Extracts original submission XML of CZ tax statement from digitally signed confirmation P7S

extract_tax_confirmation.py

"""
Extracts original submission XML from digitally signed confirmation P7S
(Potvrzení o podání písemnosti) of the Czech tax return statement
(Daň z příjmů fyzických osob).

The confirmation is a sigitally signed XML file (P7S extension) and can be
opened using openssl (kudos to
[moldovan.IT blog](http://moldovan.it/blog/open-and-read-p7s-files-in-ubuntu-linux/)):

```
openssl smime -inform DER -verify -noverify -in input.p7s -out output.xml
```

Then the confirmation is just a wrapper over the original submission XML,
In the `<Pisemnost><Data>` element it just contains the hex-encoded binary data.

Usage:

```
pip install xmltodict
```

```
python extract_tax_confirmation.py DPFDP5-0000000000-20180331-094412-000000000-potvrzeni.p7s
```

And it extracts the submission XML to:
`DPFDP5-0000000000-20180331-094412-000000000-potvrzeni_submission.xml`.

See: https://adisepo.mfcr.cz/adistc/adis/idpr_epo/epo2/spol/odeslani.faces
"""

import argparse
import binascii
import os
import subprocess

import xmltodict

def extract_signed_xml(p7s_path):
    print('Extracting signed confirmation XML from P7S:', p7s_path)
    xml_path = os.path.splitext(p7s_path)[0] + '.xml'
    err_code = subprocess.call([
        'openssl', 'smime', '-inform', 'DER', '-verify', '-noverify',
        '-in', p7s_path,
        '-out', xml_path
    ])
    if err_code > 0:
        raise ValueError('Could not extract signed XML file from %s' % p7s_path)
    return xml_path

def extract_submission_from_confirmation(confirmation_xml_path, submission_xml_path):
    print('Reading confirmation XML:', confirmation_xml_path)
    with open(confirmation_xml_path, 'rb') as f:
        doc = xmltodict.parse(f)
    # b'<?xml version="1.0" encoding="UTF-8"?><Pisemnost><Data>3c3f786d6c2076657273696f6e3d22312e302220656e6'
    data_hex = doc['Pisemnost']['Data']
    data_bin = binascii.a2b_hex(data_hex)
    # b'<?xml version="1.0" encoding="UTF-8"?>\n<Pisemnost nazevSW="EPO MF \xc4\x8cR" verzeSW="40.7.3">\n<DPFDP5 ver'
    with open(submission_xml_path, 'wb') as f:
        f.write(data_bin)
    print('Original submission XML written to:', submission_xml_path)

def extract_submission_from_signed_confirmation(confirmation_p7s_path, submission_xml_path):
    confirmation_xml_path = extract_signed_xml(confirmation_p7s_path)
    extract_submission_from_confirmation(confirmation_xml_path, submission_xml_path)

def make_submission_path(confirmation_xml_path):
    name = os.path.splitext(confirmation_xml_path)[0]
    return '%s_submission.xml' % name

def parse_args():
    parser = argparse.ArgumentParser(description=
        'Extracts tax statement submission from signed confirmation.')
    parser.add_argument('p7s_path', metavar='P7S_PATH',
        help='Signed confirmation file (P7S)')
    return parser.parse_args()

if __name__ == '__main__':
    args = parse_args()
    confirmation_p7s_path = args.p7s_path
    submission_xml_path = make_submission_path(confirmation_p7s_path)
    extract_submission_from_signed_confirmation(confirmation_p7s_path,
        submission_xml_path)