Add certificate validation capability.

gemini-demo.py

@@ -8,6 +8,12 @@ import ssl
 import tempfile
 import textwrap
 import urllib.parse
+import socks
+
+timeout = 5
+
+tor_validation = True
+accepted_certs = {}
 
 caps = mailcap.getcaps()
 menu = []
@@ -47,11 +53,72 @@ while True:
     # Do the Gemini transaction
     try:
         while True:
-            s = socket.create_connection((parsed_url.netloc, 1965))
             context = ssl.SSLContext()
             context.check_hostname = False
             context.verify_mode = ssl.CERT_NONE
-            s = context.wrap_socket(s, server_hostname = parsed_url.netloc)
+
+            host = parsed_url.hostname
+            port = parsed_url.port or 1965
+
+            s = socket.create_connection((host, port), timeout)
+            s = context.wrap_socket(s, server_hostname = host)
+            cert = s.getpeercert(True)
+
+            if tor_validation \
+                    and (host not in accepted_certs \
+                        or accepted_certs[host] != cert):
+
+                # Verify TLS certificate from the vantage point
+                # of a random Tor exit node.
+                print("Validating certificate...")
+
+                # Get certificate via Tor (SOCKS5 @ localhost:9050).
+                # DNS lookup is done over Tor.
+                tor_socket = socks.socksocket()
+                tor_socket.set_proxy(socks.SOCKS5, "127.0.0.1", 9050, rdns=True)
+                tor_socket.settimeout(timeout)
+                tor_connection_successful = True
+                validation_problem_encountered = False
+                try:
+                    tor_socket.connect((host, port))
+                except socks.ProxyConnectionError:
+                    validation_problem_encountered = True
+                    tor_connection_successful = False
+                    print("Tor proxy not available on localhost:9050.")
+                    print("Certificate validation can't be performed.")
+                    print("Continue browsing with validation disabled? [y/N]")
+                    if input(">> ").strip().lower() == "y":
+                        tor_validation = False
+                    else:
+                        raise Exception("Connection cancelled.")
+                except socks.GeneralProxyError:
+                    validation_problem_encountered = True
+                    tor_connection_successful = False
+                    print("Tor connection timed out.")
+                    print("Certificate validation can't be performed.")
+                    print("Continue connection? [y/N]")
+                    if input(">> ").strip().lower() != "y":
+                        raise Exception("Connection cancelled.")
+                if tor_connection_successful:
+                    tor_socket = context.wrap_socket(tor_socket, server_hostname = host)
+                    cert_via_tor = tor_socket.getpeercert(True)
+                    tor_socket.shutdown(socket.SHUT_RDWR)
+                    tor_socket.close()
+                    # Compare certs.
+                    if cert != cert_via_tor:
+                        validation_problem_encountered = True
+                        print("[SECURITY WARNING] Certificate validation failed!")
+                        print("This MIGHT be a Man-in-the-Middle attack.")
+                        print("Continue connection? [y/N]")
+                        if input(">> ").strip().lower() != "y":
+                            raise Exception("Connection cancelled.")
+
+                accepted_certs[host] = cert
+
+                if not validation_problem_encountered:
+                    print("OK")
+                    print()
+
             s.sendall((url + '\r\n').encode("UTF-8"))
             # Get header and check for redirects
             fp = s.makefile("rb")