it is not necessary to use or to implement all of the data types and other features of ASN.1; you can implement only the features that you are using. Since DER uses the same framing for all data types, it is possible to skip past any fields that you do not care about (although in some cases you will still need to check its type, to determine whether or not an optional field is present; fortunately the type can be checked easily, even if it is not a type you implement).