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diff --git a/doc/dev/session_authentication.rst b/doc/dev/session_authentication.rst new file mode 100644 index 00000000..48fab623 --- /dev/null +++ b/doc/dev/session_authentication.rst @@ -0,0 +1,160 @@ +============================================== +Session Authentication for the Cephx Protocol +============================================== +Peter Reiher +7/30/12 + +The original Cephx protocol authenticated the client to the authenticator and set up a session +key used to authenticate the client to the server it needs to talk to. It did not, however, +authenticate the ongoing messages between the client and server. Based on the fact that they +share a secret key, these ongoing session messages can be easily authenticated by using the +key to sign the messages. + +This document describes changes to the code that allow such ongoing session authentication. +The changes allow for future changes that permit other authentication protocols (and the +existing null NONE and UNKNOWN protocols) to handle signatures, but the only protocol that +actually does signatures, at the time of the writing, is the Cephx protocol. + +Introduction +------------- + +This code comes into play after the Cephx protocol has completed. At this point, the client and +server share a secret key. This key will be used for authentication. For other protocols, there +may or may not be such a key in place, and perhaps the actual procedures used to perform +signing will be different, so the code is written to be general. + +The "session" here is represented by an established pipe. For such pipes, there should be a +``session\_security`` structure attached to the pipe. Whenever a message is to be sent on the +pipe, code that handles the signature for this kind of session security will be called. On the +other end of the pipe, code that checks this kind of session security's message signatures will +be called. Messages that fail the signature check will not be processed further. That implies +that the sender had better be in agreement with the receiver on the session security being used, +since otherwise messages will be uniformly dropped between them. + +The code is also prepared to handle encryption and decryption of session messages, which would +add secrecy to the integrity provided by the signatures. No protocol currently implemented +encrypts the ongoing session messages, though. + +For this functionality to work, several steps are required. First, the sender and receiver must have +a successful run of the cephx protocol to establish a shared key. They must store that key somewhere +that the pipe can get at later, to permit messages to be signed with it. Sent messages must be +signed, and received messages must have their signatures checked. + +The signature could be computed in a variety of ways, but currently its size is limited to 64 bits. +A message's signature is placed in its footer, in a field called ``sig``. + +The signature code in Cephx can be turned on and off at runtime, using a Ceph boolean option called +``cephx\_sign\_messages``. It is currently set to false, by default, so no messages will be signed. It +must be changed to true to cause signatures to be calculated and checked. + +Storing the Key +--------------- + +The key is needed to create signatures on the sending end and check signatures on the receiving end. +In the future, if asymmetric crypto is an option, it's possible that two keys (a private one for +this end of the pipe and a public one for the other end) would need to be stored. At this time, +messages going in both directions will be signed with the same key, so only that key needs to be +saved. + +The key is saved when the pipe is established. On the client side, this happens in ``connect()``, +which is located in ``msg/Pipe.cc``. The key is obtained from a run of the Cephx protocol, +which results in a successfully checked authorizer structure. If there is such an authorizer +available, the code calls ``get\_auth\_session\_handler()`` to create a new authentication session handler +and stores it in the pipe data structure. On the server side, a similar thing is done in +``accept()`` after the authorizer provided by the client has been verified. + +Once these things are done on either end of the connection, session authentication can start. + +These routines (``connect()`` and ``accept()``) are also used to handle situations where a new +session is being set up. At this stage, no authorizer has been created yet, so there's no key. +Special cases in the code that calls the signature code skip these calls when the +``CEPH\_AUTH\_UNKNOWN`` protocol is in use. This protocol label is on the pre-authorizer +messages in a session, indicating that negotiation on an authentication protocol is ongoing and +thus signature is not possible. There will be a reliable authentication operation later in this +session before anything sensitive should be passed, so this is not a security problem. + +Signing Messages +---------------- + +Messages are signed in the ``write\_message`` call located in ``msg/Pipe.cc``. The actual +signature process is to encrypt the CRCs for the message using the shared