--- /dev/null
+============================================================
+A Detailed Description of the Cephx Authentication Protocol
+============================================================
+Peter Reiher
+7/13/12
+
+This document provides deeper detail on the Cephx authorization protocol whose high level flow
+is described in the memo by Yehuda (12/19/09). Because this memo discusses details of
+routines called and variables used, it represents a snapshot. The code might be changed
+subsequent to the creation of this document, and the document is not likely to be updated in
+lockstep. With luck, code comments will indicate major changes in the way the protocol is
+implemented.
+
+Introduction
+-------------
+
+The basic idea of the protocol is based on Kerberos. A client wishes to obtain something from
+a server. The server will only offer the requested service to authorized clients. Rather
+than requiring each server to deal with authentication and authorization issues, the system
+uses an authorization server. Thus, the client must first communicate with the authorization
+server to authenticate itself and to obtain credentials that will grant it access to the
+service it wants.
+
+Authorization is not the same as authentication. Authentication provides evidence that some
+party is who it claims to be. Authorization provides evidence that a particular party is
+allowed to do something. Generally, secure authorization implies secure authentication
+(since without authentication, you may authorize something for an imposter), but the reverse
+is not necessarily true. One can authenticate without authorizing. The purpose
+of this protocol is to authorize.
+
+The basic approach is to use symmetric cryptography throughout. Each client C has its own
+secret key, known only to itself and the authorization server A. Each server S has its own
+secret key, known only to itself and the authorization server A. Authorization information
+will be passed in tickets, encrypted with the secret key of the entity that offers the service.
+There will be a ticket that A gives to C, which permits C to ask A for other tickets. This
+ticket will be encrypted with A's key, since A is the one who needs to check it. There will
+later be tickets that A issues that allow C to communicate with S to ask for service. These
+tickets will be encrypted with S's key, since S needs to check them. Since we wish to provide
+security of the communications, as well, session keys are set up along with the tickets.
+Currently, those session keys are only used for authentication purposes during this protocol
+and the handshake between the client C and the server S, when the client provides its service
+ticket. They could be used for authentication or secrecy throughout, with some changes to
+the system.
+
+Several parties need to prove something to each other if this protocol is to achieve its
+desired security effects.
+
+1. The client C must prove to the authenticator A that it really is C. Since everything
+is being done via messages, the client must also prove that the message proving authenticity
+is fresh, and is not being replayed by an attacker.
+
+2. The authenticator A must prove to client C that it really is the authenticator. Again,
+proof that replay is not occurring is also required.
+
+3. A and C must securely share a session key to be used for distribution of later
+authorization material between them. Again, no replay is allowable, and the key must be
+known only to A and C.
+
+4. A must receive evidence from C that allows A to look up C's authorized operations with
+server S.
+
+5. C must receive a ticket from A that will prove to S that C can perform its authorized
+operations. This ticket must be usable only by C.
+
+6. C must receive from A a session key to protect the communications between C and S. The
+session key must be fresh and not the result of a replay.
+
+Getting Started With Authorization
+-----------------------------------
+
+When the client first needs to get service, it contacts the monitor. At the moment, it has
+no tickets. Therefore, it uses the "unknown" protocol to talk to the monitor. This protocol
+is specified as ``CEPH_AUTH_UNKNOWN``. The monitor also takes on the authentication server
+role, A. The remainder of the communications will use the cephx protocol (most of whose code
+will be found in files in ``auth/cephx``). This protocol is responsible for creating and
+communicating the tickets spoken of above.
+
+Currently, this document does not follow the pre-cephx protocol flow. It starts up at the
+point where the client has contacted the server and is ready to start the cephx protocol itself.
+
+Once we are in the cephx protocol, we can get the tickets. First, C needs a ticket that
+allows secure communications with A. This ticket can then be used to obtain other tickets.
+This is phase I of the protocol, and consists of a send from C to A and a response from A to C.
