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What is pycrate

Pycrate is a french word for qualifying bad wine (when it's close to vinegar !). The present software library has nothing to do with wine (except it is developped in France), it is simply a Python library for manipulating various digital formats in an easy way, with a funny name. It is the glorious successor of libmich, which was started back in 2009, served well and retired in 2017.

It provides basically a runtime for encoding and decoding data structures, including CSN.1 and ASN.1. Additionally, it features a 3G and LTE mobile core network.

License

The whole library is licensed under LGPL v2.1 and is compatible with more recent version of the LGPL: all licensed files have an header making it self-explanatory. For more details, please report to the license.txt file.

Wiki

Pycrate has a growing wiki. Use it as much as possible before opening an issue. Feel free also to propose some additional content.

Installation

Operating systems and Python version

The library is designed to work with both Python 2 (2.7) and Python 3 (3.4, 3.5 and greater), from the official Python implementation CPython. It is also supporting alternative Python engine such as pypy, nuitka or Cython. It is regularly tested both on Linux and Windows, and should actually work on any operating system which has [r|d]ecent Python support (as in 2017, 2018 and more...).

Starting from 2022, full Python2 support will not be abandonned slowly but progressively.

Dependencies

Currently none. Only the Python builtins and few internal modules of Python (e.g. os, system, re, struct, datetime) are required for most of the features. The json internal module is required for supporting the JSON API. If you want to run pycrate in Python2 (which is bad !), you will however need to install the enum34 package.

The pycrate_ether/SCTP module can optionally use the external crc32c module from ICRAR.

The pycrate_mobile/TS24301_EMM and pycrate_mobile/TS24501_FGMM modules use CryptoMobile as optional dependency to encrypt and decrypt LTE and 5G NAS messages.

The pycrate_corenet part requires also pysctp and CryptoMobile to run.

The pycrate_diameter/parse_iana_diameter_xml.py file uses lxml to translate xml files from IANA to Python dictionnaries ; this is however not required for standard runtime.

The pycrate_osmo/SEDebugMux.py module relies on the crcmod to compute CRC in the frame format.

Automatic installation

An installation script is available. As soon as you have cloned or downloaded the repository, you can use it to install the library within your Python package directory:

python setup.py install

Run it as superuser for a system-wide install, or as-is for a user home-directory level install. You can also run develop instead of install if you want a developer-friendly installation.

It is also possible to test the library before installing it (this will create two local directories ./test_asn/ and ./pycrate.egg-info/ that you can just delete afterwards):

python -m unittest test.test_pycrate

Or to build the library without installing it in the system:

python setup.py build

It is also possible to recompile all ASN.1 modules, this will take few minutes (but if I did not do any mistake, all ASN.1 modules provided in ./pycrate_asn1dir/ should have been compiled with the latest version of the compiler):

python -m pycrate_asn1c.asnproc

More generally, installation is not required, and simply having all pycrate_* subdirectories into the PYTHONPATH enables to use the entire library.

Installation with pip

Alternatively, you can install the library with the pip command:

pip install pycrate

The install package is available on pypi. It contains the library from the last tagged release on github.

Contributing

Contact and support

This library is free software, and you are free to use it (or not to use it). In case you encounter a problem with it, first read this readme completely and check the wiki ; moreover many classes, methods and functions are documented with docstrings, and finally you can have a look at the source code.

If after all those steps, you still have a question or you think you found a bug, please open an issue (see below). Specific support requires time and may not be always possible. In case you require such support, please consider also contributing in one way or another (see below, too).

In case you are using this library in any of your project and you find it useful, do not hesitate to send me an email. It is always a pleasure to know where code provided on the Internet can end up...

Filling an issue

When filling an issue, please provide precise and contextual information about your case and the error you potentially encounter:

  • indicate the version (or commit-level) of pycrate your are using, together with the version of Python.
  • provide a code snippet that leads to the error you are facing, so that it can be reproduced.
  • provide the eventual stacktrace you are getting from Python
  • provide additional and contextual information as needed (e.g. a specific ASN.1 specification being used...)

This is the bare minimum if you want to get help. And when you consider your issue has been addressed, please close it: "A good issue is a closed one !" as would have said my great grandmother.

