Challenge in hex breaks keycloak authentication

[floriansipp]

Hello,

This commit breaks keycloak authentication since the challenge is now send in hex and we get this error :

2025-02-28 07:59:06,526 check(b'\x02\x07\x07\x00\r\x08SW\x03\x05\x0cT\x01 … 53144a')
2025-02-28 07:59:06,526 Error: keycloak authentication failed
2025-02-28 07:59:06,526  invalid response received from authorization endpoint
2025-02-28 07:59:06,526 failed to parse json: b'\x02\x07\x07\x00\r\x08SW\x03\x05\x0cT\x01\x00QY …. 5a000606505353144a'
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/xpra/server/auth/keycloak.py", line 88, in check
    response = json.loads(response_json)
  File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

I suppose we should decode the hex on the other side, but can you point to a function for that ?

Thank you