| Network Working Group | S. Hartman |
| Internet-Draft | Painless Security |
| Intended status: Standards Track | D. Zhang |
| Expires: September 15, 2011 | Huawei |
| March 14, 2011 |
Multicast Router Key Management Protocol (MRKMP)
draft-hartman-karp-mrkmp-01.txt
Several routing protocols engage in one-to-many communication. In order to authenticate these communications using symmetric cryptography, a group key needs to be established. This specification defines a group protocol for establishing and managing such keys.
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Many routing protocols such as OSPF and IS-Is use a one-to-many or multicast model of communications. The same message is sent to a number of recipients.
These protocols have cryptographic authentication mechanisms that use a key shared among all members of a communicating group in order to protect messages sent within that group. From a security standpoint, all routers in a group are considered equal. Protecting against a misbehaving router that is part of the group is out of scope for this protocol.
Routers need to be provisioned with some credentials for a one-to-one authentication protocol. Preshared keys or asymmetric keys and an authorization list are expected to be common deployments.
The members of a group elect a Group Controller/Key Server (GCKS). Potentially any member of the group may act as a GCKS. Since protecting against misbehaving routers is out of scope, there is no need to protect against an entity that is not currently the GCKS impersonating the GCKS.
To prove membership in the group, a router authenticates using its provisioned credentials to the current GCKS. If successful, the router is given the current key material for the group. Group size is relatively small and need for forced eviction of members is rare. If a GCKS needs to evict a member, then it can simply re-authenticate with the existing members and provide them new key material.
IKEv2 provides a protocol for authenticating IPsec security associations between two peers. It currently provides no group keying. IKev2 is attractive as a basis for this protocol because while it is much simpler than IKE, it provides all the needed flexibility in one-to-one authentication.
Unlike IKE, IKEv2 is explicitly designed for IPsec. The document does not separate handling of aspects of the protocol that would be needed for IPsec from those that apply to general key management. IPsec specific rules are combined with more general requirements. While concepts and protocol payloads can be used in a different key management protocol, the current structure of IKEv2 does not provide a mechanism for applying IKEv2 to a domain of interpretation other than IPsec. In addition, the complexity required in the IKE specification when compared to IKEv2 suggests that the generality of IKE may not be worth the complexity cost.
So this protocol borrows concepts and payloads from IKEv2 but does not normatively depend on the IKEv2 specification.
The IPsec GDOI provides a protocol that is structurally very similar to this one. As specified, IKE can be used to provide phase 1 authentication to a GCKS. After that, GDOI provides phase 2 messages to establish key-encryption keys and traffic keys. Key management operations can be accomplished via GDOI messages sent to the group after the phase 2 exchange.
GDOI is defined for IKE not for IKEv2. In addition, GDOI's phase 2 uses its own hashing mechanism and nonce mechanism to provide integrity protection and replay protection. Like IKE, GDOI has significant complexity to support phase 2 identities that are different than the phase 1 identity. GDOI requires a GCKS to have a signature key used to sign GDOI messages. Since attacks caused by members of the group masquerading as the GCKS are out of scope, this is significant unnecessary complexity in the protocol.
So, this protocol can be thought of as a simplified GDOI based on IKEv2 rather than IKE. However, integrity and replay mechanisms are taken from IKEv2. Support for phase 2 identities is removed as unneeded complexity. Security for the group key management messages is provided using symmetric primitives rather than asymmetric signatures. Phase 1 authentication will often still involve asymmetric signatures.
MRKMP manipulates several different types of symmetric keys:
When a router wishes to join a group, the router performs the mrkmp_init and mrkmp_auth exchange with a GCKS. During this process the router can establish an association with a specific group. Part of that association will be delivery of a KEK and associated parameters.
Group key management messages are sent to a group address not unicast to an individual peer. The group key management messages are protected using the KEK. The group key management messages need to provide both integrity and confidentiality protection using the KEK.
As part of establishing the association, the router joining the group is given an expiration time for the KEK. A group key management message may establish a new KEK with new parameters.