key. Thus, we must +defer signing until all CRCs have been computed. The header CRC is computed last, so we +call ``sign\_message()`` as soon as we've calculated that CRC. + +``sign\_message()`` is a virtual function defined in ``auth/AuthSessionHandler.h``. Thus, +a specific version of it must be written for each authentication protocol supported. Currently, +only UNKNOWN, NONE and CEPHX are supported. So there is a separate version of ``sign\_message()`` in +``auth/unknown/AuthUnknownSessionHandler.h``, ``auth/none/AuthNoneSessionHandler.h`` and +``auth/cephx/CephxSessionHandler.cc``. The UNKNOWN and NONE versions simply return 0, indicating +success. + +The CEPHX version is more extensive. It is found in ``auth/cephx/CephxSessionHandler.cc``. +The first thing done is to determine if the run time option to handle signatures (see above) is on. +If not, the Cephx version of ``sign\_message()`` simply returns success without actually calculating +a signature or inserting it into the message. + +If the run time option is enabled, ``sign\_message()`` copies all of the message's CRCs (one from the +header and three from the footer) into a buffer. It calls ``encode\_encrypt()`` on the buffer, +using the key obtained from the pipe's ``session\_security`` structure. 64 bits of the encrypted +result are put into the message footer's signature field and a footer flag is set to indicate that +the message was signed. (This flag is a sanity check. It is not regarded as definitive +evidence that the message was signed. The presence of a ``session\_security`` structure at the +receiving end requires a signature regardless of the value of this flag.) If this all goes well, +``sign\_message()`` returns 0. If there is a problem anywhere along the line and no signature +was computed, it returns ``SESSION\_SIGNATURE\_FAILURE``. + +Checking Signatures +------------------- + +The signature is checked by a routine called ``check\_message\_signature()``. This is also a +virtual function, defined in ``auth/AuthSessionHandler.h``. So again there are specific versions +for supported authentication protocols, such as UNKNOWN, NONE and CEPHX. Again, the UNKNOWN and +NONE versions are stored in ``auth/unknown/AuthUnknownSessionHandler.h`` and +``auth/none/AuthNoneSessionHandler.h``, respectively, and again they simply return 0, indicating +success. + +The CEPHX version of ``check\_message\_signature()`` performs a real signature check. This routine +(stored in ``auth/cephx/CephxSessionHandler.cc``) exits with success if the run time option has +disabled signatures. Otherwise, it takes the CRCs from the header and footer, encrypts the result, +and compares it to the signature stored in the footer. Since an earlier routine has checked that +the CRCs actually match the contents of the message, it is unnecessary to recompute the CRCs +on the raw data in the message. The encryption is performed with the same ``encode\_encrypt()`` +routine used on the sending end, using the key stored in the local ``session\_security`` +data structure. + +If everything checks out, the CEPHX routine returns 0, indicating success. If there is a +problem, the routine returns ``SESSION\_SIGNATURE\_FAILURE``. + +Adding New Session Authentication Methods +----------------------------------------- + +For the purpose of session authentication only (not the basic authentication of client and +server currently performed by the Cephx protocol), in addition to adding a new protocol, that +protocol must have a ``sign\_message()`` routine and a ``check\_message\_signature`` routine. +These routines will take a message pointer as a parameter and return 0 on success. The procedure +used to sign and check will be specific to the new method, but probably there will be a +``session\_security`` structure attached to the pipe that contains a cryptographic key. This +structure will be either an ``AuthSessionHandler`` (found in ``auth/AuthSessionHandler.h``) +or a structure derived from that type. + +Adding Encryption to Sessions +----------------------------- + +The existing code is partially, but not fully, set up to allow sessions to have their packets +encrypted. Part of adding encryption would be similar to adding a new authentication method. +But one would also need to add calls to the encryption and decryption routines in ``write\_message()`` +and ``read\_message()``. These calls would probably go near where the current calls for +authentication are made. You should consider whether you want to replace the existing calls +with something more general that does whatever the chosen form of session security requires, +rather than explicitly saying ``sign`` or ``encrypt``. + +Session Security Statistics +--------------------------- + +The existing Cephx authentication code keeps statistics on how many messages were signed, how +many message signature were checked, and how many checks succeeded and failed. It is prepared +to keep similar statistics on encryption and decryption. These statistics can be accessed through +the call ``printAuthSessionHandlerStats`` in ``auth/AuthSessionHandler.cc``. + +If new authentication or encryption methods are added, they should include code that keeps these +statistics. |