diff options
Diffstat (limited to '')
| -rw-r--r-- | doc/dev/cephx_protocol.rst | 341 |
1 files changed, 341 insertions, 0 deletions
diff --git a/doc/dev/cephx_protocol.rst b/doc/dev/cephx_protocol.rst new file mode 100644 index 00000000..7b8c1787 --- /dev/null +++ b/doc/dev/cephx_protocol.rst @@ -0,0 +1,341 @@ +.. _cephx_2012_peter: + +============================================================ +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 ``CEPHX_GET_AUTH_SESSION_KEY``. We need to authenticate +this message with C's secret key, so we fetch that from the local key repository. We create +a random challenge, whose purpose is to prevent replays. We encrypt that challenge using +``cephx_calc_client_server_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_GET_AUTH_SESSION_KEY``. 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_GET_AUTH_SESSION_KEY``. We need the +secret key A shares with C, so we call ``get_secret()`` from out local key repository to get +it. (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 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, and the global ID provided in the +method call (unless there was an old ticket). 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. |