From time to time, a GCKS may wish to either force early expiration of a KEK or allow a KEK to expire. Protocol master keys are permitted to be valid for somewhat longer than the KEK that created them so as to avoid disrupting routing when this happens. When a KEK is retired or expires without being replaced by a new KEK announced in the old KEK, group members need to perform a new initial exchange to the GCKS. This is useful for example if a router is no longer authorized to be part of the group.
Other mechanisms such as LKH (section 5.4 [RFC2627]) could be used to permit removal of a group member while avoiding new initial authentications. However these mechanisms come at a complexity cost that is not justified for a small number of routers participating in a single multicast link.
Current routing protocols directly use the protocol master key to integrity protect messages. One advantage for this approach is that the initial hello messages used for discovery and capability exchange can be protected using the same mechanism as other messages. Typically a sequence number is used for replay detection. Without changing the key, the existing protocols are vulnerable to a number of serious denial of service attacks from replays.
The MRKMP can solve this replay problem by changing the protocol master key whenever a peer is about to exhaust its sequence number space or whenever a peer loses information about what sequence numbers it used. This could potentially involve changing the protocol master key whenever a router reboots that was part of the group using the current protocol master key. Since key changes will not disrupt active adjacencies and can be accomplished relatively quickly, this is not expected to be a huge problem. Note that after one key change, others routers can boot without causing additional key changes; a flurry of key changes would not be required if several routers reboot near each other.
Another approach would be to separate the protocol master key from the transport keys. For example the transport key used by a given peer could be a fresh key derived from the protocol master key and nonces announced by that peer. Some mechanism would need to make sure that the peer's announcement of its nonce was fresh; this mechanism would almost certainly involve some form of interaction with the router wishing to guarantee freshness. There are two key advantages of this separation between transport keys and protocol master keys. The first is that the interaction between the MRKMP and routing protocol can be simplified significantly. The second is that even when manually configured protocol master keys are used, replay and adequate DOS protection can be achieved.
Before a MRKMP system actually starts working, the routers in the multicast group need to select a GCKS so that they can obtain cryptographic keys to secure subsequent exchanges of routing information. MRKMP specifies an election protocol that dynamically assigns the responsibility of key management to one of the group members. Note that there are already announcer-electing mechanisms provided in some routing protocols (e.g., OSPF and IS-IS). However, much involvement between a MRKMP system and a routing protocol implementation will be introduced if the MRKMP system reuses the announcer-electing mechanism for the election of the GCKS. The state machine of the routing protocol also has to be modified. For instance, in OSPF, after a DR has been elected, routers need to halt their OSPF executions, and carry out the initial exchange to authenticate the DR and collect the keys for subsequent communications. After this step, the routers need to re-start their OSPF state machines so as to exchange routing information. As a consequence of such cases, an individual GCKS electing solution within MRKMP is preferable.
Each router has a GCKS priority. Higher priorities are more preferred GCKSes. As discussed in Section 8, the routing protocol can influence the GCKS election protocol by manipulating the priority so that it is likely that the same router will be the announcer for the routing protocol and the GCKS. Even if two different routers are elected as the announcer and GCKS, then the routing protocol and MRKMP will function correctly.
A key design goal of the election protocol is to maximize the chance that some router permitted to take on the role of GCKS will be elected to that role even when attackers are injecting messages into the election process. The election process can be attacked to cause a router other than the most preferred router to be elected.
The initial exchange is based on IKEv2's IKE_SA_INIT and IKE_SA_AUTH exchanges. During this exchange, an initiating router attempts to authenticate to the router it believes is a GCKS for a group that the initiating router wants to join. Messages are unicast from the initiator to the responding GCKS. Unicast MRKMP P messages form a request/response protocol; the party sending the messages is responsible for retransmissions.
The initial exchange provides capability negotiation, specifically including supported cryptographic suites for the key management protocol. Identification of the initiator and responder is also exchanged. A symmetric key is established to integrity protect and encrypt key management messages. While routing security does not typically require confidentiality, the key management protocol does because keys are exchanged and these must be protected.