Extending the library

If you are willing to extend the library, do not hesitate to contact me by email or preferably through the github service (ideally, open a pull request). For important changes, please elaborate about your need and provide some justification. Any patch or submission is always very welcome!

Other contributions

In case you do not want to deep dive in the code, you can still contribute in many ways:

  • highlighting specific issues in the inner-working of the library, and opening an issue with concrete debugging information
  • writing new test cases for more coverage (have a look at the test/ directory)
  • sending captures / real-world data that can be used for writing new test cases
  • writing new parts of the wiki (have a look at the pycrate wiki)

Getting contributions is extremely important to encourage the continuous development of the library, and to confirm the choice made to open-source it.

Components

Pycrate is actually more a software suite than a single library. It is composed of several subdirectories, each providing specific services.

pycrate_core

The core of the library.

  • utils provides basics functions to manipulate integers, bytes and bits
  • charpy provides the Charpy class to handle easily the consumption of a bit-stream
  • elt and base are providing several classes to help when building complex data structures
  • repr provides simple functions to help with the representation of instances from the elt and base modules

Some of the most useful features are provided by the pack_val() functions from the utils module and the Charpy class from the charpy module. They help to deal easily with packing and unpacking bytes and integers (signed / unsigned, little / big endian) in an aligned and unaligned way. All lengths of fields are provided in bits, hence facilitating the handling of unaligned structures.

pycrate_ether

The modules provided here implement Ethernet and IP-oriented protocols and formats.

  • MPLS with structures for MPLS label and header
  • Ethernet with structures for Ethernet and VLAN headers
  • ARP simply providing the structure for ARP
  • IP with structures for IPv4, IPv6, ICMP, UDP and TCP
  • SCTP with structures for SCTP headers and various chunks
  • PCAP with structures for the PCAP global header and the record header

pycrate_media

The modules here implement various multimedia formats.

  • JPEG with detailed structures used in the JPEG format
  • GIF with detailed structures used in the GIF format
  • TIFF with detailed structures used in the TIFF format
  • BMP with structures used in the BMP format
  • PNG with the basic structure used in the PNG format
  • MPEG4 with the basic structure used in the MPEG4 file format
  • MP3 with detailed structures used in the MP3 format, including ID3v1 and ID3v2 tags

Most of the classes here implement a complete recipe to parse all of those format in a single shot, by using their from_char() method.

pycrate_asn1c

All the modules here serve the sole purpose of compiling ASN.1 specifications. The most important ones are:

  • asnobj which is the almighty class when parsing any ASN.1 definition
  • generator which provides two distinct generators to produce source files from the ASN.1 objects processed in Python: PycrateGenerator which produces source file to be used with the pycrate ASN.1 runtime (in pycrate_asn1rt), and JSONDepGraphGenerator which produces json file listing object dependencies (which then can be browsed dynamically thanks to D3).
  • asnproc which is the top-level module for the compiler, it contains for example the compile_text() function which compiles a serie of ASN.1 modules into Python objects

This compiler support most of the ASN.1 language features, including parameterization and class objects and sets (especially useful when working with table constraints). It has however few restrictions, the biggest being the need for the left part of the ASN.1 assignment ::= being on a single line. Also, old-school ASN.1 macros are not supported ; hence, the compiler cannot parse SNMP MIBs.

pycrate_asn1dir

This subdirectory contains several ASN.1 specifications that are supported and precompiled for pycrate. Very few specifications have been changed in order to work with pycrate :

  • Q.775, in which the terrible AllPackagesAS is commented out
  • Q.773 and Q.775, in which the TCInvokeIdSet constraint is modified to be used as a set of values That's all !

pycrate_asn1rt

This subdirectory contains the ASN.1 runtime, that is loaded and used by the ASN.1 specifications compiled with the compiler in pycrate_asn1c. It supports the PER encoding rules (aligned and not, canonical also), and the BER, CER, DER and JER encoding rules.

pycrate_csn1

This subdirectory contains a CSN.1 to Python translater in the file trans.py, and a CSN.1 runtime in the file csnobj.py, in order to encode and decode CSN.1 structures translated to Python objects.

