Hey,

I was doing some fuzzing and I found out that the idna.decode() method can raise a UnicodeDecodeError leading to a crash of the running program.

This could be problematic if users of the library are dealing with untrusted data since this issue will lead to a DoS. This should be detected and an idna.IDNAError should be triggered as detailed in the Readme Exception section.

Reproduction

import idna

idna.decode(b'\x8d\xd2')

Traceback

python3 replay_crash.py
Traceback (most recent call last):
  File "replay_crash.py", line 3, in <module>
    idna.decode(b'\x8d\xd2')
  File "/home/scop/.local/lib/python3.8/site-packages/idna/core.py", line 387, in decode
    s = s.decode('ascii')
UnicodeDecodeError: 'ascii' codec can't decode byte 0x8d in position 0: ordinal not in range(128)

Happy to help if you have any questions ;)