RFC 4535
GSAKMP: Group Secure Association Key Management Protocol
Pages: 106
Proposed Standard
Proposed Standard
Part 2 of 4 – Pages 15 to 45
4. Architecture
This architecture presents a trust model for GSAKMP and a concept of operations for establishing a trusted distributed infrastructure for group key and policy distribution. GSAKMP conforms to the IETF MSEC architectural concepts as specified in the MSEC Architecture document [RFC3740]. GSAKMP uses the MSEC components to create a trust model for operations that implement the security principles of mutual suspicion and trusted policy creation authorities.4.1. Trust Model
4.1.1. Components
The trust model contains four key components: - Group Owner (GO), - Group Controller Key Server (GC/KS), - Subordinate GC/KS (S-GC/KS), and - Group Member (GM). The goal of the GSAKMP trust model is to derive trust from a common trusted policy creation authority for a group. All security-relevant decisions and actions implemented by GSAKMP are based on information that ultimately is traceable to and verified by the trusted policy creation authority. There are two trusted policy creation authorities for GSAKMP: the GO (policy creation authority) and the PKI root that allows us to verify the GO.
4.1.2. GO
The GO is the policy creation authority for the group. The GO has a well-defined identity that is relevant to the group. That identity can be of a person or of a group-trusted component. All potential entities in the group have to recognize the GO as the individual with authority to specify policy for the group. The policy reflects the protection requirements of the data in a group. Ultimately, the data and the application environment drives the security policy for the group. The GO has to determine the security rules and mechanisms that are appropriate for the data being protected by the group keys. All this information is captured in a policy token (PT). The GO creates the PT and signs it.4.1.3. GC/KS
The GC/KS is authorized to perform several functions: key creation, key distribution, rekey, and group membership management. As the key creation authority, the GC/KS will create the set of keys for the group. These keys include the Group Traffic Protection Keys (GTPKs) and first-tier rekey keys. There may be second-tier rekey trees if a distributed rekey management structure is required for the group. As the key distribution (registration) authority, it has to notify the group of its location for registration services. The GC/KS will have to enforce key access control as part of the key distribution and registration processes. As the group rekey authority, it performs rekey in order to change the group's GTPK. Change of the GTPK limits the exposure of data encrypted with any single GTPK. Finally, as the group membership management authority, the GC/KS can manage the group membership (registration, eviction, de-registration, etc.). This may be done in part by using a key tree approach, such as Logical Key Hierarchies (LKH), as an optional approach.
4.1.4. Subordinate GC/KS
A subordinate GC/KS is used to distribute the GC/KS functionality across multiple entities. The S-GC/KS will have all the authorities of the GC/KS except one: it will not create the GTPK. It is assumed here that the group will transmit data with a single GTPK at any one time. This GTPK comes from the GC/KS. Note that relative to the GC/KS, the S-GC/KS is responsible for an additional security check: the S-GC/KS must register as a member with the GC/KS, and during that process it has to verify the authority of the GC/KS.4.1.5. GM
The GM has two jobs: to make sure all security-relevant actions are authorized and to use the group keys properly. During the registration process, the GM will verify that the PT is signed by a recognized GO. In addition, it will verify that the GC/KS or S-GC/KS engaged in the registration process is authorized, as specified in the PT. If rekey and new PTs are distributed to the group, the GM will verify that they are proper and all actions are authorized. The GM is granted access to group data through receipt of the group keys This carries along with it a responsibility to protect the key from unauthorized disclosure. GSAKMP does not offer any enforcement mechanisms to control which GMs are multicast speakers at a given moment. This policy and its enforcement depend on the multicast application and its protocols. However, GSAKMP does allow a group to have one of three Group Security Association multicast speaker configurations: - There is a single GM authorized to be the group's speaker. There is one multicast application SA allocated by the GO in support of that speaker. The PT initializes this multicast application SA and identifies the GM that has been authorized to be speaker. All GMs share a single TPK with that GM speaker. Sequence number checking for anti-replay protection is feasible and enabled by default. This is the default group configuration. GSAKMP implementations MUST support this configuration. - The GO authorizes all of the GMs to be group speakers. The GO allocates one multicast application SA in support of these speakers. The PT initializes this multicast application SA and indicates that any GM can be a speaker. All of the GMs share a single GTPK and other SA state information. Consequently, some SA security features such as sequence number checking for anti-replay
protection cannot be supported by this configuration. GSAKMP
implementations MUST support this group configuration.