pycrate_csn1dir

This subdirectory contains CSN.1 structures extracted from 3GPP specifications (in the .csn files), and translated into Python objects. The following specifications have been used: TS 44.018, TS 44.060 and TS 24.008.

pycrate_mobile

This subdirectory implements most of the 3GPP NAS protocol formats:

  • GSMTAP: gsmtap header format
  • MCC_MNC: dictionnaries for MCC and MNC look-up
  • NAS: provides two functions to parse any uplink and downlink mobile NAS messages
  • NASLTE: provides two functions to parse LTE uplink and downlink NAS messages
  • NAS5G: provides one function to parse 5G uplink and downlink mobile NAS messages
  • PPP: structures for NCP and LCP protocols used for PPP connection estabishment
  • SCCP: structures for SCCP user-data and management messages
  • SIGTRAN: structures for SIGTRAN (mostly M2PA and M3UA) messages
  • TS102225: structures for SIM card's Secured Packets from ETSI TS 102.225
  • TS23038: structures and routines for SMS encoding from TS 23.038
  • TS23040_SMS: structures for the SMS transport protocol from TS 23.040
  • TS23041_CBS: structures for the Cell Broadcast Service protocol from TS 23.041
  • TS24007: basic structures from the TS 24.007 specification, reused in most of the NAS protocols
  • TS24008_CC : structures for call control messages from TS 24.008
  • TS24008_GMM: structures for GPRS mobility management messages from TS 24.008
  • TS24008_IE: structures for many information elements from TS 24.008
  • TS24008_MM: structures for mobility management messages from TS 24.008
  • TS24008_SM: structures for GPRS session management messages from TS 24.008
  • TS24011_PPSMS: structures for the SMS point-to-point protocol from TS 24.011
  • TS24080_SS: structures for the Supplementary Services protocol from TS 24.080, wrapping some MAP ASN.1 objects
  • TS24301_EMM: structures for the EPS mobility management messages from TS 24.301
  • TS24301_ESM: structures for the EPS session management messages from TS 24.301
  • TS24301_IE: structures for many information elements from TS 24.301
  • TS24501_FGMM: structures for the 5G mobility management messages from TS 24.501
  • TS24501_FGSM: structures for the 5G session management messages from TS 24.501
  • TS24501_IE: structures for many information elements from TS 24.501
  • TS24501_UEPOL, TS24526_UEPOL and TS24588_UEPOL: structures for the 5G UE policy protocol from TS 24.501, 526 and 588
  • TS29002_MAPAppCtx: functions that relies on the Pycrate_TCAP_MAPv2v3 ASN.1 module, dealing mostly with MAP application-contexts
  • TS29002_MAPIE: structure for the MAP AddressString object from TS 29.002
  • TS29244_PFCP: structure for PFCP messages from TS 29.244
  • TS29274_GTPC: structures for LTE/EPC GTP-C messages from TS 29.274
  • TS29281_GTPU: structures for LTE/EPC GTP-U messages from TS 29.281
  • TS31111_SAT: basic structures and dict for the SIM application toolkit from TS 31.111
  • TS31115: structures for SIM card's Secured Packets over SMS from TS 31.115
  • TS38415_PDUSess: structure used in 5G user-place traffic (i.e. GTP-U) from TS 38.415
  • TS44018_GTTP: structure for the single GSM GTTP message from TS 44.018
  • TS44018_IE: structures for many information elements from TS 44.018
  • TS44018_RR: structures for the GSM and GPRS radio ressources messages from TS 44.018

pycrate_diameter

This subdirectory contains the following modules:

  • parse_iana_diameter_xml: to translate XML Diameter structures from IANA to Python
  • iana_diameter_dicts.py: that is automatically created by the former, containing Diameter Python dicts
  • Diameter: a generic Diameter module which implements DiameterGeneric and AVPGeneric structures
  • DiameterIETF: a Diameter module which relies on AVP types provided in all IETF RFC
  • Diameter3GPP: a Diameter module which relies on AVP types provided in all 3GPP TS

pycrate_osmo

This subdirectory contains the following modules:

  • L1CTL: structures used by osmocom-bb to communicate with the embedded stack from the host
  • SEDebugMux: structure used by Sony-Ericsson SoC and basebands to wrap logs

pycrate_corenet

This subdirectory implements a signaling server that supports IuCS and IuPS over Iuh interfaces (including HNBAP and RUA/RANAP) for interfacing with 3G femtocells, and S1 interfaces (including S1AP) for interfacing with LTE eNodeBs. It handles many procedures to drive femtocells, eNodeBs and mobile terminals connecting through them. In terms of services, it mostly support short messages and data connectivity. It does not handle call services, neither active mobility procedures (handovers).