Then the identities of each party are cryptographically verified. This can be done using a preshared key or symmetric keys. Other mechanisms may be added as a future extension.
The authentication exchange also provides an opportunity to join a group as part of the initial exchange. In the typical case, a router can obtain the needed key material for a group in two round-trips.
The primary purpose of the unicast MRKMP messages is to get an initiator the information it needs to join a group and participate in a routing protocol. The initiator indicates what group it wants to join. XXX we need to discuss group naming--if MRKMP is limited to a subnet this may be as simple as saying that initiator wants to join the OSPF group or the IS-IS group.
The responder performs several checks. First, the responder confirms that the responder is currently acting as GCKS for the group in question. Then, the responder confirms that the initiator is permitted to join the group. If these checks pass, then the responder provides a key download payload to the initiator encrypted in the peer key management key. As discussed in Section 2.1.2, the GCKS MUST change the protocol master key if a router was part of the group under the current protocol master key and reboots. In this case, the GCKS SHOULD provide the new and old protocol master key to the initiator, setting the validity times for the old key to permit reception but not transmission. The GCKS MUST use the mechanism in the next section to flood the new key to the rest of the group.
A group association created by this exchange may last beyond the unicast MRKMP association used to create it. Once membership in a group is established, resources are not required to maintain the unicast association with the GCKS.
A member of a group can also use the unicast exchange to request a GCKS to change the protocol master key because that group has exhausted its available sequence space. For protocols where the protocol master key is the same as the transport key, it is critical that no two messages be sent by the same router with the same sequence number and protocol master key. The sequence number space is finite. So if a router is running low on available sequence space it needs to request a new protocol master key be generated.
The GCKS shares a KEK with all members of a group. The GCKS can send a multicast message to the group to update the set of protocol master keys, update the KEK, or retire the KEK and request new group join exchanges.
Typically the protocol master key is changed only when needed to provide replay protection or when the KEK changes. The KEK changes whenever a new GCKS is elected or whenever it is administratively desirable to change the keys. For example if an employee leaves an organization it might be desirable to change the KEKs. A KEK is retired whenever forward security is desired: whenever the authorization of who is permitted to be in a group changes and the GCKS needs to make sure that the router is no longer participating. Most authorization changes such as removing a router from service do not require forward security in practical deployments.
The GCKS election process selects a single router to act as GCKS for a group.Similar with other popular announcer electing mechanisms (e.g., VRRP, HSRP), in MRKMP, only GCKSes use multicast to periodically send Advertisement messages. Such advertisements can be used as heart beat packets to indicate the aliveness of GCKSes. In addition, a state machine with six states (Initial, Validate, GCKS, GCKS2, Follower, and Member) is specified for GCKS election. When a router is initially connected to a multicast network, its state is set as Initial. The router then sends a multicast initial advertisement. If a GCKS is working on the network, it will reply to the router with an advertisement. After receiving the advertisement from the GCKS, the router will try to register with the GCKS using the initial exchange. Typically this registration will succeed, and the state of the router is transferred to Member. After a certain period, if the router still does not receive any advertisement from a GCKS or other group members, the router then believes there is no other group member on the network and sets its state as GCKS. If during the period the router does not receive any advertisement from a GCKS but receives advertisements from other more preferred routers on the network, the router believes that the group is involved in a GCKS election process. The router then puts these routers into its candidate list. When the timer to end the Initial state expires, the router tries to authenticate the most preferred router in the candidate list and validate whether it can be a GCKS. If the validation result is possitive, the router then transfer its state to Member, and the router being validated transfers its state to GCKS.