+Then, C needs a ticket to allow it to talk to S to get services. This is phase II of the
+protocol, and consists of a send from C to A and a response from A to C.
+
+Phase I:
+--------
+
+The client is set up to know that it needs certain things, using a variable called ``need``,
+which is part of the ``AuthClientHandler`` class, which the ``CephxClientHandler`` inherits
+from. At this point, one thing that's encoded in the ``need`` variable is
+``CEPH_ENTITY_TYPE_AUTH``, indicating that we need to start the authentication protocol
+from scratch. Since we're always talking to the same authorization server, if we've gone
+through this step of the protocol before (and the resulting ticket/session hasn't timed out),
+we can skip this step and just ask for client tickets. But it must be done initially, and
+we'll assume that we are in that state.
+
+The message C sends to A in phase I is build in ``CephxClientHandler::build_request()`` (in
+``auth/cephx/CephxClientHandler.cc``). This routine is used for more than one purpose.
+In this case, we first call ``validate_tickets()`` (from routine
+``CephXTicektManager::validate_tickets()`` which lives in ``auth/cephx/CephxProtocol.h``).
+This code runs through the list of possible tickets to determine what we need, setting values
+in the ``need`` flag as necessary. Then we call ``ticket.get_handler()``. This routine
+(in ``CephxProtocol.h``) finds a ticket of the specified type (a ticket to perform
+authorization) in the ticket map, creates a ticket handler object for it, and puts the
+handler into the right place in the map. Then we hit specialized code to deal with individual
+cases. The case here is when we still need to authenticate to A (the
+``if (need & CEPH_ENTITY_TYPE_AUTH)`` branch).
+
+We now create a message of type ``CEPH_AUTH_UNKNOWN``. We need to authenticate
+this message with C's secret key, so we fetch that from the local key repository. (It's
+called a key server in the code, but it's not really a separate machine or processing entity.
+It's more like the place where locally used keys are kept.) We create a
+random challenge, whose purpose is to prevent replays. We encrypt that challenge. We already
+have a server challenge (a similar set of random bytes, but created by the server and sent to
+the client) from our pre-cephx stage. We take both challenges and our secret key and
+produce a combined encrypted challenge value, which goes into ``req.key``.
+
+If we have an old ticket, we store it in ``req.old_ticket``. We're about to get a new one.
+
+The entire ``req`` structure, including the old ticket and the cryptographic hash of the two
+challenges, gets put into the message. Then we return from this function, and the
+message is sent.
+
+We now switch over to the authenticator side, A. The server receives the message that was
+sent, of type ``CEPH_AUTH_UNKNOWN``. The message gets handled in ``prep_auth()``,
+in ``mon/AuthMonitor.cc``, which calls ``handle_request()`` is ``CephxServiceHandler.cc`` to
+do most of the work. This routine, also, handles multiple cases.
+
+The control flow is determined by the ``request_type`` in the ``cephx_header`` associated
+with the message. Our case here is ``CEPH_AUTH_UNKNOWN``. We need the
+secret key A shares with C, so we call ``get_secret()`` from out local key repository to get
+it. We should have set up a server challenge already with this client, so we make sure
+we really do have one. (This variable is specific to a ``CephxServiceHandler``, so there
+is a different one for each such structure we create, presumably one per client A is
+dealing with.) If there is no challenge, we'll need to start over, since we need to
+check the client's crypto hash, which depends on a server challenge, in part.
+
+We now call the same routine the client used to calculate the hash, based on the same values:
+the client challenge (which is in the incoming message), the server challenge (which we saved),
+and the client's key (which we just obtained). We check to see if the client sent the same
+thing we expected. If so, we know we're talking to the right client. We know the session is
+fresh, because it used the challenge we sent it to calculate its crypto hash. So we can
+give it an authentication ticket.
+
+We fetch C's ``eauth`` structure. This contains an ID, a key, and a set of caps (capabilities).