It can be easily (common, running a mobile core network is not that easy) configured and used thanks to the corenet project, also open-source.

Usage

Most of the modules have doc strings. I try also to write readable sources and to comment them as much as possible for understanding them easily (and to allow also myself to understand my own code years after...). A wiki is provided and extended from time to time, to bring examples and methods on how to use the different modules (any contribution on this would be very welcome, too). Finally, the code provided in the test/ subdirectory is also representative on how to use the different modules.

Basically, a pycrate's object exposes the following methods:

  • set_val() / get_val(), which sets and gets a value into the object
  • from_bytes() / to_bytes(), which converts a buffer into values according to the internal structure of the object, and back
  • from_json() / to_json(), for working with JSON-encoded values
  • hex() / bin(), for getting hexadecimal and binary representation of the serialized obect's value
  • repr() / show(), for providing nice python's internal representation, and printable representation of the object's value

ASN.1 usage

When a Python module from pycrate_asn1dir/ is loaded, it creates Python classes corresponding to ASN.1 modules (all dash characters are converted to underscore). Each ASN.1 object has a corresponding Python instance, exposing the following methods:

  • from_asn1() / to_asn1(), which converts ASN.1 textual value to Python value and back
  • from_aper() / to_aper(), which converts aligned PER encoded value to Python value and back
  • from_uper() / to_uper(), which converts unaligned PER
  • from_ber() / to_ber(), which converts BER
  • from_cer() / to_cer(), which converts CER
  • from_der() / to_der(), which converts DER
  • from_jer() / to_jer(), which converts JER
  • set_val() / get_val(), to set and get Python's values into the ASN.1 object
  • get_proto(), to return to internal structure of the ASN.1 object

All the methods useful for working with ASN.1 objects at runtime can be found in the file pycrate_asn1rt/asnobj.py.

Tools

Four different tools are provided (yet):

  • pycrate_showmedia.py parses some media files (jpg, bmp, gif, mp3, png, tiff, mpeg4) and pretty print the file structure on the standard output.
  • pycrate_asn1compile.py compiles ASN.1 source file(s) and produce a Python source file that makes use of the ASN.1 runtime. This source file is then usable to encode / decode any ASN.1 object from the compiled ASN.1 specification.
  • pycrate_berdecode.py parses any BER/CER/DER encoded binary value of ASN.1 objects and prints the corresponding structure.
  • pycrate_map_op_info.py prints prototypes and various information related to TCAP-MAP (Mobile Application Part) and CAMEL operations and application-contexts.

Examples

It is possible to test the pycrate_showmedia.py tool with media test files provided in ./test/res/, or any other supported media file.

$ ./tools/pycrate_showmedia.py --help
usage: pycrate_showmedia.py [-h] [-bl BL] [-wt] input

print the internal structure of the input media file,supported formats are:
BMP, GIF, JPEG, MP3, MPEG4, PNG, TIFF

positional arguments:
  input       input media file

optional arguments:
  -h, --help  show this help message and exit
  -bl BL      maximum length for buffer representation
  -wt         show also absent / transparent fields