- The GO authorizes a subset of the GMs to be group speakers (which
may be the subset composed of all GMs). The GO allocates a
distinct multicast application SA for each of these speakers. The
PT identifies the authorized speakers and initializes each of
their multicast application Security Associations. The speakers
still share a common TPK across their SA, but each speaker has a
separate SA state information instance at every peer GM.
Consequently, this configuration supports SA security features,
such as sequence number checking for anti-replay protection, or
source authentication mechanisms that require per-speaker state at
the receiver. The drawback of this configuration is that it does
not scale to a large number of speakers. GSAKMP implementations
MAY support this group configuration.
4.1.6. Assumptions
The assumptions for this trust model are that:
- the GCKS is never compromised,
- the GO is never compromised,
- the PKI, subject to certificate validation, is trustworthy,
- The GO is capable of creating a security policy to meet the
demands of the group,
- the compromises of a group member will be detectable and reported
to the GO in a trusted manner,
- the subsequent recovery from a compromise will deny inappropriate
access to protected data to the compromised member,
- no security-relevant actions depend on a precise network time,
- there are confidentiality, integrity, multicast source
authentication, and anti-replay protection mechanisms for all
GSAKMP control messages.
4.2. Rule-Based Security Policy
The trust model for GSAKMP revolves around the definition and
enforcement of the security policy. In fact, the use of the key is
only relevant, in a security sense, if it represents the successful
enforcement of the group security policy.
Group operations lend themselves to rule-based security policy. The need for distribution of data to many endpoints often leads to the defining of those authorized endpoints based on rules. For example, all IETF attendees at a given conference could be defined as a single group. If the security policy rules are to be relevant, they must be coupled with validation mechanisms. The core principle here is that the level of trust one can afford a security policy is exactly equal to the level of trust one has in the validation mechanism used to prove that policy. For example, if all IETF attendees are allowed in, then they could register their identity from their certificate upon check-in to the meetings. That certificate is issued by a trusted policy creation authority (PKI root) that is authorized to identify someone as an IETF attendee. The GO could make admittance rules to the IETF group based on the identity certificates issued from trusted PKIs. In GSAKMP, every security policy rule is coupled with an explicit validation mechanism. For interoperability considerations, GSAKMP requires that its supporting PKI implementations MUST be compliant to RFC 3280. If a GM's public key certificate is revoked, then the entity that issues that revocation SHOULD signal the GO, so that the GO can expel that GM. The method that signals this event to the GO is not standardized by this specification. A direct mapping of rule to validation mechanism allows the use of multiple rules and PKIs to create groups. This allows a GO to define a group security policy that spans multiple PKI domains, each with its own Certificate Authority public key certificate.4.2.1. Access Control
The access control policy for the group keys is equivalent to the access control policy for the multicast application data the keys are protecting. In a group, each data source is responsible for ensuring that the access to the source's data is appropriate. This implies that every data source should have knowledge of the access control policy for the group keys. In the general case, GSAKMP offers a suite of security services to its applications and does not prescribe how they use those services.
GSAKMP supports the creation of GSAs with multiple data sources. It also supports architectures where the GC/KS is not itself a data source. In the multiple data source architectures GSAKMP requires that the access control policy is precisely defined and distributed to each data source. The reference for this data structure is the GSAKMP Policy Token [RFC4534].4.2.2. Authorizations for Security-Relevant Actions
A critical aspect of the GSAKMP trust model is the authorization of security-relevant actions. These include download of group key, rekey, and PT creation and updates. These actions could be used to disrupt the secure group, and all entities in the group must verify that they were instigated by authorized entities within the group.4.3. Distributed Operation
Scalability is a core feature of GSAKMP. GSAKMP's approach to scalable operations is the establishment of S-GC/KSes. This allows the GSAKMP systems to distribute the workload of setting up and managing very large groups. Another aspect of distributed S-GC/KS operations is the enabling of local management authorities. In very large groups, subordinate enclaves may be best suited to provide local management of the enclaves' group membership, due to a direct knowledge of the group members. One of the critical issues involved with distributed operation is the discovery of the security infrastructure location and security suite. Many group applications that have dynamic interactions must "find" each other to operate. The discovery of the security infrastructure is just another piece of information that has to be known by the group in order to operate securely. There are several methods for infrastructure discovery: - Announcements - Anycast - Rendezvous points / Registration One method for distributing the security infrastructure location is to use announcements. The SAP is commonly used to announce the existence of a new multicast application or service. If an
