Require ECDH key exchange & remove dh_params (#4429)

* remove dh_params and set better cipher string

README.rst

@@ -220,7 +220,7 @@ is configured to use TLS with a self-signed certificate. If you would like
 to do initial test with a client without having to setup a reverse proxy,
 you can temporarly use another certificate. (Note that a self-signed
 certificate is fine for `Federation`_). You can do so by changing
-``tls_certificate_path``, ``tls_private_key_path`` and ``tls_dh_params_path``
+``tls_certificate_path`` and ``tls_private_key_path``
 in ``homeserver.yaml``; alternatively, you can use a reverse-proxy, but be sure
 to read `Using a reverse proxy with Synapse`_ when doing so.
 

changelog.d/4229.feature

@@ -0,0 +1 @@
+Synapse's cipher string has been updated to require ECDH key exchange. Configuring and generating dh_params is no longer required, and they will be ignored.

debian/homeserver.yaml

@@ -9,9 +9,6 @@ tls_certificate_path: "/etc/matrix-synapse/homeserver.tls.crt"
 # PEM encoded private key for TLS
 tls_private_key_path: "/etc/matrix-synapse/homeserver.tls.key"
 
-# PEM dh parameters for ephemeral keys
-tls_dh_params_path: "/etc/matrix-synapse/homeserver.tls.dh"
-
 # Don't bind to the https port
 no_tls: False
 

docker/conf/homeserver.yaml

@@ -4,7 +4,6 @@
 
 tls_certificate_path: "/data/{{ SYNAPSE_SERVER_NAME }}.tls.crt"
 tls_private_key_path: "/data/{{ SYNAPSE_SERVER_NAME }}.tls.key"
-tls_dh_params_path: "/data/{{ SYNAPSE_SERVER_NAME }}.tls.dh"
 no_tls: {{ "True" if SYNAPSE_NO_TLS else "False" }}
 tls_fingerprints: []
 

synapse/config/tls.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 
 import os
-import subprocess
 from hashlib import sha256
 
 from unpaddedbase64 import encode_base64
@@ -23,8 +22,6 @@
 
 from ._base import Config
 
-GENERATE_DH_PARAMS = False
-
 
 class TlsConfig(Config):
     def read_config(self, config):
@@ -42,10 +39,6 @@ def read_config(self, config):
                 config.get("tls_private_key_path")
             )
 
-        self.tls_dh_params_path = self.check_file(
-            config.get("tls_dh_params_path"), "tls_dh_params"
-        )
-
         self.tls_fingerprints = config["tls_fingerprints"]
 
         # Check that our own certificate is included in the list of fingerprints
@@ -72,7 +65,6 @@ def default_config(self, config_dir_path, server_name, **kwargs):
 
         tls_certificate_path = base_key_name + ".tls.crt"
         tls_private_key_path = base_key_name + ".tls.key"
-        tls_dh_params_path = base_key_name + ".tls.dh"
 
         return """\
         # PEM encoded X509 certificate for TLS.
@@ -85,9 +77,6 @@ def default_config(self, config_dir_path, server_name, **kwargs):
         # PEM encoded private key for TLS
         tls_private_key_path: "%(tls_private_key_path)s"
 
-        # PEM dh parameters for ephemeral keys
-        tls_dh_params_path: "%(tls_dh_params_path)s"
-
         # Don't bind to the https port
         no_tls: False
 
@@ -131,7 +120,6 @@ def read_tls_private_key(self, private_key_path):
     def generate_files(self, config):
         tls_certificate_path = config["tls_certificate_path"]
         tls_private_key_path = config["tls_private_key_path"]
-        tls_dh_params_path = config["tls_dh_params_path"]
 
         if not self.path_exists(tls_private_key_path):
             with open(tls_private_key_path, "wb") as private_key_file:
@@ -165,31 +153,3 @@ def generate_files(self, config):
                 cert_pem = crypto.dump_certificate(crypto.FILETYPE_PEM, cert)
 
                 certificate_file.write(cert_pem)
-
-        if not self.path_exists(tls_dh_params_path):
-            if GENERATE_DH_PARAMS:
-                subprocess.check_call([
-                    "openssl", "dhparam",
-                    "-outform", "PEM",
-                    "-out", tls_dh_params_path,
-                    "2048"
-                ])
-            else:
-                with open(tls_dh_params_path, "w") as dh_params_file:
-                    dh_params_file.write(
-                        "2048-bit DH parameters taken from rfc3526\n"
-                        "-----BEGIN DH PARAMETERS-----\n"
-                        "MIIBCAKCAQEA///////////JD9qiIWjC"
-                        "NMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
-                        "IlFKCHmONATd75UZs806QxswKwpt8l8U"
-                        "N0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
-                        "awv/XLb0Brft7jhr+1qJn6WunyQRfEsf"
-                        "5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
-                        "mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVS"
-                        "u57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
-                        "fDKQXkYuNs474553LBgOhgObJ4Oi7Aei"
-                        "j7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
-                        "5RXSJhiY+gUQFXKOWoqsqmj/////////"
-                        "/wIBAg==\n"
-                        "-----END DH PARAMETERS-----\n"
-                    )

synapse/crypto/context_factory.py

@@ -46,8 +46,10 @@ def configure_context(context, config):
         if not config.no_tls:
             context.use_privatekey(config.tls_private_key)
 
-        context.load_tmp_dh(config.tls_dh_params_path)
-        context.set_cipher_list("!ADH:HIGH+kEDH:!AECDH:HIGH+kEECDH")
+        # https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
+        context.set_cipher_list(
+            "ECDH+AESGCM:ECDH+CHACHA20:ECDH+AES256:ECDH+AES128:!aNULL:!SHA1"
+        )
 
     def getContext(self):
         return self._context

tests/config/test_generate.py

@@ -51,7 +51,6 @@ def test_generate_config_generates_files(self):
                     "lemurs.win.log.config",
                     "lemurs.win.signing.key",
                     "lemurs.win.tls.crt",
-                    "lemurs.win.tls.dh",
                     "lemurs.win.tls.key",
                 ]
             ),