Add Connection.get_certificate method

This makes it possible to retrieve the local certificate (if any)
for a Connection.

An example where this is useful is when negotiating a DTLS-SRTP
connection, the fingerprint of the local certificate needs to be
communicated to the remote party out-of-band via SDP.

CHANGELOG.rst

@@ -23,7 +23,8 @@ Deprecations:
 Changes:
 ^^^^^^^^
 
-*none*
+- Added ``Connection.get_certificate`` to retrieve the local certificate.
+  `#733 </~https://github.com/pyca/pyopenssl/pull/733>`_
 
 
 ----

src/OpenSSL/SSL.py

@@ -2164,6 +2164,18 @@ def sock_shutdown(self, *args, **kwargs):
         """
         return self._socket.shutdown(*args, **kwargs)
 
+    def get_certificate(self):
+        """
+        Retrieve the local certificate (if any)
+
+        :return: The local certificate
+        """
+        cert = _lib.SSL_get_certificate(self._ssl)
+        if cert != _ffi.NULL:
+            _lib.X509_up_ref(cert)
+            return X509._from_raw_x509_ptr(cert)
+        return None
+
     def get_peer_certificate(self):
         """
         Retrieve the other side's certificate (if any)

tests/test_ssl.py

@@ -2392,6 +2392,31 @@ def test_makefile(self):
         with pytest.raises(NotImplementedError):
             conn.makefile()
 
+    def test_get_certificate(self):
+        """
+        `Connection.get_certificate` returns the local certificate.
+        """
+        chain = _create_certificate_chain()
+        [(cakey, cacert), (ikey, icert), (skey, scert)] = chain
+
+        context = Context(TLSv1_METHOD)
+        context.use_certificate(scert)
+        client = Connection(context, None)
+        cert = client.get_certificate()
+        assert cert is not None
+        assert "Server Certificate" == cert.get_subject().CN
+
+    def test_get_certificate_none(self):
+        """
+        `Connection.get_certificate` returns the local certificate.
+
+        If there is no certificate, it returns None.
+        """
+        context = Context(TLSv1_METHOD)
+        client = Connection(context, None)
+        cert = client.get_certificate()
+        assert cert is None
+
     def test_get_peer_cert_chain(self):
         """
         `Connection.get_peer_cert_chain` returns a list of certificates