185 lines
6.2 KiB
Markdown
185 lines
6.2 KiB
Markdown
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ASN.1 library for Python
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------------------------
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[](https://pypi.org/project/pyasn1)
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[](https://pypi.org/project/pyasn1/)
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[](https://secure.travis-ci.org/etingof/pyasn1)
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[](https://codecov.io/github/etingof/pyasn1)
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[](https://raw.githubusercontent.com/etingof/pyasn1/master/LICENSE.txt)
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This is a free and open source implementation of ASN.1 types and codecs
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as a Python package. It has been first written to support particular
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protocol (SNMP) but then generalized to be suitable for a wide range
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of protocols based on
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[ASN.1 specification](https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.208-198811-W!!PDF-E&type=items).
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Features
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--------
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* Generic implementation of ASN.1 types (X.208)
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* Standards compliant BER/CER/DER codecs
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* Dumps/loads ASN.1 structures from Python types
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* 100% Python, works with Python 2.4 up to Python 3.7
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* MT-safe
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* Contributed ASN.1 compiler [Asn1ate](https://github.com/kimgr/asn1ate)
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Why using pyasn1
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----------------
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ASN.1 solves the data serialisation problem. This solution was
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designed long ago by the wise Ancients. Back then, they did not
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have the luxury of wasting bits. That is why ASN.1 is designed
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to serialise data structures of unbounded complexity into
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something compact and efficient when it comes to processing
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the data.
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That probably explains why many network protocols and file formats
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still rely on the 30+ years old technology. Including a number of
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high-profile Internet protocols and file formats.
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Quite a number of books cover the topic of ASN.1.
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[Communication between heterogeneous systems](http://www.oss.com/asn1/dubuisson.html)
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by Olivier Dubuisson is one of those high quality books freely
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available on the Internet.
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The pyasn1 package is designed to help Python programmers tackling
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network protocols and file formats at the comfort of their Python
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prompt. The tool struggles to capture all aspects of a rather
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complicated ASN.1 system and to represent it on the Python terms.
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How to use pyasn1
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-----------------
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With pyasn1 you can build Python objects from ASN.1 data structures.
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For example, the following ASN.1 data structure:
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```bash
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Record ::= SEQUENCE {
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id INTEGER,
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room [0] INTEGER OPTIONAL,
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house [1] INTEGER DEFAULT 0
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}
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```
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Could be expressed in pyasn1 like this:
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```python
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class Record(Sequence):
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componentType = NamedTypes(
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NamedType('id', Integer()),
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OptionalNamedType(
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'room', Integer().subtype(
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implicitTag=Tag(tagClassContext, tagFormatSimple, 0)
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)
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),
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DefaultedNamedType(
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'house', Integer(0).subtype(
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implicitTag=Tag(tagClassContext, tagFormatSimple, 1)
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)
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)
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)
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```
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It is in the spirit of ASN.1 to take abstract data description
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and turn it into a programming language specific form.
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Once you have your ASN.1 data structure expressed in Python, you
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can use it along the lines of similar Python type (e.g. ASN.1
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`SET` is similar to Python `dict`, `SET OF` to `list`):
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```python
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>>> record = Record()
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>>> record['id'] = 123
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>>> record['room'] = 321
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>>> str(record)
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Record:
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id=123
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room=321
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>>>
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```
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Part of the power of ASN.1 comes from its serialisation features. You
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can serialise your data structure and send it over the network.
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```python
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>>> from pyasn1.codec.der.encoder import encode
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>>> substrate = encode(record)
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>>> hexdump(substrate)
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00000: 30 07 02 01 7B 80 02 01 41
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```
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Conversely, you can turn serialised ASN.1 content, as received from
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network or read from a file, into a Python object which you can
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introspect, modify, encode and send back.
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```python
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>>> from pyasn1.codec.der.decoder import decode
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>>> received_record, rest_of_substrate = decode(substrate, asn1Spec=Record())
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>>>
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>>> for field in received_record:
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>>> print('{} is {}'.format(field, received_record[field]))
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id is 123
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room is 321
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house is 0
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>>>
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>>> record == received_record
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True
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>>> received_record.update(room=123)
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>>> substrate = encode(received_record)
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>>> hexdump(substrate)
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00000: 30 06 02 01 7B 80 01 7B
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```
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The pyasn1 classes struggle to emulate their Python prototypes (e.g. int,
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list, dict etc.). But ASN.1 types exhibit more complicated behaviour.
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To make life easier for a Pythonista, they can turn their pyasn1
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classes into Python built-ins:
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```python
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>>> from pyasn1.codec.native.encoder import encode
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>>> encode(record)
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{'id': 123, 'room': 321, 'house': 0}
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```
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Or vice-versa -- you can initialize an ASN.1 structure from a tree of
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Python objects:
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```python
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>>> from pyasn1.codec.native.decoder import decode
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>>> record = decode({'id': 123, 'room': 321, 'house': 0}, asn1Spec=Record())
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>>> str(record)
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Record:
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id=123
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room=321
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>>>
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```
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With ASN.1 design, serialisation codecs are decoupled from data objects,
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so you could turn every single ASN.1 object into many different
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serialised forms. As of this moment, pyasn1 supports BER, DER, CER and
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Python built-ins codecs. The extremely compact PER encoding is expected
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to be introduced in the upcoming pyasn1 release.
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More information on pyasn1 APIs can be found in the
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[documentation](http://snmplabs.com/pyasn1/),
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compiled ASN.1 modules for different protocols and file formats
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could be found in the pyasn1-modules
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[repo](https://github.com/etingof/pyasn1-modules).
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How to get pyasn1
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-----------------
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The pyasn1 package is distributed under terms and conditions of 2-clause
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BSD [license](http://snmplabs.com/pyasn1/license.html). Source code is freely
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available as a GitHub [repo](https://github.com/etingof/pyasn1).
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You could `pip install pyasn1` or download it from [PyPI](https://pypi.org/project/pyasn1).
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If something does not work as expected,
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[open an issue](https://github.com/etingof/pyasn1/issues) at GitHub or
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post your question [on Stack Overflow](https://stackoverflow.com/questions/ask)
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or try browsing pyasn1
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[mailing list archives](https://sourceforge.net/p/pyasn1/mailman/pyasn1-users/).
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Copyright (c) 2005-2019, [Ilya Etingof](mailto:etingof@gmail.com).
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All rights reserved.
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