[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[Antoine Pitrou]

Change by Antoine Pitrou :


--
versions: +Python 3.11 -Python 3.9

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[Miguel Brito]

Miguel Brito  added the comment:

I had a look at the HMAC RFC and apparently empty bytes sequence can be used as 
secret key.

"The definition of HMAC requires a cryptographic hash function, which
we denote by H, and a secret key K. 

...

The authentication key K can be of any length up to B, the
block length of the hash function."
   
https://tools.ietf.org/html/rfc2104.html#section-2

Assuming that is the case, the fix would be to change the Listener to:

```
if self._authkey is not None:
deliver_challenge(c, self._authkey)
answer_challenge(c, self._authkey)
return c
```


I created a PR for that, if anyone can review it, I appreciate it.
https://github.com/python/cpython/pull/25845

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[Miguel Brito]

Change by Miguel Brito :


--
keywords: +patch
pull_requests: +24528
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/25845

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[Miguel Brito]

Miguel Brito  added the comment:

I tried debugging this and from what I can see it's because there's an if that 
checks if the authkey is not None in the Client constructor:

https://github.com/python/cpython/blob/v3.9.4/Lib/multiprocessing/connection.py#L512

```
if authkey is not None:
answer_challenge(c, authkey)
deliver_challenge(c, authkey)
```

Whereas in the Listener, the check is different:

https://github.com/python/cpython/blob/v3.9.4/Lib/multiprocessing/connection.py#L469

```
c = self._listener.accept()
if self._authkey:
deliver_challenge(c, self._authkey)
answer_challenge(c, self._authkey)
return c
```

If I change the Listener to:

```
if self._authkey is not None:
deliver_challenge(c, self._authkey)
answer_challenge(c, self._authkey)
return c
```

it works.


The docs say:

"""
If authkey is given and not None, it should be a byte string and will be used 
as the secret key for an HMAC-based authentication challenge. No authentication 
is done if authkey is None. AuthenticationError is raised if authentication 
fails. See Authentication keys.
"""


Now the question is, if None is OK because no auth will be done what about 
empty bytes? Can it be used as secret key? If empty bytes is not accepted 
shouldn't Listener/Client raise an exception in the constructor?

--
nosy: +miguendes

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[anon01]

Change by anon01 :


--
type:  -> behavior

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[anon01]

Change by anon01 :


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nosy:  -anon01

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[issue43952] Multiprocessing UNIX socket connection: client freeze if authkey is an empty byte string

[anon01]

New submission from anon01 :

When I run this code (on a UNIX system with temporary directory `/tmp/`):

```
from multiprocessing import connection
import threading
key=b"1"

def run():
c=connection.Client("/tmp/xxx", authkey=key)
c.send("data")

l=connection.Listener("/tmp/xxx", authkey=key)
threading.Thread(target=run).start()
c=l.accept()
print("receive", c.recv())
```

it prints out "receive data" normally.

However, in the special case that `key=b""` the doesn't print anything, and can 
only be interrupted with Ctrl+C.

`key=None` doesn't have that issue.

Note that this issue does happen when the client uses the key `b""` and the 
server uses the key `None`, but the program works normally if the reverse 
situation.

Python version: Python 3.9.3.

--
components: Library (Lib)
messages: 392056
nosy: anon01
priority: normal
severity: normal
status: open
title: Multiprocessing UNIX socket connection: client freeze if authkey is an 
empty byte string
versions: Python 3.9

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