Support for writing IMPLICIT values

[ivan-tkatchev]

It seems it's impossible to write an [APPLICATION 1] IMPLICIT INTEGER value due to this:

  File ".../python3.13/site-packages/asn1.py", line 243, in write
    self._emit_length(len(value))
                      ~~~^^^^^^^
TypeError: object of type 'int' has no len()

and this:

    def _encode_value(self, cls, nr, value):  # type: (int, int, any) -> bytes
        """Encode a value."""
        if cls != Classes.Universal:
            return value

How is this supposed to work? There is no documentation for IMPLICIT tagging. In fact, judging by the code it seems like only Universal class is supported for primitive types.

[andrivet]

There is no notion of implicit tagging in the library. When encoding an integer (and more generally any value), a Universal class is assumed.

Do you have a concrete example where it is a problem?

[ivan-tkatchev]

In my case I need it for encoding values in SNMP messages.

[andrivet]

Can you be more precise and share some code.

[ivan-tkatchev]

In SNMP 32-bit integers are represented like this:

encoder.write(encoder._encode_integer(value), 1, asn1.Types.Primitive, asn1.Classes.Application)

Other primitive types (e.g. timestamps or 64-bit integers) use other numbers besides 1.

It would be nice if I could use standard ASN.1 functionality without resorting to undocumented private interfaces.

[andrivet]

Thank. I will look at it and see what can be done in the library.