About

A Python package for ASN.1 parsing, encoding and decoding.

This project is under development and does only support a subset of the ASN.1 specification syntax.

Codecs under development:

• Basic Encoding Rules (BER)
• Distinguished Encoding Rules (DER)
• Generic String Encoding Rules (GSER)
• JSON Encoding Rules (JER)
• Aligned Packed Encoding Rules (PER)
• Unaligned Packed Encoding Rules (UPER)
• XML Encoding Rules (XER)

Planned codecs:

• Octet Encoding Rules (OER)

Project homepage: https://github.com/eerimoq/asn1tools

Documentation: http://asn1tools.readthedocs.org/en/latest

Known limitations

• The CLASS keyword and its friends are not supported.

Installation

pip install asn1tools

Example Usage

This is an example ASN.1 specification defining the messages of a fictitious Foo protocol (based on the FooProtocol on Wikipedia).

Foo DEFINITIONS ::= BEGIN

    Question ::= SEQUENCE {
        id        INTEGER,
        question  IA5String
    }

    Answer ::= SEQUENCE {
        id        INTEGER,
        answer    BOOLEAN
    }

END

Scripting

Compile the ASN.1 specification, and encode and decode a question using the default codec (BER).

>>> import asn1tools
>>> foo = asn1tools.compile_files('tests/files/foo.asn')
>>> encoded = foo.encode('Question', {'id': 1, 'question': 'Is 1+1=3?'})
>>> encoded
bytearray(b'0\x0e\x02\x01\x01\x16\x09Is 1+1=3?')
>>> foo.decode('Question', encoded)
{'id': 1, 'question': 'Is 1+1=3?'}

The same ASN.1 specification, but using the PER codec.

>>> import asn1tools
>>> foo = asn1tools.compile_files('tests/files/foo.asn', 'per')
>>> encoded = foo.encode('Question', {'id': 1, 'question': 'Is 1+1=3?'})
>>> encoded
bytearray(b'\x01\x01\tIs 1+1=3?')
>>> foo.decode('Question', encoded)
{'id': 1, 'question': 'Is 1+1=3?'}

See the examples folder for additional examples.

Command line tool

The shell subcommand

Use the command line shell to convert data between given formats. The default input codec is BER and output codec is GSER (produces human readable text).

> asn1tools shell

Welcome to the asn1tools shell!

$ help
Commands:
  compile
  convert
$ compile tests/files/foo.asn
$ convert Question 300e0201011609497320312b313d333f
question Question ::= {
    id 1,
    question "Is 1+1=3?"
}
$ compile --output-codec xer tests/files/foo.asn
$ convert Question 300e0201011609497320312b313d333f
<Question>
    <id>1</id>
    <question>Is 1+1=3?</question>
</Question>
$ compile -o uper tests/files/foo.asn
$ convert Question 300e0201011609497320312b313d333f
01010993cd03156c5eb37e
$ exit
>

The convert subcommand

Convert given encoded Question from BER to GSER (produces human readable text).

> asn1tools convert tests/files/foo.asn Question 300e0201011609497320312b313d333f
question Question ::= {
    id 1,
    question "Is 1+1=3?"
}
>

Convert given encoded Question from UPER to XER (xml).

> asn1tools convert -i uper -o xer tests/files/foo.asn Question 01010993cd03156c5eb37e
<Question>
    <id>1</id>
    <question>Is 1+1=3?</question>
</Question>
>

Convert given encoded Question from UPER to JER (json).

> asn1tools convert -i uper -o jer tests/files/foo.asn Question 01010993cd03156c5eb37e
{
    "id": 1,
    "question": "Is 1+1=3?"
}
>

Continuously convert encoded Questions read from standard input. Any line that cannot be converted is printed as is, in this example the dates.

> cat encoded.txt
2018-02-24 11:22:09
300e0201011609497320312b313d333f
2018-02-24 11:24:15
300e0201021609497320322b323d353f
> cat encoded.txt | asn1tools convert tests/files/foo.asn Question -
2018-02-24 11:22:09
question Question ::= {
    id 1,
    question "Is 1+1=3?"
}
2018-02-24 11:24:15
question Question ::= {
    id 2,
    question "Is 2+2=5?"
}
>

Contributing

1. Fork the repository.

2. Install prerequisites.

   pip install -r requirements.txt
   

3. Implement the new feature or bug fix.

4. Implement test case(s) to ensure that future changes do not break legacy.

5. Run the tests.

   make test
   

6. Create a pull request.

Specifications

ASN.1 specifications released by ITU and IETF.

General

• X.680: Specification of basic notation
• X.681: Information object specification
• X.682: Constraint specification
• X.683: Parameterization of ASN.1 specifications

Encodings

• X.690: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)
• X.691: Specification of Packed Encoding Rules (PER)
• X.693: XML Encoding Rules (XER)
• RFC 3641: Generic String Encoding Rules (GSER) for ASN.1