Is Python SSL API thread-safe?

Christian Heimes christian at
Sun Jan 22 16:34:38 EST 2017

On 2017-01-22 21:18, Grant Edwards wrote:
> Is the Python SSL API thread-safe with respect to recv() and send()?
> IOW, can I have one thread doing blocking recv() calls on an SSL
> connection object while "simultaneously" a second thread is calling
> send() on that same connection object?
> I assumed that was allowed, but I can't find anything in the
> documentation that actually says it is.

OpenSSL and Python's ssl module are thread-safe. However IO is not safe
concerning reentrancy. You cannot safely share a SSLSocket between
threads without a mutex. Certain aspects of the TLS protocol can cause
interesting side effects. A recv() call can send data across a wire and
a send() call can receive data from the wire, e.g. during re-keying.

In order to archive reentrancy, you have to do all IO yourself by
operating the SSL connection in non-blocking mode or with a Memorio-BIO

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