+
+The client sent us its old ticket in the message, if it had one. If so, we set a flag,
+``should_enc_ticket``, to true and set the global ID to the global ID in that old ticket.
+If the attempt to decode its old ticket fails (most probably because it didn't have one),
+``should_enc_ticket`` remains false. Now we set up the new ticket, filling in timestamps,
+the name of C, the global ID provided in the method call (unless there was an old ticket), and
+his ``auid``, obtained from the ``eauth`` structure obtained above. We need a new session key
+to help the client communicate securely with us, not using its permanent key. We set the
+service ID to ``CEPH_ENTITY_TYPE_AUTH``, which will tell the client C what to do with the
+message we send it. We build a cephx response header and call
+``cephx_build_service_ticket_reply()``.
+
+``cephx_build_service_ticket_reply()`` is in ``auth/cephx/CephxProtocol.cc``. This
+routine will build up the response message. Much of it copies data from its parameters to
+a message structure. Part of that information (the session key and the validity period)
+gets encrypted with C's permanent key. If the ``should_encrypt_ticket`` flag is set,
+encrypt it using the old ticket's key. Otherwise, there was no old ticket key, so the
+new ticket is not encrypted. (It is, of course, already encrypted with A's permanent key.)
+Presumably the point of this second encryption is to expose less material encrypted with
+permanent keys.
+
+Then we call the key server's ``get_service_caps()`` routine on the entity name, with a
+flag ``CEPH_ENTITY_TYPE_MON``, and capabilities, which will be filled in by this routine.
+The use of that constant flag means we're going to get the client's caps for A, not for some
+other data server. The ticket here is to access the authorizer A, not the service S. The
+result of this call is that the caps variable (a parameter to the routine we're in) is
+filled in with the monitor capabilities that will allow C to access A's authorization services.
+
+``handle_request()`` itself does not send the response message. It builds up the
+``result_bl``, which basically holds that message's contents, and the capabilities structure,
+but it doesn't send the message. We go back to ``prep_auth()``, in ``mon/AuthMonitor.cc``,
+for that. This routine does some fiddling around with the caps structure that just got
+filled in. There's a global ID that comes up as a result of this fiddling that is put into
+the reply message. The reply message is built here (mostly from the ``response_bl`` buffer)
+and sent off.
+
+This completes Phase I of the protocol. At this point, C has authenticated itself to A, and A has generated a new session key and ticket allowing C to obtain server tickets from A.
+
+Phase II
+--------
+
+This phase starts when C receives the message from A containing a new ticket and session key.
+The goal of this phase is to provide C with a session key and ticket allowing it to
+communicate with S.
+
+The message A sent to C is dispatched to ``build_request()`` in ``CephxClientHandler.cc``,
+the same routine that was used early in Phase I to build the first message in the protocol.
+This time, when ``validate_tickets()`` is called, the ``need`` variable will not contain
+``CEPH_ENTITY_TYPE_AUTH``, so a different branch through the bulk of the routine will be
+used. This is the branch indicated by ``if (need)``. We have a ticket for the authorizer,
+but we still need service tickets.
+
+We must send another message to A to obtain the tickets (and session key) for the server
+S. We set the ``request_type`` of the message to ``CEPHX_GET_PRINCIPAL_SESSION_KEY`` and
+call ``ticket_handler.build_authorizer()`` to obtain an authorizer. This routine is in
+``CephxProtocol.cc``. We set the key for this authorizer to be the session key we just got
+from A,and create a new nonce. We put the global ID, the service ID, and the ticket into a
+message buffer that is part of the authorizer. Then we create a new ``CephXAuthorize``
+structure. The nonce we just created goes there. We encrypt this ``CephXAuthorize``
+structure with the current session key and stuff it into the authorizer's buffer. We
+return the authorizer.