$ ./tools/pycrate_showmedia.py ./test/res/xkcd_wireless_signal.png 
### PNG ###
 <sig [PNG signature] : '\x89PNG\r\n\x1a\n'>
     ### PNGBody ###
      ### PNGChunk ###
       <len : 13>
       <type : 'IHDR'>
       ### IHDR ###
        <width : 238>
        <height : 415>
        <depth [bit depth] : 8>
        <color [color type] : 0 (Greyscale)>
        <comp [compression method] : 0 (inflate/deflate with sliding window)>
        <filter [filter method] : 0 (no interlace)>
        <interlace [interlace method] : 0 (no interlace)>
       <crc : 0x7d8cb12e>
      ### PNGChunk ###
       <len : 9>
       <type : 'pHYs'>
       <data :
        00 00 0c 4e 00 00 0c 4e 01                      | '\x00\x00\x0cN\x00\x00\x0cN\x01'>
       <crc : 0x7f778c23>
      ### PNGChunk ###
       <len : 792>
       <type : 'iCCP'>
       <data :
        50 68 6f 74 6f 73 68 6f 70 20 49 43 43 20 70 72 | 'Photoshop ICC pr'
        6f 66 69 6c 65 00 00 78 da 63 60 60 9e e0 e8 e2 | 'ofile\x00\x00x\xdac``\x9e\xe0\xe8\xe2'
        [...]
        32 fd fc ea eb 82 ef e1 3f 05 7e 9d fa d3 fa cf | '2\xfd\xfc\xea\xeb\x82\xef\xe1?\x05~\x9d\xfa\xd3\xfa\xcf'
        f1 ff 7f 00 0d 00 0f 34                         | '\xf1\xff\x7f\x00\r\x00\x0f4'>
       <crc : 0xfa96f15d>
      ### PNGChunk ###
       <len : 32>
       <type : 'cHRM'>
       <data :
        00 00 6e 27 00 00 73 af 00 00 df f2 00 00 83 30 | "\x00\x00n'\x00\x00s\xaf\x00\x00\xdf\xf2\x00\x00\x830"
        00 00 77 43 00 00 c8 0a 00 00 34 95 00 00 2e dc | '\x00\x00wC\x00\x00\xc8\n\x00\x004\x95\x00\x00.\xdc'>
       <crc : 0x20bf171a>
      ### PNGChunk ###
       <len : 21130>
       <type : 'IDAT'>
       <data :
        78 da ed bd 79 50 8d fd 1f ff ff bc ce 39 73 4e | 'x\xda\xed\xbdyP\x8d\xfd\x1f\xff\xff\xbc\xce9sN'
        db b4 37 95 32 b4 19 94 06 2d 7e 11 26 b2 fc 10 | '\xdb\xb47\x952\xb4\x19\x94\x06-~\x11&\xb2\xfc\x10'
        [...]
        91 a3 d8 5b fc e1 cb 51 fd ab fb c9 cc ec ee 21 | '\x91\xa3\xd8[\xfc\xe1\xcbQ\xfd\xab\xfb\xc9\xcc\xec\xee!'
        7d 70 6e f3 18 ce c1 c1 6d 8c 81 44 32 cf 51 ba | '}pn\xf3\x18\xce\xc1\xc1m\x8c\x81D2\xcfQ\xba'
        ...>
       <crc : 0xa9fbdd38>
      ### PNGChunk ###
       <len : 0>
       <type : 'IEND'>
       <data : >
       <crc : 0xae426082>

It is possible to test the pycrate_asn1compile.py tool with some test ASN.1 specification from ./test/res/, or any other valid ASN.1 specification of your choice.

$ ./tools/pycrate_asn1compile.py --help
usage: pycrate_asn1compile.py [-h] [-s SPEC] [-i INPUT [INPUT ...]] [-o OUTPUT] [-g GENERATOR_PATH] [-j] [-fautotags] [-fextimpl] [-fverifwarn]

compile ASN.1 input file(s) for the pycrate ASN.1 runtime

optional arguments:
  -h, --help            show this help message and exit
  -s SPEC               provide a specification shortname, instead of ASN.1 input file(s)
  -i INPUT [INPUT ...]  ASN.1 input file(s) or directory
  -o OUTPUT             compiled output Python (and json) source file(s)
  -g GENERATOR_PATH, --generator GENERATOR_PATH
                        provide an alternative python generator file path
  -j                    output a json file with information on ASN.1 objects dependency
  -fautotags            force AUTOMATIC TAGS for all ASN.1 modules
  -fextimpl             force EXTENSIBILITY IMPLIED for all ASN.1 modules
  -fverifwarn           force warning instead of raising during the verification stage

$ ./tools/pycrate_asn1compile.py -i ./test/res/Hardcore.asn -o Hardcore
[proc] [./test/res/Hardcore.asn] module HardcoreSyntax (oid: []): 116 ASN.1 assignments found
--- compilation cycle ---
--- compilation cycle ---
--- compilation cycle ---
--- verifications ---
[proc] ASN.1 modules processed: ['HardcoreSyntax']
[proc] ASN.1 objects compiled: 75 types, 3 sets, 37 values
[proc] done

After compiling a module, it is possible to load it in Python and use it for encoding / decoding any objects defined in it.

