bpo-17258: use sha256 instead of md5 within multiprocessing.connection

[stratakis]

Within multiprocessing.connection, deliver_challenge() and
answer_challenge() use hmac for a challenge/response, however
hmac implicitly defaults to using MD5, which fails under FIPS
mode. Hardcode the digest value to sha256.

This PR is adapted from the patch provided at bpo-17258 and pulled from encukou@b819ca6

https://bugs.python.org/issue17258

[stratakis]

cc @tiran

[tiran]

I would prefer a more flexible solution that allows us to change algorithms more easily in the future and perhaps allows different auth mechanisms than HMAC challenge and response.

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[encukou]

I would prefer a more flexible solution that allows us to change algorithms more easily in the future

As fas as I can see, changing the algorithm would mean setting HMAC_DIGEST_NAME to a new value. That sounds very easy.
What am I missing?

and perhaps allows different auth mechanisms than HMAC challenge and response.

I don't see how this PR is blocking someone from working on that.

[tiran]

The change breaks backward compatibility. multiprocessing supports distributed computing across multiple machines and works with multiple Python versions. With the change a controller with Python 3.9 would no longer be able to talk to a 3.8 server or the other way around.

[stratakis]

@tiran What would you propose to move this forward, essentially moving away from md5? Any approaches that would make sense? Something like implementing a client-server handshake negotiation to agree on a hash?

[stratakis]

Closing this. Superseded by #20380