application uses SAP [RFC2974] to announce the existence of a service on a multicast channel, that service could be extended to include the security infrastructure location for a particular group. Announcements can also be used by GSAKMP in one of two modes: expanding ring searches (ERSes) of security infrastructure and ERSes for infrastructure discovery. In either case, the GSAKMP would use a multicast broadcast that would slowly increase in its range by incremental multicast hops. The multicast source controls the packet's multicast range by explicitly setting its Time To Live count. An expanding ring announcement operates by the GC/KS announcing its existence for a particular group. The number of hops this announcement would travel would be a locally configured number. The GMs would listen on a well-known multicast address for GC/KSes that provide service for groups of interest. If multiple GC/KSes are found that provide service, then the GM would pick the closest one (in terms of multicast hops). The GM would then send a GSAKMP Request to Join message (RTJ) to the announced GC/KS. If the announcement is found to be spurious, then that is reported to the appropriate management authorities. The ERA concept is slightly different from SAP in that it could occur over the data channel multicast address, instead of a special multicast address dedicated for the SAP service. An expanding ring search operates in the reverse order of the ERA. In this case, the GM is the announcing entity. The (S-)GC/KSes listen for the requests for service, specifically the RTJ. The (S-)GC/KS responds to the RTJ. If the GM receives more than one response, it would either ignore the responses or send NACKs based on local configuration. Anycast is a service that is very similar to ERS. It also can be used to provide connection to the security infrastructure. In this case, the GM would send the RTJ to a well-known service request address. This anycast service would route the RTJ to an appropriate GC/KS. The anycast service would have security infrastructure and network connectivity knowledge to facilitate this connection. Registration points can be used to distribute many group-relevant data, including security infrastructure. Many group applications rely on well-known registration points to advertise the availability of groups. There is no reason that GSAKMP could not use the same approach for advertising the existence and location of the security infrastructure. This is a simple process if the application being supported already supports registration. The GSAKMP infrastructure can always provide a registration site if the existence of this
security infrastructure discovery hub is needed. The registration of S-GC/KSes at this site could be an efficient way to allow GM registration. GSAKMP infrastructure discovery can use whatever mechanism suits a particular multicast application's requirements, including mechanisms that have not been discussed by this architecture. However, GSAKMP infrastructure discovery is not standardized by this version of the GSAKMP specification.4.4. Concept of Operation
This concept of operation shows how the different roles in GSAKMP interact to set up a secure group. This particular concept of operation focuses on a secure group that utilizes the distributed key dissemination services of the S-GC/KS.4.4.1. Assumptions
The most basic assumption here is that there is one or more trustworthy PKIs for the group. That trusted PKI will be used to create and verify security policy rules. There is a GO that all GMs recognize as having group policy creation authority. All GM must be securely pre-configured to know the GO public key. All GMs have access to the GO PKI information, both the trusted anchor public keys and the certificate path validation rules. There is sufficient connectivity between the GSAKMP entities. - The registration SA requires that GM can connect to the GC/KS or S-GC/KS using either TCP or UDP. - The Rekey SA requires that the data-layer multicast communication service be available. This can be multicast IP, overlay networks using TCP, or NAT tunnels. - GSAKMP can support many different data-layer secure applications, each with unique connectivity requirements.4.4.2. Creation of a Policy Token
The GO creates and signs the policy token for a group. The policy token contains the rules for access control and authorizations for a particular group.
The PT consists of the following information:
- Identification: This allows an unambiguous identification of the
PT and the group.
- Access Control Rules: These rules specify who can have access to
the group keys.
- Authorization Rules: These rules specify who can be a S-GC/KS.
- Mechanisms: These rules specify the security mechanisms that will
be used by the group. This is necessary to ensure there is no
weak link in the group security profile. For example, for IPsec,
this could include SPD/SAD configuration data.
- Source authentication of the PT to the GO: The PT is a CMS signed
object, and this allows all GMs to verify the PT.
4.4.3. Creation of a Group
The PT is sent to a potential GC/KS. This can occur in several ways,
and the method of transmittal is outside the scope of GSAKMP. The
potential GC/KS will verify the GO signature on the PT to ensure that
it comes from a trusted GO. Next, the GC/KS will verify that it is
authorized to become the GC/KS, based on the authorization rules in
the PT. Assuming that the GC/KS trusts the PT, is authorized to be a
GC/KS, and is locally configured to become a GC/KS for a given group
and the GO, then the GC/KS will create the keys necessary to start
the group. The GC/KS will take whatever action is necessary (if any)
to advertise its ability to distribute key for the group. The GC/KS
will then listen for RTJs.
