Untyped fields of constructed types

To aid data structures flexibility, ASN.1 allows the designer to leave incomplete field type specification in the Sequence and Set types.

To figure out field’s type at the run time, a type selector field must accompany the open type field. The open type situation can be captured by the OpenType object.

algo_map = {
    ObjectIdentifier('1.2.840.113549.1.1.1'): rsaEncryption(),
    ObjectIdentifier('1.2.840.113549.1.1.2'): md2WithRSAEncryption()

class Algorithm(Sequence):
    Algorithm ::= SEQUENCE {
            algorithm OBJECT IDENTIFIER,
            parameters ANY DEFINED BY algorithm OPTIONAL
    componentType = NamedTypes(
        NamedType('algorithm', ObjectIdentifier()),
        OptionalNamedType('parameters', Any(),
                  openType=OpenType('algorithm', algo_map))