Python 3.8.5 (default, Jul 28 2020, 12:59:40) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from Hardcore import HardcoreSyntax
>>> HardcoreSyntax # this is the only ASN.1 module provided in Hardcore.asn
<class 'Hardcore.HardcoreSyntax'>
>>> Final = HardcoreSyntax.Final # this is the Final object defined at line 115
>>> Final
<Final (SEQUENCE)>
>>> Final.get_proto() # warning: this can return very laaaaaaarge definitions
('SEQUENCE', {
w1: ('SEQUENCE', {
 r10: ('SEQUENCE', {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null (OPT): 'NULL'
  }),
 r90: ('SEQUENCE', {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null (OPT): 'NULL'
  })
 }),
w2: ('SEQUENCE', {
 r10: ('SEQUENCE', {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null (OPT): 'NULL'
  }),
 r90: ('SEQUENCE', {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null (OPT): 'NULL'
  })
 }),
bool: 'BOOLEAN'
})
>>> V = {
... 'w1':{'r10':{'low':5, 'high':50, 'bool':False}, 'r90':{'low':50, 'high':95, 'bool':False, 'null':0}},
... 'w2':{'r10':{'low':1, 'high':10, 'bool':False}, 'r90':{'low':90, 'high':100, 'bool':True}},
... 'bool': True}
>>> Final.set_val(V)
>>> print(Final.to_asn1()) # .to_asn1() returns a printable ASN.1 representation of the value
{
  w1 {
    r10 {
      low 5,
      high 50,
      bool FALSE
    },
    r90 {
      low 50,
      high 95,
      bool FALSE,
      null NULL
    }
  },
  w2 {
    r10 {
      low 1,
      high 10,
      bool FALSE
    },
    r90 {
      low 90,
      high 100,
      bool TRUE
    }
  },
  bool TRUE
}
>>> Final.to_aper() # aligned PER
b'*\x85\x92\x80@\x01\x00\x08\x02\xd5`'
>>> Final.to_uper() # unaligned PER
b'*\x85\x92\x80@@\x02\x00\xb5X'
>>> Final.to_ber()
b'05\xa0\x18\xa0\t\x80\x01\x05\x81\x012\x82\x01\x00\xa1\x0b\x80\x012\x81\x01_\x82\x01\x00\x83\x00\xa1\x16\xa0\t\x80\x01\x01\x81\x01\n\x82\x01\x00\xa1\t\x80\x01Z\x81\x01d\x82\x01\xff\x82\x01\xff'
>>> Final.to_cer()
b'0\x80\xa0\x80\xa0\x80\x80\x01\x05\x81\x012\x82\x01\x00\x00\x00\xa1\x80\x80\x012\x81\x01_\x82\x01\x00\x83\x00\x00\x00\x00\x00\xa1\x80\xa0\x80\x80\x01\x01\x81\x01\n\x82\x01\x00\x00\x00\xa1\x80\x80\x01Z\x81\x01d\x82\x01\xff\x00\x00\x00\x00\x82\x01\xff\x00\x00'
>>> Final.to_der()
b'05\xa0\x18\xa0\t\x80\x01\x05\x81\x012\x82\x01\x00\xa1\x0b\x80\x012\x81\x01_\x82\x01\x00\x83\x00\xa1\x16\xa0\t\x80\x01\x01\x81\x01\n\x82\x01\x00\xa1\t\x80\x01Z\x81\x01d\x82\x01\xff\x82\x01\xff'
>>> Final.from_ber( Final.to_ber() )
>>> Final() == V # or Final._val == V
True

For more information about the API exposed for each ASN.1 object, you can check the docstrings of all ASN.1 objects, and also read the source file pycrate_asn1rt/asnobj.py. Do not forget to have a look at the wiki, too!