Extraneous whitespace around the signing key results in an opaque failure

This case leads to a DecodeException from the underlying library. It's roughly like:

  File "challenge_bypass_ristretto/__init__.py", line 61, in decode_base64
    raise DecodeException()
challenge_bypass_ristretto.DecodeException

This is the same exception as any key decoding problem provokes. Since the key itself is quite opaque (not quite a purely random byte string but close) it's far from obvious that this signals a whitespace problem. Debugging the failure then involves serious digging or making a lucky guess or bumping into someone who has already figured this out.

The underlying library could be more informative in its exceptions but, failing that, ZKAPAuthorizer can likely improve the user experience for this scenario.