In the absence of attacks, this process functions similar to designated router election protocols in existing routing protocols. Because the election process happens before group keys are established, the initial election process is not integrity-protected. An attacker can inject fake GCKS announcements or initial announcements from fake routers that are more preferred than any router actually in the group. Such attacks can create a denial of service situation. If the election process does not converge within the expected time, or if an authentication attempt fails, then the group is probably under attack. A new state called GCKS2 is introduced. A router permitted to be the GCKS can enter the GCKS2 state after failing to validate a received announcement in the expected time. GCKS2 is used to increase the convergence speed while the system is under attack. If an initial router receives a GCKS2 announcement, the initial router can authenticate and validate the sender, and transfer its own state to Follower, similar to how it would respond to a GCKS announcement. GCKS2 routers attempt to validate each other and to use the resulting security keys to establish a router to act as GCKS. The GCKS2 state does not generate protocol master keys: until the election result in a GCKS only keying material needed for the election is produced. In the subsequent election, the router will wait for the election results from its GCKS2 router until its GCKS2 end timer expires. In this way, the authenticated entities generate a tree structure and avoid generating large amount of keks and protocol master keys when a adversary keeps sending fake GCKS announcements to distrupt election.
Apart from the initialization of a multicast network, the fail-over of a GCKS can also trigger an election process. For instance, if a router does not receive the heart beat advertisement for a certain period, it will transfer its state to Initial and try to elect a new one. In a GCKS electing process, a router has to stay in the Initial state until a new GCKS is allocated. Particularly, the router first sends its initial advertisement with its priority and waits for a certain period. During the period, if a router receives an initial advertisement which consists of a lower priority, the router then sends the advertisement again with a limited rate. After period, if the router does not find any router with a higher priority, it announces itself as the GCKS. If two routers have the same priority, the one with the lowest IP source address used for messages on the link will be the GCKS. After a router transfers its state to GCKS, it will reply to the initial advertisements from other routers with GCKS advertisements, even when the initial advertisements consist of higher priorities than its priority. This approach guarantees that a GCKS will not be changed frequently after it has been elected. After receiving the GCKS advertisement of the new elected GCKS, other routers transfer their states to Member. However, if a GCKS G1 receives a GCKS advertisement from another router G2 and G2 is a more preferred GCKS, G1 follows the procedure in Section 3.2.
If a node in state member fails to perform an initial exchange with the router it believes to be GCKS, it resets its state to initial but ignores advertisements from that router. This way an attacker cannot disrupt communications indefinitely by masquerading as a GCKS.
If a node transitions to GCKS state, it performs the procedure in Section 3.1.
This section is a detailed description of the election process.
In the following discussion, the packets are identified by all upper case characters.
Before going into detailed discussion, several parameters are introduced:
Correspondingly, each router in MRKMP has several timers, Initial_Anno_Timer, Initial_End_Timer, Validate_End_Timer, GCKS_Down_Timer, GCKS2_Down_Timer, GCKS2_End_Timer, GCKS_Anno_Timer, GCKS2_Anno_Timer. Initial_Anno_Timer fires to trigger sending of an INITIAL_ANNOUNCEMENT based on Initial_Announcement_Interval. Initial_End_Timer fires to trigger the transition of a router state from Initial to some other state. Validate_End_Timer fires to trigger the transition of a router state from Validate to GCKS2. GCKS_Down_Timer fires when no GCKS_ANNOUNCEMENT has been heard for GCKS_Down_Interval. GCKS2_Down_Timer fires when no GCKS2_ANNOUNCEMENT has not been heard for GCKS2_Down_Interval. GCKS2_End_Timer fires to trigger the transition of the state of a router from GCKS2 to GCKS. GCKS_Anno_Timer fires to trigger sending of a GCKS_ANNOUNCEMENT based on GCKS_Announcement_Interval. GCKS2_Anno_Timer fires to trigger sending of a GCKS2_ANNOUNCEMENT based on GCKS2_Anno_Interval.
During an election process, a MRKMP router may have to deal with following types of events:
The timers utilized in this state are Initial_Anno_Timer and Initial_End_Timer.
On entry:
Events:
The timer utilized in this state is Validate_End_Timer
Entering this state means that we have a router we believe should be GCKS. The purpose of this state is to confirm that e can establish a security association with that router and that router's policy permits it to be a GCKS for this group. The two normal paths through the state machine are Initial leading to GCKS for the most preferred router and Initial leading to Validate leading to Member for other routers.
