
+			}
+		}
 	}
 }