What is pycrate

Pycrate is a french word for qualifying bad wine. The present software library has nothing to do with bad wine, it is simply a Python library for manipulating various digital formats in an easy way. It is the glorious successor of libmich, which was started 8 years ago and served well.

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 or nuitka. 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).

Dependencies

Currently none. Only the Python builtins and few internal modules of Python (e.g. os, system, re) are required.

Automatic installation

An installation script is available. It installs the library within your Python package directory:

python setup.py install

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 setup.py test

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.proc

License

The whole library is licensed under GPLv2 and is compatible with more recent version of the GPL: all licensed files have an header making it self-explanatory.

Contact and support

As the unique developper of the library, I am the only person to contact: michau [dot] benoit [at] gmail [dot] com

Extending the library

If you are willing to extend the library, do not hesitate to contact me by email or through the github service. Any patch or submission is very welcome! Moreover, in case you are using this library in any of your project and you find it useful, do not hesitate to drop me an email. It is always a pleasure to know where code provided on the Internet can end up...

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.

• 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
• 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).
• proc 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 easier 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 and DER 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 dubdirectory contains some CSN.1 structures extracted from 3GPP specifications (in the .csn files), and translated into Python objects.

pycrate_mobile

This subdirectory implements several 3GPP NAS protocol formats:

• GSMTAP with the gsmtap header format
• MCC_MNC with dictionnaries for MCC and MNC look-ups
• TS24007 with basic formats from the TS 24.007 specification
• TS24008_IE with formats supporting many information elements from TS 24.008
• TS24008_MM with formats for encoding / decoding mobility management messages from TS 24.008
• TS24008_GMM with formats for encoding / decoding GPRS mobility management messages from TS 24.008
• TS24301_IE with formats supporting some information elements from TS 24.301

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...). In a near future, a wiki may be provided to bring examples and methods on how to use the different modules (any contribution on this would be very welcomed, too). Finally, the code provided in the test/ subdirectory is also representative on how to use the different modules.

ASN.1 usage

When a Python module from pycrate_asn1dir/ is loaded, it creates Python classes corresponding to ASN.1 module (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

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

Tools

Three 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.

