ANY / ANY DEFINED BY

The ASN.1 ANY and ANY DEFINED BY types are flexible container types intended to hold arbitrary ASN.1 values. They differ mainly in how the contained type is determined:

• ANY — accepts any ASN.1 value, without further constraints.

• ANY DEFINED BY — links the actual type to another field’s value (usually an enumerated field, OID, or similar), allowing dynamic type selection.

In modern ASN.1 usage, ANY and ANY DEFINED BY are considered legacy constructs and are replaced by OPEN TYPE in the X.680 ASN.1 standard. However, they still appear in many older ASN.1 specifications and thus remain relevant.

Python Representation

In Python, generated ANY and ANY DEFINED BY classes store their contents as raw encoded bytes. These bytes represent the full BER/DER/CER/… encoding of the underlying value.

The generated classes will not automatically decode these bytes — instead, you must explicitly invoke the target class’ decoding method to interpret the value.

Example ASN.1 definitions:

MyAny ::= ANY

MyAnyDefinedBy ::= SEQUENCE {
    typeId OBJECT IDENTIFIER,
    value ANY DEFINED BY typeId
}

Generated Python representations:

class MyAny(_BasicAsn1Type[bytes]):
    pass

class MyAnyDefinedBy(_Asn1Type):
    typeId: str
    value: bytes  # raw encoding of the referenced type

Conceptual Representation

class _BasicAsn1Type[bytes]

Represents an ASN.1 type holding raw encoded bytes.

__init__(self, value: bytes | None = None) → None

Initializes the object with optional raw ASN.1-encoded data.

property value: bytes

Gets or sets the raw encoded bytes.

Assigning a new value replaces the underlying stored encoding.

Decoding Contents

To interpret the contents of an ANY or ANY DEFINED BY value, use the decoding API of the target type:

# Suppose we expect the value to be a BOOLEAN
decoded_bool = MyBoolean.ber_decode(my_any_instance.value)

# Or with ANY DEFINED BY, decide based on typeId
if obj.typeId == some_oid:
    decoded_value = TargetType.ber_decode(obj.value)