read-qr.py

"""
Install dependencies with:

brew install zbar
virtualenv venv ; source venv/bin/activate
pip install pyzbar pillow

run with

python read-qr.py qr.png

copies heavily from https://github.com/marcan2020/shc-decoder-poc/blob/main/shc-decoder-poc.py
"""

import json
import sys
import base64
import re
import zlib

from pyzbar.pyzbar import decode
from PIL import Image

qrfile = sys.argv[1]

qr = decode(Image.open(qrfile))
assert len(qr) == 1
qr_data = qr[0].data.decode('utf-8')

parts = re.findall("..", qr_data[5:])

jws = ""
for p in parts:
  jws += chr(int(p)+ 45)

def decode(data):
  missing_padding = len(data) % 4
  if missing_padding:
      data += "="* (4 - missing_padding)
  return base64.urlsafe_b64decode(data)

jws_parts = list(map(decode, jws.split(".")))

print("JWS Header:")
print(json.dumps(json.loads(jws_parts[0]), indent=2))

# https://bugs.python.org/issue5784
shc_data = zlib.decompress(jws_parts[1], wbits=-15)

print("SHC Data:")
print(json.dumps(json.loads(shc_data), indent=2))