On entry:
Events:
The timers utilized in this state include GCKS2_Anno_Timer and GCKS2_End_Timer.
This state is not expected to be used in normal operation. This state indicates there has been some authentication/validation problem or another node is behaving in a manner inconsistent with the election state. The purpose of this state is to establish sufficient security keys to integrity protect the election process. It is possible during normal operation to send a brief time in this state if the router being elected GCKS gets an authentication request before Initial_End_timer expires.
On entry:
Events:
The timer utilized in this state is GCKS_Anno_Timer.
On entry:
Events:
The timer utilized in this state is GCKS_Down_Timer.
On entry:
Events:
The timer utilized in this state is GCKS2_Down_Timer.
On entry:
Events:
Whenever a GCKS finds that a more preferred router is also acting as a GCKS for the same group, then the group is partitioned. Typically if there is already an active GCKS for a group, even if a more preferred GCKS joins, the GCKS will not change. Two situations can result in multiple GCKSes active for a group. The first is that members of the group do not share common authentication credentials. The second is that the group was previously partitioned so that some nodes could not see election messages from other nodes. After the problem resulting in the partition is fixed, then both active GCKSes will see each others election announcements. The group needs to merge.
The less preferred GCKS performs a unicast mrkmp_merge_sa unicast key management message to the more preferred GCKS. In this message the less preferred GCKS includes its key download payload, so the more preferred GCKS learns the protocol master keys of the less preferred GCKS.
The more preferred GCKS generates a new key download payload including a KEK and the union of all the protocol master keys. The GCKS SHOULD mark the existing protocol master keys as expiring for usage in transmitted packets in a relatively short time. The GCKS SHOULD introduce a new protocol master key. This key download payload is returned to the less preferred GCKS and is sent out in the current KEK using a group key management message.
The less preferred GCKS sends the received key download payload encrypted in its existing KEK. XXX how many retransmits. After all retransmissions of this payload the less preferred GCKS sets its state to member.
As a result of this procedure, members learn the protocol master keys of both GCKSes and converge on a single KEK and GCKS. Changing the protocol master keys during a merge is important for protocols that use the protocol master key as a transport key. The new GCKS does not know which routers have joined the group with the other GCKS. Therefore, it could not correctly detect one of these routers rebooting and change the protocol master key at that point. If the key is changed as part of the merge, replays are handled.
When a router attempts to join an election process, it may have a valid kek. For instance, when a GCKS cannot work properly, the routers on the link need to transfer their state to Initial and raise an election to find a new valid GCKS. If there is Still a valid kek shared by the router, they can use the kek to secure the packets transmitted during the election until a new kek is distributed by the new GCKS. A router holding the valid kek is regarded to be more preferred than a router which doesn't have the key. By using the kek, it is able to prevent an attacker from disturbing the election process by broadcasting fake announcements. Therefore, after an initial router does not find any more preferred router holding the valid key, it then can transfer its state to GCKS directly.
Therefore, the operations on receiving a packet are as follows:
It is notable this approach limits the scope of the election within the routers managed by the failed GCKS. If there are routers newly accessing the link during the election, no router with a KEK will process their packets. However these routers can process packets from routers with the KEK. In many cases one of the routers with a KEK will be elected GCKS and the other routers can authenticate and join. In the worst case, two independent GCKSes will be elected and then merge.
What all is actually in the message you get at the end of phase 2 and that is sent out periodically during group key management
For the KEK, this needs to include the key itself, the algorithm (presumably drawn from the IKEv2 symmetric algorithms), key ID, group ID transmit start time, receive start time, and expire time.
The protocol master keys include the key, an algorithm ID, the key ID and thelifetimes.
If a router receives a group join exchange for a group for which it is not the GCKS, it MUST return a notification. If it knows the GCKS for the group then it returns MRKMP_WRONG_GCKS including the address of the GCKS or GCKS2 in the notification payload along with an indication of whether the router is a GCKS or GCKS2. The initiator tries the group join exchange (probably with a new initial exchange) with the indicated router. If the responder does not know the GCKS for the group, either because it is not a member of the group or because its GCKS election state is initial, it returns the MRKMP_GCKS_UNKNOWN notification.
