python api: allow control over the server certificate verification

The --insecure option ("insecure=true" in .zuliprc) disables
verification entirely, similar to other tools like curl.

The --cert_bundle ("cert_bundle=<file>" in .zuliprc) allows
a file to be specified containing the CA certificates to verify
against.
When using self-signed certificates, the server's public key
can be used as the only cerificate in the file.

This change incidentally fixes an issue where the "site" parameter
in .zuliprc was ignored when specifying --user and --api-key on
the command line.

Fixes: #104

README.md

@@ -31,11 +31,24 @@ file is as follows:
     key=<api key from the web interface>
     email=<your email address>
     site=<your Zulip server's URI>
+    insecure=<true or false, true means do not verify the server certificate>
+    cert_bundle=<path to a file containing CA or server certificates to trust>
+
+If omitted, these settings have the following defaults:
+
+    site=https://api.zulip.com
+    insecure=false
+    cert_bundle=<the default CA bundle trusted by Python>
 
 Alternatively, you may explicitly use "--user" and "--api-key" in our
 examples, which is especially useful if you are running several bots
-which share a home directory.  There is also a "--site" option for
+which share a home directory.
-setting the Zulip server on the command line.
+
+The command line equivalents for other configuration options are:
+
+    --site=<your Zulip server's URI>
+    --insecure
+    --cert-bundle=<file>
 
 You can obtain your Zulip API key, create bots, and manage bots all
 from your Zulip [settings page](https://zulip.com/#settings).
@@ -101,3 +114,46 @@ Alternatively, if you don't want to use your ~/.zuliprc file:
         --api-key a0b1c2d3e4f5a6b7c8d9e0f1a2b3c4d5 \
         hamlet@example.com cordelia@example.com -m \
         "Conscience doth make cowards of us all."
+
+#### Working with an untrusted server certificate
+
+If your server has either a self-signed certificate, or a certificate signed
+by a CA that you don't wish to globally trust then by default the API will
+fail with an SSL verification error.
+
+You can add `insecure=true` to your .zuliprc file.
+
+    [api]
+    site=https://zulip.example.com
+    insecure=true
+
+This disables verification of the server certificate, so connections are
+encrypted but unauthenticated. This is not secure, but may be good enough
+for a development environment.
+
+
+You can explicitly trust the server certificate using `cert_bundle=<filename>`
+in your .zuliprc file.
+
+    [api]
+    site=https://zulip.example.com
+    cert_bundle=/home/bots/certs/zulip.example.com.crt
+
+You can also explicitly trust a different set of Certificate Authorities from
+the default bundle that is trusted by Python. For example to trust a company
+internal CA.
+
+    [api]
+    site=https://zulip.example.com
+    cert_bundle=/home/bots/certs/example.com.ca-bundle
+
+Save the server certificate (or the CA certificate) in its own file,
+converting to PEM format first if necessary.
+Verify that the certificate you have saved is the same as the one on the
+server.
+
+The `cert_bundle` option trusts the server / CA certificate only for
+interaction with the zulip site, and is relatively secure.
+
+Note that a certificate bundle is merely one or more certificates combined
+into a single file.

zulip/__init__.py

@@ -117,6 +117,19 @@ def generate_option_group(parser, prefix=''):
                      default=None,
                      dest="zulip_client",
                      help=optparse.SUPPRESS_HELP)
+    group.add_option('--insecure',
+                     action='store_true',
+                     dest='insecure',
+                     help='''Do not verify the server certificate.
+                          The https connection will not be secure.''')
+    group.add_option('--cert-bundle',
+                     action='store',
+                     dest='cert_bundle',
+                     help='''Specify a file containing either the
+                          server certificate, or a set of trusted
+                          CA certificates. This will be used to
+                          verify the server's identity. All
+                          certificates should be PEM encoded.''')
     return group
 
 def init_from_options(options, client=None):
@@ -126,7 +139,8 @@ def init_from_options(options, client=None):
         client = _default_client()
     return Client(email=options.zulip_email, api_key=options.zulip_api_key,
                   config_file=options.zulip_config_file, verbose=options.verbose,
-                  site=options.zulip_site, client=client)
+                  site=options.zulip_site, client=client,
+                  cert_bundle=options.cert_bundle, insecure=options.insecure)
 
 def get_default_config_filename():
     config_file = os.path.join(os.environ["HOME"], ".zuliprc")
@@ -138,15 +152,14 @@ def get_default_config_filename():
 class Client(object):
     def __init__(self, email=None, api_key=None, config_file=None,
                  verbose=False, retry_on_errors=True,
-                 site=None, client=None):
+                 site=None, client=None,
+                 cert_bundle=None, insecure=None):
         if client is None:
             client = _default_client()
-        if None in (api_key, email):
+
-            if config_file is None:
+        if config_file is None:
-                config_file = get_default_config_filename()
+            config_file = get_default_config_filename()
-            if not os.path.exists(config_file):
+        if os.path.exists(config_file):
-                raise RuntimeError("api_key or email not specified and %s does not exist"
-                                   % (config_file,))
             config = SafeConfigParser()
             with file(config_file, 'r') as f:
                 config.readfp(f, config_file)
@@ -156,6 +169,22 @@ class Client(object):
                 email = config.get("api", "email")
             if site is None and config.has_option("api", "site"):
                 site = config.get("api", "site")
+            if cert_bundle is None and config.has_option("api", "cert_bundle"):
+                cert_bundle = config.get("api", "cert_bundle")
+            if insecure is None and config.has_option("api", "insecure"):
+                # Be quite strict about what is accepted so that users don't
+                # disable security unintentionally.
+                insecure_setting = config.get("api", "insecure").lower()
+                if insecure_setting == "true":
+                    insecure = True
+                elif insecure_setting == "false":
+                    insecure = False
+                else:
+                    raise RuntimeError("insecure is set to '%s', it must be 'true' or 'false' if it is used in %s"
+                                       % (insecure_setting, config_file))
+        elif None in (api_key, email):
+            raise RuntimeError("api_key or email not specified and %s does not exist"
+                               % (config_file,))
 
         self.api_key = api_key
         self.email = email
@@ -175,6 +204,17 @@ class Client(object):
         self.retry_on_errors = retry_on_errors
         self.client_name = client
 
+        if insecure:
+            self.tls_verification=False
+        elif cert_bundle is not None:
+            if not os.path.isfile(cert_bundle):
+                raise RuntimeError("tls bundle '%s' does not exist"
+                                   %(cert_bundle,))
+            self.tls_verification=cert_bundle
+        else:
+            # Default behavior: verify against system CA certificates
+            self.tls_verification=True
+
     def get_user_agent(self):
         vendor = ''
         vendor_version = ''
@@ -249,7 +289,7 @@ class Client(object):
                         urlparse.urljoin(self.base_url, url),
                         auth=requests.auth.HTTPBasicAuth(self.email,
                                                          self.api_key),
-                        verify=True, timeout=90,
+                        verify=self.tls_verification, timeout=90,
                         headers={"User-agent": self.get_user_agent()},
                         **kwargs)