ASN.1 library for Python

This is a free and open source implementation of ASN.1 types and codecs
as a Python package. It has been first written to support particular
protocol (SNMP) but then generalized to be suitable for a wide range
of protocols based on
ASN.1 specification.

NOTE: The package is now maintained by Christian Heimes and
Simon Pichugin in project https://github.com/pyasn1/pyasn1.

Features

Generic implementation of ASN.1 types (X.208)
Standards compliant BER/CER/DER codecs
Can operate on streams of serialized data
Dumps/loads ASN.1 structures from Python types
100% Python, works with Python 3.8+
MT-safe
Contributed ASN.1 compiler Asn1ate

Why using pyasn1

ASN.1 solves the data serialisation problem. This solution was
designed long ago by the wise Ancients. Back then, they did not
have the luxury of wasting bits. That is why ASN.1 is designed
to serialise data structures of unbounded complexity into
something compact and efficient when it comes to processing
the data.

That probably explains why many network protocols and file formats
still rely on the 30+ years old technology. Including a number of
high-profile Internet protocols and file formats.

Quite a number of books cover the topic of ASN.1.
Communication between heterogeneous systems
by Olivier Dubuisson is one of those high quality books freely
available on the Internet.

The pyasn1 package is designed to help Python programmers tackling
network protocols and file formats at the comfort of their Python
prompt. The tool struggles to capture all aspects of a rather
complicated ASN.1 system and to represent it on the Python terms.

How to use pyasn1

With pyasn1 you can build Python objects from ASN.1 data structures.
For example, the following ASN.1 data structure:

Record ::= SEQUENCE {
  id        INTEGER,
  room  [0] INTEGER OPTIONAL,
  house [1] INTEGER DEFAULT 0
}

Could be expressed in pyasn1 like this:

class Record(Sequence):
    componentType = NamedTypes(
        NamedType('id', Integer()),
        OptionalNamedType(
            'room', Integer().subtype(
                implicitTag=Tag(tagClassContext, tagFormatSimple, 0)
            )
        ),
        DefaultedNamedType(
            'house', Integer(0).subtype(
                implicitTag=Tag(tagClassContext, tagFormatSimple, 1)
            )
        )
    )

It is in the spirit of ASN.1 to take abstract data description
and turn it into a programming language specific form.
Once you have your ASN.1 data structure expressed in Python, you
can use it along the lines of similar Python type (e.g. ASN.1
SET is similar to Python dict, SET OF to list):

>>> record = Record()
>>> record['id'] = 123
>>> record['room'] = 321
>>> str(record)
Record:
 id=123
 room=321
>>>

Part of the power of ASN.1 comes from its serialisation features. You
can serialise your data structure and send it over the network.

>>> from pyasn1.codec.der.encoder import encode
>>> substrate = encode(record)
>>> hexdump(substrate)
00000: 30 07 02 01 7B 80 02 01 41

Conversely, you can turn serialised ASN.1 content, as received from
network or read from a file, into a Python object which you can
introspect, modify, encode and send back.

>>> from pyasn1.codec.der.decoder import decode
>>> received_record, rest_of_substrate = decode(substrate, asn1Spec=Record())
>>>
>>> for field in received_record:
>>>    print('{} is {}'.format(field, received_record[field]))
id is 123
room is 321
house is 0
>>>
>>> record == received_record
True
>>> received_record.update(room=123)
>>> substrate = encode(received_record)
>>> hexdump(substrate)
00000: 30 06 02 01 7B 80 01 7B

The pyasn1 classes struggle to emulate their Python prototypes (e.g. int,
list, dict etc.). But ASN.1 types exhibit more complicated behaviour.
To make life easier for a Pythonista, they can turn their pyasn1
classes into Python built-ins:

>>> from pyasn1.codec.native.encoder import encode
>>> encode(record)
{'id': 123, 'room': 321, 'house': 0}

Or vice-versa -- you can initialize an ASN.1 structure from a tree of
Python objects:

>>> from pyasn1.codec.native.decoder import decode
>>> record = decode({'id': 123, 'room': 321, 'house': 0}, asn1Spec=Record())
>>> str(record)
Record:
 id=123
 room=321
>>>

With ASN.1 design, serialisation codecs are decoupled from data objects,
so you could turn every single ASN.1 object into many different
serialised forms. As of this moment, pyasn1 supports BER, DER, CER and
Python built-ins codecs. The extremely compact PER encoding is expected
to be introduced in the upcoming pyasn1 release.

More information on pyasn1 APIs can be found in the
documentation,
compiled ASN.1 modules for different protocols and file formats
could be found in the pyasn1-modules
repo.

How to get pyasn1

The pyasn1 package is distributed under terms and conditions of 2-clause
BSD license. Source code is freely
available as a GitHub repo.

You could pip install pyasn1 or download it from PyPI.

If something does not work as expected,
open an issue at GitHub or
post your question on Stack Overflow
or try browsing pyasn1
mailing list archives.

Copyright (c) 2005-2020, Ilya Etingof.
All rights reserved.

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