ASN.1 DER (Abstract Syntax Notation One, Distinguished Encoding Rules): Python parsing library

ASN.1 (Abstract Syntax Notation One) DER (Distinguished Encoding Rules) is a standard-backed serialization scheme used in many different use-cases. Particularly popular usage scenarios are X.509 certificates and some telecommunication / networking protocols.

DER is self-describing encoding scheme which allows representation of simple, atomic data elements, such as strings and numbers, and complex objects, such as sequences of other elements.

DER is a subset of BER (Basic Encoding Rules), with an emphasis on being non-ambiguous: there's always exactly one canonical way to encode a data structure defined in terms of ASN.1 using DER.

This spec allows full parsing of format syntax, but to understand the semantics, one would typically require a dictionary of Object Identifiers (OIDs), to match OID bodies against some human-readable list of constants. OIDs are covered by many different standards, so typically it's simpler to use a pre-compiled list of them, such as:

  • https://www.cs.auckland.ac.nz/~pgut001/dumpasn1.cfg
  • http://oid-info.com/
  • https://www.alvestrand.no/objectid/top.html

File extension

der

KS implementation details

License: CC0-1.0

References

This page hosts a formal specification of ASN.1 DER (Abstract Syntax Notation One, Distinguished Encoding Rules) using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.

The Python runtime library can be installed from PyPI:

python3 -m pip install kaitaistruct

Code

Parse a local file and get structure in memory:

data = Asn1Der.from_file("path/to/local/file.der")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

raw = b"\x00\x01\x02..."
data = Asn1Der(KaitaiStream(BytesIO(raw)))

After that, one can get various attributes from the structure by invoking getter methods like:

data.type_tag # => get type tag

Python source code to parse ASN.1 DER (Abstract Syntax Notation One, Distinguished Encoding Rules)

asn1_der.py

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

from pkg_resources import parse_version
import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO
from enum import Enum


if parse_version(kaitaistruct.__version__) < parse_version('0.9'):
    raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))

class Asn1Der(KaitaiStruct):
    """ASN.1 (Abstract Syntax Notation One) DER (Distinguished Encoding
    Rules) is a standard-backed serialization scheme used in many
    different use-cases. Particularly popular usage scenarios are X.509
    certificates and some telecommunication / networking protocols.
    
    DER is self-describing encoding scheme which allows representation
    of simple, atomic data elements, such as strings and numbers, and
    complex objects, such as sequences of other elements.
    
    DER is a subset of BER (Basic Encoding Rules), with an emphasis on
    being non-ambiguous: there's always exactly one canonical way to
    encode a data structure defined in terms of ASN.1 using DER.
    
    This spec allows full parsing of format syntax, but to understand
    the semantics, one would typically require a dictionary of Object
    Identifiers (OIDs), to match OID bodies against some human-readable
    list of constants. OIDs are covered by many different standards,
    so typically it's simpler to use a pre-compiled list of them, such
    as:
    
    * https://www.cs.auckland.ac.nz/~pgut001/dumpasn1.cfg
    * http://oid-info.com/
    * https://www.alvestrand.no/objectid/top.html
    
    .. seealso::
       Source - https://www.itu.int/rec/T-REC-X.690-201508-I/en
    """

    class TypeTag(Enum):
        end_of_content = 0
        boolean = 1
        integer = 2
        bit_string = 3
        octet_string = 4
        null_value = 5
        object_id = 6
        object_descriptor = 7
        external = 8
        real = 9
        enumerated = 10
        embedded_pdv = 11
        utf8string = 12
        relative_oid = 13
        sequence_10 = 16
        printable_string = 19
        ia5string = 22
        sequence_30 = 48
        set = 49
    def __init__(self, _io, _parent=None, _root=None):
        self._io = _io
        self._parent = _parent
        self._root = _root if _root else self
        self._read()

    def _read(self):
        self.type_tag = KaitaiStream.resolve_enum(Asn1Der.TypeTag, self._io.read_u1())
        self.len = Asn1Der.LenEncoded(self._io, self, self._root)
        _on = self.type_tag
        if _on == Asn1Der.TypeTag.printable_string:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodyPrintableString(_io__raw_body, self, self._root)
        elif _on == Asn1Der.TypeTag.sequence_10:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodySequence(_io__raw_body, self, self._root)
        elif _on == Asn1Der.TypeTag.set:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodySequence(_io__raw_body, self, self._root)
        elif _on == Asn1Der.TypeTag.sequence_30:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodySequence(_io__raw_body, self, self._root)
        elif _on == Asn1Der.TypeTag.utf8string:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodyUtf8string(_io__raw_body, self, self._root)
        elif _on == Asn1Der.TypeTag.object_id:
            self._raw_body = self._io.read_bytes(self.len.result)
            _io__raw_body = KaitaiStream(BytesIO(self._raw_body))
            self.body = Asn1Der.BodyObjectId(_io__raw_body, self, self._root)
        else:
            self.body = self._io.read_bytes(self.len.result)

    class BodySequence(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.entries = []
            i = 0
            while not self._io.is_eof():
                self.entries.append(Asn1Der(self._io))
                i += 1



    class BodyUtf8string(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.str = (self._io.read_bytes_full()).decode(u"UTF-8")


    class BodyObjectId(KaitaiStruct):
        """
        .. seealso::
           Source - https://docs.microsoft.com/en-us/windows/desktop/SecCertEnroll/about-object-identifier
        """
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.first_and_second = self._io.read_u1()
            self.rest = self._io.read_bytes_full()

        @property
        def first(self):
            if hasattr(self, '_m_first'):
                return self._m_first if hasattr(self, '_m_first') else None

            self._m_first = self.first_and_second // 40
            return self._m_first if hasattr(self, '_m_first') else None

        @property
        def second(self):
            if hasattr(self, '_m_second'):
                return self._m_second if hasattr(self, '_m_second') else None

            self._m_second = (self.first_and_second % 40)
            return self._m_second if hasattr(self, '_m_second') else None


    class LenEncoded(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.b1 = self._io.read_u1()
            if self.b1 == 130:
                self.int2 = self._io.read_u2be()

            if self.b1 == 129:
                self.int1 = self._io.read_u1()


        @property
        def result(self):
            if hasattr(self, '_m_result'):
                return self._m_result if hasattr(self, '_m_result') else None

            self._m_result = (self.int1 if self.b1 == 129 else (self.int2 if self.b1 == 130 else self.b1))
            return self._m_result if hasattr(self, '_m_result') else None


    class BodyPrintableString(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.str = (self._io.read_bytes_full()).decode(u"ASCII")