Document SecurityContext class

This commit adds documentation to the SecurityContext class.

Closes #7.

gssapi/sec_contexts.py

@@ -13,6 +13,18 @@
 
 @six.add_metaclass(_utils.CheckLastError)
 class SecurityContext(rsec_contexts.SecurityContext):
+    """A GSSAPI Security Context
+
+    This class represents a GSSAPI security context that may be used
+    with and/or returned by other GSSAPI methods.
+
+    It inherits from the low-level GSSAPI
+    :class:`~gssapi.raw.sec_contexts.SecurityContext` class,
+    and thus may used with both low-level and high-level API methods.
+
+    This class may be pickled an unpickled.
+    """
+
     def __new__(cls, base=None, token=None,
                 name=None, creds=None, desired_lifetime=None, flags=None,
                 mech_type=None, channel_bindings=None, usage=None):
@@ -25,6 +37,31 @@ def __new__(cls, base=None, token=None,
     def __init__(self, base=None, token=None,
                  name=None, creds=None, desired_lifetime=None, flags=None,
                  mech_type=None, channel_bindings=None, usage=None):
+        """
+        The constructor creates a new security context, but does not begin
+        the initiate or accept process.
+
+        If the `base` argument is used, an existing
+        :class:`~gssapi.raw.sec_contexts.SecurityContext` object from
+        the low-level API is converted into a high-level object.
+
+        If the `token` argument is passed, the security context is imported
+        using the token.
+
+        Otherwise, a new security context is created.
+
+        If the `usage` argument is not passed, the constructor will attempt
+        to detect what the appropriate usage is based on either the existing
+        security context (if `base` or `token` are used) or the argument set.
+
+        For a security context of the `initiate` usage, the `name` argument
+        must be used, and the `creds`, `mech_type`, `flags`,
+        `desired_lifetime`, and `channel_bindings` arguments may be
+        used as well.
+
+        For a security context of the `accept` usage, the `creds` and
+        `channel_bindings` arguments may optionally be used.
+        """
 
         # NB(directxman12): _last_err must be set first
         self._last_err = None
@@ -84,19 +121,95 @@ def __init__(self, base=None, token=None,
     # TODO(directxman12): implement flag properties
 
     def get_signature(self, message):
+        """Calculate the signature for a message
+
+        This method calculates the signature (called a MIC) for
+        the given message, which may be then used with
+        :meth:`verify_signature` to confirm the validity of the
+        signature.  This is useful if you wish to transmit the
+        message signature and message in your own format.
+
+        Args:
+            message (bytes): the input message
+
+        Returns:
+            bytes: the message signature
+        """
+
         # TODO(directxman12): check flags?
         return rmessage.get_mic(self, message)
 
     def verify_signature(self, message, mic):
+        """Verify the signature for a message
+
+        This method verifies that a signature (generated by
+        :meth:`get_signature` is valid for the given method.
+
+        If the signature is valid, the method will return.
+        Otherwise, it will raise an error.
+
+        Args:
+            message (bytes): the message
+            mic (bytes): the signature to verify
+
+        Raises:
+            BadMICError: the signature was not valid
+        """
+
         return rmessage.verify_mic(self, message, mic)
 
     def wrap(self, message, encrypt):
+        """Wrap a message, optionally with encryption
+
+        This method generates a signature and uses it to
+        wrap the message, optionally encrypting it.
+
+        Args:
+            message (bytes): the message to wrap
+            encrypt (bool): whether or not to encrypt the message
+
+        Returns:
+            WrapResult: the wrapped message and details about it
+                (e.g. whether encryption was used succesfully)
+        """
+
         return rmessage.wrap(self, message, encrypt)
 
     def unwrap(self, message):
+        """Unwrap a wrapped message
+
+        This method unwraps/unencrypts a wrapped message,
+        verifying the signature along the way.
+
+        Args:
+            message (bytes): the message to unwrap/decrypt
+
+        Returns:
+            UnwrapResult: the unwrapped message and details about it
+                (e.g. wheter encryption was used)
+        """
+
         return rmessage.unwrap(self, message)
 
     def encrypt(self, message):
+        """Encrypt a message
+
+        This method wraps and encrypts a message, similarly to
+        :meth:`wrap`.  The difference is that encryption is always
+        used, and the method will raise an exception if this is
+        not possible.  Additionally, this method simply returns
+        the encrypted message directly.
+
+        Args:
+            message (bytes): the message to encrypt
+
+        Returns:
+            bytes: the encrypted message
+
+        Raises:
+            EncryptionNotUsed: the encryption could not be used
+        """
+
         res = self.wrap(message, encrypt=True)
 
         if not res.encrypted:
@@ -105,6 +218,23 @@ def encrypt(self, message):
         return res.message
 