Periodically the GCKS will send out an update message encrypted in the current KEK including the current group key download payload and parameters. If a new KEK is about to be valid for receiving messages, this is included. Any protocol master keys that are valid for sending or receiving SHOULD be included.
If a previous KEK is still valid for sending, then an update message is sent encrypted in the old KEK. This message MUST include the new KEK. This message SHOULD include the protocol master keys.
This section describes signaling between MRKMP and the routing protocol. The primary communication between these protocols is that MRKMP populates rows in the key table making protocol master keys available to the routing protocol. However additional signaling is also required from the routing protocol to MRKMP. This section discusses that signaling. All required communication from MRKMP to the routing protocol can be accomplished by manipulating the key table. However an implementation MAY wish to signal MRKMP failures to the routing protocol in order to provide consistent management feedback.
When a routing protocol instance wishes to begin communicating on a multicast group, it signals a group join event to MRKMP. This event includes the identity of the group as well as this router's priority for being a GCKS for the group. When MRKMP receives this event, it starts MRKMP for this group and attempts to find a GCKS.
It is desirable that the GCKS function track the functions within a routing protocol. For example for protocols such as OSPF that designate a router on a link to manage adjacencies for that link, it would be desirable for the GCKS role to be assigned to that router. The routing protocol provides a priority input to the GCKS election process. Initially the routing protocol should map any priority mechanism within the routing protocol to the GCKS election procedure so that routers favored as announcer for a link will also be favored as a GCKS.
However, the routing protocol SHOULD also dynamically manipulate the GCKS election priority based on what happens within the routing protocol. The router actually elected as the announcer SHOULD have a GCKS election priority higher than any other group member. Typically, by the time the routing protocol is able to elect an announcer, a GCKS will already be chosen. However, if a GCKS election is triggered when the routing protocol is already operational, then the election can choose the routing protocol's announcer.
If a routing protocol terminates on an interface, MRKMP needs to be notified that group is no longer joined. MRKMP MUST stop participating in the GCKS election process, stop monitoring for key management messages and if the current router is a GCKS, stop acting in that role.
An attacker who can suppress packets sent to the group can create a denial of service condition. One attack is to suppress GCKS election packets and cause two routers to believe they are both the GCKS for the group. If the least preferred router never hears the GCKS advertisement from the more preferred router, then the group will remain partitioned. Such an attacker is likely to be able to mount more direct denial of service, for example suppressing the actual routing protocol packets.
The election protocol has been designed to try and resist denial of service conditions. However, the election protocol maintains state in the form of a candidate list and black list. An attacker can consume state by generating fake election announcements. An implementation can discard state if it has insufficient resources. However, if legitimate routers are discarded from the candidate list, the protocol may take longer to converge or may not converge. If entries are removed from the black list, then more resources may be spent on attackers. So the solution has some residual denial of service possibilities. The election protocol requires significant analysis to confirm it meets its design goals.
The security of the election protocol depends on the denial of service resistance of the authentication protocol. It is important that an attacker not be able to cause an authentication to fail by injecting a packet. So, rather than failing an authentication if a bad packet is received, an implementation needs to wait and see if a good packet appears in some timeout.
The security of the system as a whole depends on the pair-wise security between the router currently in the GCKS role and the other routers in the group. Since any router can potentially act as GCKS, the pair-wise security between all members of the group is critical to the security of the system. In practical deployments, information used by the router acting as GCKS to authorize a member joining the group will be configured by some management application. In these deployments, the security of the system depends on the management application correctly maintaining this information on all routers potentially in the group.
The funding for Sam Hartman's work on this document is provided by Huawei.
XXX add the list of people in the lunch time group unless they are willing to be listed as authors.
| [RFC2627] | Wallner, D., Harder, E. and R. Agee, "Key Management for Multicast: Issues and Architectures", RFC 2627, June 1999. |