Remove signature check on v1 identity server lookups (#8001)

We've [decided](https://github.com/matrix-org/synapse/issues/5253#issuecomment-665976308) to remove the signature check for v1 lookups.

The signature check has been removed in v2 lookups. v1 lookups are currently deprecated. As mentioned in the above linked issue, this verification was causing deployments for the vector.im and matrix.org IS deployments, and this change is the simplest solution, without being unjustified.

Implementations are encouraged to use the v2 lookup API as it has [increased privacy benefits](https://github.com/matrix-org/matrix-doc/pull/2134).

changelog.d/8001.misc

@@ -0,0 +1 @@
+Remove redundant and unreliable signature check for v1 Identity Service lookup responses.

synapse/handlers/identity.py

@@ -22,14 +22,10 @@ import urllib.parse
 from typing import Awaitable, Callable, Dict, List, Optional, Tuple
 
 from canonicaljson import json
-from signedjson.key import decode_verify_key_bytes
-from signedjson.sign import verify_signed_json
-from unpaddedbase64 import decode_base64
 
 from twisted.internet.error import TimeoutError
 
 from synapse.api.errors import (
-    AuthError,
     CodeMessageException,
     Codes,
     HttpResponseException,
@@ -628,9 +624,9 @@ class IdentityHandler(BaseHandler):
             )
 
             if "mxid" in data:
-                if "signatures" not in data:
-                    raise AuthError(401, "No signatures on 3pid binding")
-                await self._verify_any_signature(data, id_server)
+                # note: we used to verify the identity server's signature here, but no longer
+                # require or validate it. See the following for context:
+                # https://github.com/matrix-org/synapse/issues/5253#issuecomment-666246950
                 return data["mxid"]
         except TimeoutError:
             raise SynapseError(500, "Timed out contacting identity server")
@@ -751,30 +747,6 @@ class IdentityHandler(BaseHandler):
         mxid = lookup_results["mappings"].get(lookup_value)
         return mxid
 
-    async def _verify_any_signature(self, data, server_hostname):
-        if server_hostname not in data["signatures"]:
-            raise AuthError(401, "No signature from server %s" % (server_hostname,))
-        for key_name, signature in data["signatures"][server_hostname].items():
-            try:
-                key_data = await self.blacklisting_http_client.get_json(
-                    "%s%s/_matrix/identity/api/v1/pubkey/%s"
-                    % (id_server_scheme, server_hostname, key_name)
-                )
-            except TimeoutError:
-                raise SynapseError(500, "Timed out contacting identity server")
-            if "public_key" not in key_data:
-                raise AuthError(
-                    401, "No public key named %s from %s" % (key_name, server_hostname)
-                )
-            verify_signed_json(
-                data,
-                server_hostname,
-                decode_verify_key_bytes(
-                    key_name, decode_base64(key_data["public_key"])
-                ),
-            )
-            return
-
     async def ask_id_server_for_third_party_invite(
         self,
         requester: Requester,