     def decrypt(self, message):
+        """Decrypt a message
+
+        This method decrypts and unwraps a message, verifying the signature
+        along the way, similarly to :meth:`unwrap`.  The difference is that
+        this method will raise an exception if encryption was by the context
+        and not used, and simply returns the decrypted message directly.
+
+        Args:
+            message (bytes): the encrypted message
+
+        Returns:
+            bytes: the decrypted message
+
+        Raises:
+            EncryptionNotUsed: encryption was expected, but not used
+        """
+
         res = self.unwrap(message)
 
         if (not res.encrypted and
@@ -118,26 +248,80 @@ def decrypt(self, message):
 
     def get_wrap_size_limit(self, desired_output_size,
                             encrypted=True):
+        """Get the maximum message size for a given wrapped message size
+
+        This method calculates the maximum input message size for a given
+        wrapped/encrypted message size.
+
+        Args:
+            desired_output_size (int): the maximum output message size
+            encrypted (bool): whether or not encryption should be taken
+                into account
+
+        Returns:
+            int: the maximum input message size
+        """
+
         return rmessage.wrap_size_limit(self, desired_output_size,
                                         encrypted)
 
     def process_token(self, token):
+        """Process an output token asynchronously
+
+        This method processes an output token even when the security context
+        was not expecting it.
+
+        Args:
+            token (bytes): the token to process
+        """
+
         rsec_contexts.process_context_token(self, token)
 
     def export(self):
+        """Export a security context
+
+        This method exports a security context, allowing it to be passed
+        between processes.
+
+        Returns:
+            bytes: the exported security context
+        """
+
         return rsec_contexts.export_sec_context(self)
 
-    INQUIRE_ARGS = ('initiator_name', 'target_name', 'lifetime',
-                    'mech_type', 'flags', 'locally_init', 'complete')
+    _INQUIRE_ARGS = ('initiator_name', 'target_name', 'lifetime',
+                     'mech_type', 'flags', 'locally_init', 'complete')
 
     @_utils.check_last_err
     def _inquire(self, **kwargs):
+        """Inspect the security context for information
+
+        This method inspects the security context for information.
+
+        If no keyword arguments are passed, all available information
+        is returned.  Otherwise, only the keyword arguments that
+        are passed and set to `True` are returned.
+
+        Args:
+            initiator_name (bool): get the initiator name for this context
+            target_name (bool): get the target name for this context
+            lifetime (bool): get the remaining lifetime for this context
+            mech_type (bool): get the mechanism used by this context
+            flags (bool): get the flags set on this context
+            locally_init (bool): get whether this context was locally initiated
+            complete (bool): get whether negotiation on this context has
+                been completed
+
+        Returns:
+            InquireContextResult: the results of the inquiry, with unused
+                fields set to None
+        """
         if not kwargs:
             default_val = True
         else:
             default_val = False
 
-        for arg in self.INQUIRE_ARGS:
+        for arg in self._INQUIRE_ARGS:
             kwargs[arg] = kwargs.get(arg, default_val)
 
         res = rsec_contexts.inquire_context(self, **kwargs)
@@ -161,24 +345,58 @@ def _inquire(self, **kwargs):
 
     @property
     def lifetime(self):
+        """Get the amount of time for which the context remains valid"""
         return rsec_contexts.context_time(self)
 
-    initiator_name = _utils.inquire_property('initiator_name')
-    target_name = _utils.inquire_property('target_name')
-    mech_type = _utils.inquire_property('mech_type')
-    actual_flags = _utils.inquire_property('flags')
-    locally_initiated = _utils.inquire_property('locally_init')
+    initiator_name = _utils.inquire_property(
+        'initiator_name', 'Get the Name of the initiator of this context')
+    target_name = _utils.inquire_property(
+        'target_name', 'Get the Name of the target of this context')
+    mech_type = _utils.inquire_property(
+        'mech_type', 'Get the mechanism in use by this context')
+    actual_flags = _utils.inquire_property(
+        'flags', 'Get the flags set on this context')
+    locally_initiated = _utils.inquire_property(
+        'locally_init', 'Get whether this context was locally intiated')
 
     @property
     @_utils.check_last_err
     def complete(self):
+        """Get whether negotiation for this context has been completed"""
         if self._started:
             return self._inquire(complete=True).complete
         else:
             return False
 
     @_utils.catch_and_return_token
     def step(self, token=None):
+        """Perform a negotation step
+
+        This method performs a negotiation step based on the usage type
+        of this context.  If `__DEFER_STEP_ERRORS__` is set to true,
+        this method will return a token, even when exceptions would be
+        thrown.  The generated exception will be thrown on the next
+        method call or property lookup on the context.
+
+        This method should be used in a while loop, as such:
+
+        .. code-block:: python
+
+           input_token = None
+           try:
+               while not ctx.complete:
+                   output_token = ctx.step(input_token)
+                   input_token = send_and_receive(output_token)
+           except GSSError as e:
+                handle_the_issue()
+
+        Args:
+            token (bytes): the input token from the other participant's step
+
+        Returns:
+            bytes: the output token to send to the other participant
+        """
+
         if self.usage == 'accept':
             return self._acceptor_step(token=token)
         else: