Google Protocol Buffers (protobuf): Python parsing library

Google Protocol Buffers (AKA protobuf) is a popular data serialization scheme used for communication protocols, data storage, etc. There are implementations are available for almost every popular language. The focus points of this scheme are brevity (data is encoded in a very size-efficient manner) and extensibility (one can add keys to the structure, while keeping it readable in previous version of software).

Protobuf uses semi-self-describing encoding scheme for its messages. It means that it is possible to parse overall structure of the message (skipping over fields one can't understand), but to fully understand the message, one needs a protocol definition file (.proto). To be specific:

  • "Keys" in key-value pairs provided in the message are identified only with an integer "field tag". .proto file provides info on which symbolic field names these field tags map to.
  • "Keys" also provide something called "wire type". It's not a data type in its common sense (i.e. you can't, for example, distinguish sint32 vs uint32 vs some enum, or string from bytes), but it's enough information to determine how many bytes to parse. Interpretation of the value should be done according to the type specified in .proto file.
  • There's no direct information on which fields are optional / required, which fields may be repeated or constitute a map, what restrictions are placed on fields usage in a single message, what are the fields' default values, etc, etc.

KS implementation details

License: MIT
Minimal Kaitai Struct required: 0.7

References

This page hosts a formal specification of Google Protocol Buffers (protobuf) 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 = GoogleProtobuf.from_file("path/to/local/file.Google Protocol Buffers (protobuf)")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

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

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

data.pairs # => Key-value pairs which constitute a message

Python source code to parse Google Protocol Buffers (protobuf)

google_protobuf.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__))

import vlq_base128_le
class GoogleProtobuf(KaitaiStruct):
    """Google Protocol Buffers (AKA protobuf) is a popular data
    serialization scheme used for communication protocols, data storage,
    etc. There are implementations are available for almost every
    popular language. The focus points of this scheme are brevity (data
    is encoded in a very size-efficient manner) and extensibility (one
    can add keys to the structure, while keeping it readable in previous
    version of software).
    
    Protobuf uses semi-self-describing encoding scheme for its
    messages. It means that it is possible to parse overall structure of
    the message (skipping over fields one can't understand), but to
    fully understand the message, one needs a protocol definition file
    (`.proto`). To be specific:
    
    * "Keys" in key-value pairs provided in the message are identified
      only with an integer "field tag". `.proto` file provides info on
      which symbolic field names these field tags map to.
    * "Keys" also provide something called "wire type". It's not a data
      type in its common sense (i.e. you can't, for example, distinguish
      `sint32` vs `uint32` vs some enum, or `string` from `bytes`), but
      it's enough information to determine how many bytes to
      parse. Interpretation of the value should be done according to the
      type specified in `.proto` file.
    * There's no direct information on which fields are optional /
      required, which fields may be repeated or constitute a map, what
      restrictions are placed on fields usage in a single message, what
      are the fields' default values, etc, etc.
    
    .. seealso::
       Source - https://developers.google.com/protocol-buffers/docs/encoding
    """
    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.pairs = []
        i = 0
        while not self._io.is_eof():
            self.pairs.append(GoogleProtobuf.Pair(self._io, self, self._root))
            i += 1


    class Pair(KaitaiStruct):
        """Key-value pair."""

        class WireTypes(Enum):
            varint = 0
            bit_64 = 1
            len_delimited = 2
            group_start = 3
            group_end = 4
            bit_32 = 5
        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.key = vlq_base128_le.VlqBase128Le(self._io)
            _on = self.wire_type
            if _on == GoogleProtobuf.Pair.WireTypes.varint:
                self.value = vlq_base128_le.VlqBase128Le(self._io)
            elif _on == GoogleProtobuf.Pair.WireTypes.len_delimited:
                self.value = GoogleProtobuf.DelimitedBytes(self._io, self, self._root)
            elif _on == GoogleProtobuf.Pair.WireTypes.bit_64:
                self.value = self._io.read_u8le()
            elif _on == GoogleProtobuf.Pair.WireTypes.bit_32:
                self.value = self._io.read_u4le()

        @property
        def wire_type(self):
            """"Wire type" is a part of the "key" that carries enough
            information to parse value from the wire, i.e. read correct
            amount of bytes, but there's not enough informaton to
            interprete in unambiguously. For example, one can't clearly
            distinguish 64-bit fixed-sized integers from 64-bit floats,
            signed zigzag-encoded varints from regular unsigned varints,
            arbitrary bytes from UTF-8 encoded strings, etc.
            """
            if hasattr(self, '_m_wire_type'):
                return self._m_wire_type if hasattr(self, '_m_wire_type') else None

            self._m_wire_type = KaitaiStream.resolve_enum(GoogleProtobuf.Pair.WireTypes, (self.key.value & 7))
            return self._m_wire_type if hasattr(self, '_m_wire_type') else None

        @property
        def field_tag(self):
            """Identifies a field of protocol. One can look up symbolic
            field name in a `.proto` file by this field tag.
            """
            if hasattr(self, '_m_field_tag'):
                return self._m_field_tag if hasattr(self, '_m_field_tag') else None

            self._m_field_tag = (self.key.value >> 3)
            return self._m_field_tag if hasattr(self, '_m_field_tag') else None


    class DelimitedBytes(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.len = vlq_base128_le.VlqBase128Le(self._io)
            self.body = self._io.read_bytes(self.len.value)