+
+Back in ``build_request()``, we take the part of the authorizer that was just built (its
+buffer, not the session key or anything else) and shove it into the buffer we're creating
+for the message that will go to A. Then we delete the authorizer. We put the requirements
+for what we want in ``req.keys``, and we put ``req`` into the buffer. Then we return, and
+the message gets sent.
+
+The authorizer A receives this message which is of type ``CEPHX_GET_PRINCIPAL_SESSION_KEY``.
+The message gets handled in ``prep_auth()``, in ``mon/AuthMonitor.cc``, which again calls
+``handle_request()`` in ``CephxServiceHandler.cc`` to do most of the work.
+
+In this case, ``handle_request()`` will take the ``CEPHX_GET_PRINCIPAL_SESSION_KEY`` case.
+It will call ``cephx_verify_authorizer()`` in ``CephxProtocol.cc``. Here, we will grab
+a bunch of data out of the input buffer, including the global and service IDs and the ticket
+for A. The ticket contains a ``secret_id``, indicating which key is being used for it.
+If the secret ID pulled out of the ticket was -1, the ticket does not specify which secret
+key A should use. In this case, A should use the key for the specific entity that C wants
+to contact, rather than a rotating key shared by all server entities of the same type.
+To get that key, A must consult the key repository to find the right key. Otherwise,
+there's already a structure obtained from the key repository to hold the necessary secret.
+Server secrets rotate on a time expiration basis (key rotation is not covered in this
+document), so run through that structure to find its current secret. Either way, A now
+knows the secret key used to create this ticket. Now decrypt the encrypted part of the
+ticket, using this key. It should be a ticket for A.
+
+The ticket also contains a session key that C should have used to encrypt other parts of
+this message. Use that session key to decrypt the rest of the message.
+
+Create a ``CephXAuthorizeReply`` to hold our reply. Extract the nonce (which was in the stuff
+we just decrypted), add 1 to it, and put the result in the reply. Encrypt the reply and
+put it in the buffer provided in the call to ``cephx_verify_authorizer()`` and return
+to ``handle_request()``. This will be used to prove to C that A (rather than an attacker)
+created this response.
+
+Having verified that the message is valid and from C, now we need to build it a ticket for S.
+We need to know what S it wants to communicate with and what services it wants. Pull the
+ticket request that describes those things out of its message. Now run through the ticket
+request to see what it wanted. (He could potentially be asking for multiple different
+services in the same request, but we will assume it's just one, for this discussion.) Once we
+know which service ID it's after, call ``build_session_auth_info()``.
+
+``build_session_auth_info()`` is in ``CephxKeyServer.cc``. It checks to see if the
+secret for the ``service_ID`` of S is available and puts it into the subfield of one of
+the parameters, and calls the similarly named ``_build_session_auth_info()``, located in
+the same file. This routine loads up the new ``auth_info`` structure with the
+ID of S, a ticket, and some timestamps for that ticket. It generates a new session key
+and puts it in the structure. It then calls ``get_caps()`` to fill in the
+``info.ticket`` caps field. ``get_caps()`` is also in ``CephxKeyServer.cc``. It fills the
+``caps_info`` structure it is provided with caps for S allowed to C.
+
+Once ``build_session_auth_info()`` returns, A has a list of the capabilities allowed to
+C for S. We put a validity period based on the current TTL for this context into the info
+structure, and put it into the ``info_vec`` structure we are preparing in response to the
+message.
+
+Now call ``build_cephx_response_header()``, also in ``CephxServiceHandler.cc``. Fill in
+the ``request_type``, which is ``CEPHX_GET_PRINCIPAL_SESSION_KEY``, a status of 0,
+and the result buffer.
+
+Now call ``cephx_build_service_ticket_reply()``, which is in ``CephxProtocol.cc``. The
+same routine was used towards the end of A's handling of its response in phase I. Here,
+the session key (now a session key to talk to S, not A) and the validity period for that
+key will be encrypted with the existing session key shared between C and A.
