+++ /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.
Code Review / stor4nfv.git / blobdiff