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 :
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        fd 6d a6 0a 2f 48 96 8f c7 a6 70 05 af 00 e6 c5 | '\xfdm\xa6\n/H\x96\x8f\xc7\xa6p\x05\xaf\x00\xe6\xc5'
        a4 87 3d 13 4d 4c 2f 93 5c 85 f7 e2 1d 6a 7b 63 | '\xa4\x87=\x13ML/\x93\\\x85\xf7\xe2\x1dj{c'
        e3 3d 17 84 91 6e 50 66 93 0c c3 17 e9 e6 2f 63 | '\xe3=\x17\x84\x91nPf\x93\x0c\xc3\x17\xe9\xe6/c'
        84 ee 20 76 34 88 1b 8d 97 e2 87 62 04 5d c5 31 | '\x84\xee v4\x88\x1b\x8d\x97\xe2\x87b\x04]\xc51'
        92 8c 44 4a 7b 4c d1 70 17 fa 57 92 64 a5 95 ab | '\x92\x8cDJ{L\xd1p\x17\xfaW\x92d\xa5\x95\xab'
        36 cb bc ab 8e c9 d8 48 92 eb 10 81 b9 d5 2e f1 | '6\xcb\xbc\xab\x8e\xc9\xd8H\x92\xeb\x10\x81\xb9\xd5.\xf1'
        cc d4 e4 15 c9 49 58 ef 0f a7 80 1e f0 2d 4f c5 | '\xcc\xd4\xe4\x15\xc9IX\xef\x0f\xa7\x80\x1e\xf0-O\xc5'
        78 92 1c 2f 5d fb 1a 96 49 63 8d 89 d8 45 de c7 | 'x\x92\x1c/]\xfb\x1a\x96Ic\x8d\x89\xd8E\xde\xc7'
        0c be 75 56 a5 e5 41 95 4a d2 aa cf 72 64 91 a4 | '\x0c\xbeuV\xa5\xe5A\x95J\xd2\xaa\xcfrd\x91\xa4'
        d6 a5 fd 78 68 f6 23 0c fb 49 74 2b 70 36 be 4f | '\xd6\xa5\xfdxh\xf6#\x0c\xfbIt+p6\xbeO'
        92 da 27 d2 0f 5d c2 44 b2 a2 07 4e b1 ad 80 18 | "\x92\xda'\xd2\x0f]\xc2D\xb2\xa2\x07N\xb1\xad\x80\x18"
        92 5e 32 fd 78 ef 23 4f 65 a1 79 97 06 71 6f 60 | '\x92^2\xfdx\xef#Oe\xa1y\x97\x06qo`'
        8f f8 a1 08 41 4f b0 9d 24 17 21 35 cd 1b 63 1f | '\x8f\xf8\xa1\x08AO\xb0\x9d$\x17!5\xcd\x1bc\x1f'
        c9 dd c4 b1 7c 1b ab c9 45 b8 c1 74 2c ce ca 7c | '\xc9\xdd\xc4\xb1|\x1b\xab\xc9E\xb8\xc1t,\xce\xca|'
        30 01 1d 92 b0 aa fa 35 ee c8 ba ea 78 11 fd 0e | '0\x01\x1d\x92\xb0\xaa\xfa5\xee\xc8\xba\xeax\x11\xfd\x0e'
        62 b9 86 9c 8e f5 27 70 8e 24 63 c4 7f b8 0b d7 | "b\xb9\x86\x9c\x8e\xf5'p\x8e$c\xc4\x7f\xb8\x0b\xd7"
        c8 37 09 ef 49 26 62 1b b9 08 2f c8 eb 58 92 09 | '\xc87\t\xefI&b\x1b\xb9\x08/\xc8\xebX\x92\t'
        4c 23 35 18 b3 1d e2 24 17 89 49 c8 da 85 7b 16 | 'L#5\x18\xb3\x1d\xe2$\x17\x89I\xc8\xda\x85{\x16'
        7e a4 cc 37 0a 27 6a de 7c 98 2c 8b 64 aa 85 53 | "~\xa4\xcc7\n'j\xde|\x98,\x8bd\xaa\x85S"
        99 9d 97 6d 47 92 ae d6 fa 79 4a 18 4c 72 38 52 | '\x99\x9d\x97mG\x92\xae\xd6\xfayJ\x18Lr8R'
        1a c2 cd c2 24 e9 c9 c3 27 11 9b 49 72 0d 12 59 | "\x1a\xc2\xcd\xc2$\xe9\xc9\xc3'\x11\x9bIr\r\x12Y"
        d2 07 2a e3 57 e2 91 55 78 40 be 17 86 32 1f 32 | '\xd2\x07*\xe3W\xe2\x91Ux@\xbe\x17\x862\x1f2'
        00 e8 9d 96 04 6b 57 e7 80 8f 55 17 99 8d 83 a9 | '\x00\xe8\x9d\x96\x04kW\xe7\x80\x8fU\x17\x99\x8d\x83\xa9'
        76 36 e9 43 8c cb 49 56 d8 76 5c 85 4f 24 79 56 | 'v6\xe9C\x8c\xcbIV\xd8v\\\x85O$yV'
        ec 0d 4c 44 f7 ad 5d 00 0c cc e7 43 ec 22 bb 38 | '\xec\rLD\xf7\xad]\x00\x0c\xcc\xe7C\xec"\xbb8'
        68 5e fc b1 d9 d8 3d 05 96 b2 97 ac 44 d4 51 5c | 'h^\xfc\xb1\xd9\xd8=\x05\x96\xb2\x97\xacD\xd4Q\\'
        23 49 46 21 0c 1f 62 91 38 19 89 b4 73 56 05 d5 | '#IF!\x0c\x1fb\x918\x19\x89\xb4sV\x05\xd5'
        ba 79 3b 5f 8a f7 77 d4 ca 67 1e fe 22 15 ee d2 | '\xbay;_\x8a\xf7w\xd4\xcag\x1e\xfe"\x15\xee\xd2'
        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'
        ...>
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      ### 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] [-i INPUT [INPUT ...]] [-o OUTPUT]
                              [-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
  -i INPUT [INPUT ...]  ASN.1 input file(s) or directory
  -o OUTPUT             compiled output Python source file
  -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

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

Python 3.4.3 (default, Nov 17 2016, 01:08:31) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 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 does not show optional or extended component
{
w1: {
 r10: {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null: 'NULL'
  },
 r90: {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null: 'NULL'
  }
 },
w2: {
 r10: {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null: 'NULL'
  },
 r90: {
  low: 'INTEGER',
  high: 'INTEGER',
  bool: 'BOOLEAN',
  null: '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.