A mitmproxy pretty-printer for displaying the rfc8555 application/jose+json content type.

josejson.py

"""
This custom pretty-printer for mitmproxy will decode the base64url-encoded
'payload' and 'protected' fields.

This pretty-printer is useful for understanding the POST requests made to
an ACME server, since these requests are made using JWS as JSON, which,
contrary to JWT tokens that are very easy to decode, aren't as common.

The whole JWS as JSON and application/jose+json are detailed in
https://tools.ietf.org/html/rfc8555.

To use this pretty-printer, run mitmproxy with
  mitmproxy -s josejson.py
"""
from mitmproxy import contentviews, ctx
import json
import base64
import typing
import re
from mitmproxy.contentviews import base


def format_json(data: typing.Any) -> typing.Iterator[base.TViewLine]:
    encoder = json.JSONEncoder(indent=4, sort_keys=True, ensure_ascii=False)
    current_line: base.TViewLine = []
    for chunk in encoder.iterencode(data):
        if "\n" in chunk:
            rest_of_last_line, chunk = chunk.split("\n", maxsplit=1)
            # rest_of_last_line is a delimiter such as , or [
            current_line.append(("text", rest_of_last_line))
            yield current_line
            current_line = []
        if re.match(r'\s*"', chunk):
            current_line.append(("json_string", chunk))
        elif re.match(r"\s*\d", chunk):
            current_line.append(("json_number", chunk))
        elif re.match(r"\s*(true|null|false)", chunk):
            current_line.append(("json_boolean", chunk))
        else:
            current_line.append(("text", chunk))
    yield current_line


class ViewJoseJson(contentviews.View):
    name = "jose+json"
    content_types = ["application/jose+json"]

    def __call__(self, data: bytes, **metadata) -> contentviews.TViewResult:
        data = json.loads(data.decode("utf-8"))
        ctx.log.info("ok")
        return (
            "protected & payload base64-decoded",
            format_json(josejson(data)),
        )


def josejson(data: typing.Any) -> typing.Any:
    if len(data["protected"]) == 0:
        data["protected"] = "e30"  # '{}' in base64

    # https://stackoverflow.com/questions/2941995/python-ignore-incorrect-padding-error-when-base64-decoding
    data["protected"] += "==="

    try:
        data["protected"] = json.loads(base64.urlsafe_b64decode(data["protected"]))
    except Exception as e:
        ctx.log.info(e)

    if len(data["payload"]) == 0:
        data["payload"] = "e30"  # '{}' in base64

    # https://stackoverflow.com/questions/2941995/python-ignore-incorrect-padding-error-when-base64-decoding
    data["payload"] += "==="

    try:
        data["payload"] = json.loads(base64.urlsafe_b64decode(data["payload"]))
    except Exception as e:
        ctx.log.info(e)

    return data


view = ViewJoseJson()


def load(l):
    contentviews.add(view)


def done():
    contentviews.remove(view)