Add config option for adding additional TLS fingerprints

synapse/config/tls.py

@@ -19,6 +19,9 @@ from OpenSSL import crypto
 import subprocess
 import os
 
+from hashlib import sha256
+from unpaddedbase64 import encode_base64
+
 GENERATE_DH_PARAMS = False
 
 
@@ -42,6 +45,19 @@ class TlsConfig(Config):
             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
+        # and include it if it is not.
+        x509_certificate_bytes = crypto.dump_certificate(
+            crypto.FILETYPE_ASN1,
+            self.tls_certificate
+        )
+        sha256_fingerprint = encode_base64(sha256(x509_certificate_bytes).digest())
+        sha256_fingerprints = set(f["sha256"] for f in self.tls_fingerprints)
+        if sha256_fingerprint not in sha256_fingerprints:
+            self.tls_fingerprints.append({u"sha256": sha256_fingerprint})
+
         # This config option applies to non-federation HTTP clients
         # (e.g. for talking to recaptcha, identity servers, and such)
         # It should never be used in production, and is intended for
@@ -73,6 +89,27 @@ class TlsConfig(Config):
 
         # Don't bind to the https port
         no_tls: False
+
+        # List of allowed TLS fingerprints for this server to publish along
+        # with the signing keys for this server. Other matrix servers that
+        # make HTTPS requests to this server will check that the TLS
+        # certificates returned by this server match one of the fingerprints.
+        #
+        # Synapse automatically adds its the fingerprint of its own certificate
+        # to the list. So if federation traffic is handle directly by synapse
+        # then no modification to the list is required.
+        #
+        # If synapse is run behind a load balancer that handles the TLS then it
+        # will be necessary to add the fingerprints of the certificates used by
+        # the loadbalancers to this list if they are different to the one
+        # synapse is using.
+        #
+        # Homeservers are permitted to cache the list of TLS fingerprints
+        # returned in the key responses. It may be necessary to publish the
+        # fingerprints of a new certificate and wait for the caches on other
+        # servers to expire before deploying it.
+        tls_fingerprints: []
+        #- {"sha256": "<base64_encoded_sha256_fingerprint>"}
         """ % locals()
 
     def read_tls_certificate(self, cert_path):

synapse/rest/key/v2/local_key_resource.py

@@ -19,8 +19,6 @@ from synapse.http.server import respond_with_json_bytes
 from signedjson.sign import sign_json
 from unpaddedbase64 import encode_base64
 from canonicaljson import encode_canonical_json
-from hashlib import sha256
-from OpenSSL import crypto
 import logging
 
 
@@ -49,7 +47,8 @@ class LocalKey(Resource):
                     "key": # base64 encoded NACL verification key.
                 }
             }
-            "tls_certificate": # base64 ASN.1 DER encoded X.509 tls cert.
+            "tls_fingerprints": # Fingerprints of the TLS certs this server uses.
+                - {"sha256": "..."}
             "signatures": {
                 "this.server.example.com": {
                    "algorithm:version": # NACL signature for this server
@@ -90,21 +89,14 @@ class LocalKey(Resource):
                 u"expired_ts": key.expired,
             }
 
-        x509_certificate_bytes = crypto.dump_certificate(
+        tls_fingerprints = self.config.tls_fingerprints
-            crypto.FILETYPE_ASN1,
-            self.config.tls_certificate
-        )
-
-        sha256_fingerprint = sha256(x509_certificate_bytes).digest()
 
         json_object = {
             u"valid_until_ts": self.valid_until_ts,
             u"server_name": self.config.server_name,
             u"verify_keys": verify_keys,
             u"old_verify_keys": old_verify_keys,
-            u"tls_fingerprints": [{
+            u"tls_fingerprints": tls_fingerprints,
-                u"sha256": encode_base64(sha256_fingerprint),
-            }]
         }
         for key in self.config.signing_key:
             json_object = sign_json(