Dynamic or open type

ASN.1 allows data structure designer to leave “holes” in field type specification of Sequence or Set types.

The idea behind that feature is that there can be times, when the exact field type is not known at the design time, or it is anticipated that new field types may come up in the future.

This “hole” type is manifested in the data structure by Any type. Technically, the actual type is serialized into an octet stream and then put into Any “container”, which is in fact an (untagged, by default) specialization of ASN.1 OctetString type.

Algorithm ::= SEQUENCE {
        algorithm OBJECT IDENTIFIER,
        parameters ANY DEFINED BY algorithm OPTIONAL
}

On the receiving end, to know how to interpret the open type serialization, the receiver can rely on the supplied value in the other field of the data structure. That other field is semantically linked with the open type field. This link is expressed in ASN.1 by the DEFINE BY clause.

From ASN.1 perspective, it is not an error if the decoder does not know a type selector value it receives. In that case pyasn1 decoder just leaves serialized blob in the open type field.

Note

By default, ASN.1 ANY type has no tag. That makes it an “invisible” in serialization. However, like any other ASN.1 type, ANY type can be subtyped by tagging.

Besides scalar open type fields, ASN.1 allows using SET OF or SEQUENCE OF containers holding zero or more of ANY scalars.

AttributeTypeAndValues ::= SEQUENCE {
   type OBJECT IDENTIFIER,
   values SET OF ANY DEFINED BY type
}

Note

A single type selector field is used to guide the decoder of potentially many elements of a SET OF or SEQUENCE OF container all at once. That implies that all ANY elements must be of the same type in any given instance of a data structure.

When expressing ASN.1 type “holes” in pyasn1, the OpenType object should be used to establish a semantic link between type selector field and open type field.

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
    }
    """
    componentType = NamedTypes(
        NamedType('algorithm', ObjectIdentifier()),
        NamedType('parameters', Any(),
                  openType=OpenType('algorithm', algo_map))
    )

Similarly for SET OF ANY DEFINED BY or SEQUENCE OF ANY DEFINED BY constructs:

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 SET OF ANY DEFINED BY algorithm
    }
    """
    componentType = NamedTypes(
        NamedType('algorithm', ObjectIdentifier()),
        NamedType('parameters', SetOf(componentType=Any()),
                  openType=OpenType('algorithm', algo_map))
    )

• OpenType