The PT has a sequence number. Every time a PT is distributed to the
group, the group members verify that the sequence number on the PT is
increasing. The PT lifetime is not limited to a particular time
interval, other than by the lifetimes imposed by some of its
attributes (e.g., signature key lifetime). The current PT sequence
number is downloaded to the GM in the "Key Download" message. Also,
to avoid replay attacks, this sequence number is never reset to a
lower value (i.e., rollover to zero) as long as the group identifier
remains valid and in use. The GO MUST preserve this sequence number
across re-boots.
4.4.4. Discovery of GC/KS
Potential GMs will receive notice of the new group via some mechanism: announcement, Anycast, or registration look-up. The GM will send an RTJ to the GC/KS.4.4.5. GC/KS Registration Policy Enforcement
The GC/KS may or may not require cookies, depending on the DoS environment and the local configuration. Once the RTJ has been received, the GC/KS will verify that the GM is allowed to have access to the group keys. The GC/KS will then verify the signature on the RTJ to ensure it was sent by the claimed identity. If the checks succeed, the GC/KS will ready a Key Download message for the GM. If not, the GC/KS can notify the GM of a non- security-relevant problem.4.4.6. GM Registration Policy Enforcement
Upon receipt of the Key Download message, the GM will verify the signature on the message. Then the GM will retrieve the PT from the Key Download message and verify that the GO created and signed the PT. Once the PT is verified as valid, the GM will verify that the GC/KS is authorized to distribute key for this group. Then the GM will verify that the mechanisms used in the group are available and acceptable for protection of the GMs data (assuming the GM is a data source). The GM will then accept membership in this group. The GM will then check to see if it is allowed to be a S-GC/KS for this group. If the GM is allowed to be a S-GC/KS AND the local GM configuration allows the GM to act as a S-GC/KS for this group, then the GM changes its operating state to S-GC/KS. The GO needs to assign the authority to become a S-GC/KS in a manner that supports the overall group integrity and operations.4.4.7. Autonomous Distributed GSAKMP Operations
In autonomous mode, each S-GC/KS operates a largely self-contained sub-group for which the Primary-GC/KS delegates the sub-group's membership management responsibility to the S-GC/KS. In general, the S-GC/KS locally handles each Group Member's registration and de-registration without any interaction with the Primary-GC/KS. Periodically, the Primary-GC/KS multicasts a Rekey Event message addressed only to its one or more S-GC/KS. After a S-GC/KS successfully processes a Rekey Event message from the Primary-GC/KS, the S-GC/KS transmits to its sub-group its own Rekey
Event message containing a copy of the group's new GTPK and policy token. The S-GC/KS encrypts its Rekey Event message's sub-group key management information using Logical Key Hierarchy or a comparable rekey protocol. The S-GC/KS uses the rekey protocol to realize forward and backward secrecy, such that only the authorized sub-group members can decrypt and acquire access to the new GTPK and policy token. The frequency at which the Primary-GC/KS transmits a Rekey Event message is a policy token parameter. For the special case of a S-GC/KS detecting an expelled or compromised group member, a mechanism is defined to trigger an immediate group rekey rather than wait for the group's rekey period to elapse. See below for details. Each S-GC/KS will be registered by the GC/KS as a management node with responsibility for GTPK distribution, access control policy enforcement, LKH tree creation, and distribution of LKH key arrays. The S-GC/KS will be registered into the primary LKH tree as an endpoint. Each S-GC/KS will hold an entire LKH key array for the GC's LKH key tree. For the purpose of clarity, the process of creating a distributed GSAKMP group will be explained in chronological order. First, the Group Owner will create a policy token that authorizes a subset of the group's membership to assume the role of S-GC/KS. The GO needs to ensure that the S-GC/KS rules in the policy token will be stringent enough to ensure trust in the S-GC/KSes. This policy token is handed off to the primary GC. The GC will create the GTPK and initial LKH key tree. The GC will then wait for a potential S-GC/KS to send a Request to Join (RTJ) message. A potential S-GC/KS will eventually send an RTJ. The GC will enforce the access control policy as defined in the policy token. The S-GC/KS will accept the role of S-GC/KS and create its own LKH key tree for its sub-group membership. The S-GC/KS will then offer registration services for the group. There are local management decisions that are optional to control the scope of group members that can be served by a S-GC/KS. These are truly local management issues that allow the administrators of an S-GC/KS to restrict service to potential GMs. These local controls do not affect the overall group security policy, as defined in the policy token.