+The ``should_encrypt_ticket`` parameter is false here, and no key is provided for that
+encryption. The ticket in question, destined for S once C sends it there, is already
+encrypted with S's secret. So, essentially, this routine will put ID information,
+the encrypted session key, and the ticket allowing C to talk to S into the buffer to
+be sent to C.
+
+After this routine returns, we exit from ``handle_request()``, going back to ``prep_auth()``
+and ultimately to the underlying message send code.
+
+The client receives this message. The nonce is checked as the message passes through
+``Pipe::connect()``, which is in ``msg/SimpleMessager.cc``. In a lengthy ``while(1)`` loop in
+the middle of this routine, it gets an authorizer. If the get was successful, eventually
+it will call ``verify_reply()``, which checks the nonce. ``connect()`` never explicitly
+checks to see if it got an authorizer, which would suggest that failure to provide an
+authorizer would allow an attacker to skip checking of the nonce. However, in many places,
+if there is no authorizer, important connection fields will get set to zero, which will
+ultimately cause the connection to fail to provide data. It would be worth testing, but
+it looks like failure to provide an authorizer, which contains the nonce, would not be helpful
+to an attacker.
+
+The message eventually makes its way through to ``handle_response()``, in
+``CephxClientHandler.cc``. In this routine, we call ``get_handler()`` to get a ticket
+handler to hold the ticket we have just received. This routine is embedded in the definition
+for a ``CephXTicketManager`` structure. It takes a type (``CEPH_ENTITY_TYPE_AUTH``, in
+this case) and looks through the ``tickets_map`` to find that type. There should be one, and
+it should have the session key of the session between C and A in its entry. This key will
+be used to decrypt the information provided by A, particularly the new session key allowing
+C to talk to S.
+
+We then call ``verify_service_ticket_reply()``, in ``CephxProtocol.cc``. This routine
+needs to determine if the ticket is OK and also obtain the session key associated with this
+ticket. It decrypts the encrypted portion of the message buffer, using the session key
+shared with A. This ticket was not encrypted (well, not twice - tickets are always encrypted,
+but sometimes double encrypted, which this one isn't). So it can be stored in a service
+ticket buffer directly. We now grab the ticket out of that buffer.
+
+The stuff we decrypted with the session key shared between C and A included the new session
+key. That's our current session key for this ticket, so set it. Check validity and
+set the expiration times. Now return true, if we got this far.
+
+Back in ``handle_response()``, we now call ``validate_tickets()`` to adjust what we think
+we need, since we now have a ticket we didn't have before. If we've taken care of
+everything we need, we'll return 0.
+
+This ends phase II of the protocol. We have now successfully set up a ticket and session key
+for client C to talk to server S. S will know that C is who it claims to be, since A will
+verify it. C will know it is S it's talking to, again because A verified it. The only
+copies of the session key for C and S to communicate were sent encrypted under the permanent
+keys of C and S, respectively, so no other party (excepting A, who is trusted by all) knows
+that session key. The ticket will securely indicate to S what C is allowed to do, attested
+to by A. The nonces passed back and forth between A and C ensure that they have not been
+subject to a replay attack. C has not yet actually talked to S, but it is ready to.
+
+Much of the security here falls apart if one of the permanent keys is compromised. Compromise
+of C's key means that the attacker can pose as C and obtain all of C's privileges, and can
+eavesdrop on C's legitimate conversations. He can also pretend to be A, but only in
+conversations with C. Since it does not (by hypothesis) have keys for any services, he
+cannot generate any new tickets for services, though it can replay old tickets and session
+keys until S's permanent key is changed or the old tickets time out.
+
+Compromise of S's key means that the attacker can pose as S to anyone, and can eavesdrop on
+any user's conversation with S. Unless some client's key is also compromised, the attacker
+cannot generate new fake client tickets for S, since doing so requires it to authenticate
+himself as A, using the client key it doesn't know.
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