A potential Group Member will send an RTJ to the S-GC/KS. The S-GC/KS will enforce the entire access control policy as defined in the PT. The GM will receive an LKH key array that corresponds to the LKH tree of the S-GC/KS. The key tree generated by the S-GC/KS is independent of the key tree generated by the GC/KS; they share no common keys. The GM then has the keys it needs to receive group traffic and be subject to rekey from the S-GC/KS. For the sake of this discussion, let's assume the GM is to be expelled from the group membership. The S-GC/KS will receive notification that the GM is to be expelled. This mechanism is outside the scope of this protocol. Upon notification that a GM that holds a key array within its LKH tree is to be expelled, the S-GC/KS does two things. First, the S-GC/KS initiates a de-registration exchange with the GC/KS identifying the member to be expelled. (The S-GC/KS proxies a Group Member's de-registration informing the GC/KS that the Group Member has been expelled from the group.) Second, the S-GC/KS will wait for a rekey action by the GC/KS. The immediacy of the rekey action by the GC/KS is a management decision at the GC/KS. Security is best served by quick expulsion of untrusted members. Upon receipt of the de-registration notification from the S-GC/KS, the GC/KS will register the member to be expelled. The GC/KS will then follow group procedure for initiating a rekey action (outside the scope of this protocol). The GC/KS will communicate to the GO the expelled member's information (outside the scope of this protocol). With this information, the GO will create a new PT for the group with the expelled GM identity added to the excluded list in the group's access control rules. The GO provides this new PT to the GC/KS for distribution with the Rekey Event Message. The GC/KS will send out a rekey operation with a new PT. The S-GC/KS will receive the rekey and process it. At the same time, all other S-GC/KSes will receive the rekey and note the excluded GM identity. All S-GC/KSes will review local identities to ensure that the excluded GM is not a local member. If it is, then the S-GC/KS will create a rekey message. The S-GC/KSes must always create a rekey message, whether or not the expelled Group Member is a member of their subtrees. The S-GC/KS will then create a local rekey message. The S-GC/KS will send the wrapped Group TPK to all members of its local LKH tree, except the excluded member(s).
5. Group Life Cycle
The management of a cryptographic group follows a life cycle: group definition, group establishment, and security-relevant group maintenance. Group definition involves defining the parameters necessary to support a secure group, including its policy token. Group establishment is the process of granting access to new members. Security-relevant group maintenance messages include rekey, policy changes, member deletions, and group destruction. Each of these life cycle phases is discussed in the following sections. The use and processing of the optional Vendor ID payload for all messages can be found in Section 7.10.5.1. Group Definition
A cryptographic group is established to support secure communications among a group of individuals. The activities necessary to create a policy token in support of a cryptographic group include: - Determine Access Policy: identify the entities that are authorized to receive the group key. - Determine Authorization Policy: identify which entities are authorized to perform security-relevant actions, including key dissemination, policy creation, and initiation of security- management actions. - Determine Mechanisms: define the algorithms and protocols used by GSAKMP to secure the group. - Create Group Policy Token: format the policies and mechanisms into a policy token, and apply the GO signature.5.2. Group Establishment
GSAKMP Group Establishment consists of three mandatory-to-implement messages: the Request to Join, the Key Download, and the Key Download Ack/Failure. The exchange may also include two OPTIONAL error messages: the Request to Join Error and the Lack_of_Ack messages. Operation using the mandatory messages only is referred to as "Terse Mode", while inclusion of the error messaging is referred to as "Verbose Mode". GSAKMP implementations MUST support Terse Mode and MAY support Verbose Mode. Group Establishment is discussed in Section 5.2.1.
A group is set in Terse or Verbose Mode by a policy token parameter. All (S-)GC/KSes in a Verbose Mode group MUST support Verbose Mode. GSAKMP allows Verbose Mode groups to have GMs that do not support Verbose Mode. Candidate GMs that do not support Verbose Mode and receive a RTJ-Error or Lack-of-Ack message must handle these messages gracefully. Additionally, a GM will not know ahead of time that it is interacting with the (S-)GC/KS in Verbose or Terse Mode until the policy token is received. For denial of service protection, a Cookie Exchange MAY precede the Group Establishment exchange. The Cookie Exchange is described in Section 5.2.2. Regardless of mode, any error message sent between component members indicates the first error encountered while processing the message.5.2.1. Standard Group Establishment
After the out-of-band receipt of a policy token, a potential Group Controller Key Server (GC/KS) verifies the token and its eligibility to perform GC/KS functionality. It is then permitted to create any needed group keys and begin to establish the group. The GSAKMP Ladder Diagram, Figure 1, illustrates the process of establishing a cryptographic group. The left side of the diagram represents the actions of the GC/KS. The right side of the diagram represents the actions of the GMs. The components of each message shown in the diagram are presented in Sections 5.2.1.1 through 5.2.1.5. CONTROLLER Mandatory/ MESSAGE MEMBER Optional !<-M----------Request to Join-------------! <Process> ! ! <RTJ> ! ! !--M----------Key Download--------------->! ! !<Process KeyDL> !--O-------Request to Join Error--------->! or ! ! <Proc RTJ-Err> !<-M----Key Download - Ack/Failure--------! <Process >! ! <KeyDL-A/F>! ! !--O------Lack of Acknowledgement-------->! ! ! <Proc LOA> !<=======SHARED KEYED GROUP SESSION======>! Figure 1: GSAKMP Ladder Diagram
The Request to Join message is sent from a potential GM to the GC/KS to request admission to the cryptographic group. The message contains key creation material, freshness data, an optional selection of mechanisms, and the signature of the GM. The Key Download message is sent from the GC/KS to the GM in response to an accepted Request to Join. This GC/KS-signed message contains the identifier of the GM, freshness data, key creation material, encrypted keys, and the encrypted policy token. The policy token is used to facilitate well-ordered group creation and MUST include the group's identification, group permissions, group join policy, group controller key server identity, group management information, and digital signature of the GO. This will allow the GM to determine whether group policy is compatible with local policy. The Request to Join Error message is sent from the GC/KS to the GM in response to an unaccepted Request to Join. This message is not signed by the GC/KS for two reasons: 1) the GM, at this point, has no knowledge of who is authorized to act as a GC/KS, and so the signature would thus be meaningless to the GM, and 2) signing responses to denied join requests would provide a denial of service potential. The message contains an indication of the error condition. The possible values for this error condition are: Invalid-Payload-Type, Invalid-Version, Invalid-Group-ID, Invalid- Sequence-ID, Payload-Malformed, Invalid-ID-Information, Invalid- Certificate, Cert-Type-Unsupported, Invalid-Cert-Authority, Authentication-Failed, Certificate-Unavailable, Unauthorized-Request, Prohibited-by-Group-Policy, and Prohibited-by-Locally-Configured- Policy. The Key Download Ack/Failure message indicates Key Download receipt status at the GM. It is a GM-signed message containing freshness data and status. The Lack_of_Ack message is sent from the GC/KS to the GM in response to an invalid or absent Key Download Ack/Failure message. The signed message contains freshness and status data and is used to warn the GM of impending eviction from the group if a valid Key Download Ack/Failure is not sent. Eviction means that the member will be excluded from the group after the next Rekey Event. The policy of when a particular group needs to rekey itself is stated in the policy token. Eviction is discussed further in Section 5.3.2.1. For the following message structure sections, details about payload format and processing can be found in Section 7. Each message is identified by its exchange type in the header of the message. Nonces MUST be present in the messages unless synchronization time is available to the system.
5.2.1.1. Request to Join
The exchange type for Request to Join is eight (8). The components of a Request to Join Message are shown in Table 1. Table 1: Request to Join (RTJ) Message Definition Message Name : Request to Join (RTJ) Dissection : {HDR-GrpID, Key Creation, Nonce_I, [VendorID], : [Notif_Mechanism_Choices], [Notif_Cookie], : [Notif_IPValue]} SigM, [Cert] Payload Types : GSAKMP Header, Key Creation, [Nonce], [Vendor ID], Signature, [Certificate], [Notifications] SigM : Signature of Group Member Cert : Necessary Certificates, zero or more {}SigX : Indicates fields used in Signature [] : Indicate an optional data item As shown by Figure 1, a potential GM MUST generate and send an RTJ message to request permission to join the group. At a minimum, the GM MUST be able to manually configure the destination for the RTJ. As defined in the dissection of the RTJ message, this message MUST contain a Key Creation payload for KEK determination. A Nonce payload MUST be included for freshness and the Nonce_I value MUST be saved for potential later use. The GC/KS will use this supplied nonce only if the policy token for this group defines the use of nonces versus synchronization time. An OPTIONAL Notification payload of type Mechanism Choices MAY be included to identify the mechanisms the GM wants to use. Absence of this payload will cause the GC/KS to select appropriate default policy-token-specified mechanisms for the Key Download. In response, the GC/KS accepts or denies the request based on local configuration. <Process RTJ> indicates the GC/KS actions that will determine if the RTJ will be acted upon. The following checks SHOULD be performed in the order presented. In this procedure, the GC/KS MUST verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, then the identity of the sender is extracted from the Signature payload. This identity MUST be used to perform access control checks and find the GMs credentials (e.g., certificate) for message verification. It MUST also be used in the Key Download message. Then, the GC/KS will verify the signature on the message to ensure its authenticity. The
GC/KS MUST use verified and trusted authentication material from a known root. If the message signature verifies, the GC/KS then confirms that all required payloads are present and properly formatted based upon the mechanisms announced and/or requested. If all checks pass, the GC/KS will create and send the Key Download message as described in Section 5.2.1.2. If the GM receives no response to the RTJ within the GM's locally configured timeout value, the GM SHOULD resend the RTJ message up to three (3) times. NOTE: At any one time, a GC/KS MUST process no more than one (1) valid RTJ message from a given GM per group until its pending registration protocol exchange concludes. If any error occurs during RTJ message processing, and the GC/KS is running in Terse Mode, the registration session MUST be terminated, and all saved state information MUST be cleared. The OPTIONAL Notification payload of type Cookie is discussed in Section 5.2.2. The OPTIONAL Notification payload of type IPValue may be used for the GM to convey a specific IP value to the GC/KS.5.2.1.2. Key Download
The exchange type for Key Download is nine (9). The components of a Key Download Message are shown in Table 2: Table 2: Key Download (KeyDL) Message Definition Message Name : Key Download (KeyDL) Dissection : {HDR-GrpID, Member ID, [Nonce_R, Nonce_C], Key Creation, (Policy Token)*, (Key Download)*, [VendorID]} SigC, [Cert] Payload Types : GSAKMP Header, Identification, [Nonce], Key Creation, Policy Token, Key Download, [Vendor ID], Signature, [Certificate] SigC : Signature of Group Controller Key Server Cert : Necessary Certificates, zero or more {}SigX : Indicates fields used in Signature [] : Indicate an optional data item (data)* : Indicates encrypted information
In response to a properly formed and verified RTJ message, the GC/KS creates and sends the KeyDL message. As defined in the dissection of the message, this message MUST contain payloads to hold the following information: GM identification, Key Creation material, encrypted policy token, encrypted key information, and signature information. If synchronized time is not available, the Nonce payloads MUST be included in the message for freshness. If present, the nonce values transmitted MUST be the GC/KS's generated Nonce_R value and the combined Nonce_C value that was generated by using the GC/KS's Nonce_R value and the Nonce_I value received from the GM in the RTJ. If two-party key determination is used, the key creation material supplied by the GM and/or the GC/KS will be used to generate the key. Generation of this key is dependent on the key exchange, as defined in Section 7.11, "Key Creation Payload". The policy token and key material are encrypted in the generated key. The GM MUST be able to process the Key Download message. <Process KeyDL> indicates the GM actions that will determine how the Key Download message will be acted upon. The following checks SHOULD be performed in the order presented. In this procedure, the GM will verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, the GM MUST confirm that this message was intended for itself by comparing the Member ID in the Identification payload to its identity. After identification confirmation, the freshness values are checked. If using nonces, the GM MUST use its saved Nonce_I value, extract the received GC/KS Nonce_R value, compute the combined Nonce_C value, and compare it to the received Nonce_C value. If not using nonces, the GM MUST check the timestamp in the Signature payload to determine if the message is new. After freshness is confirmed, the signature MUST be verified to ensure its authenticity. The GM MUST use verified and trusted authentication material from a known root. If the message signature verifies, the key creation material is extracted from the Key Creation payload to generate the KEK. This KEK is then used to decrypt the policy token data. The signature on the policy token MUST be verified. Access control checks MUST be performed on both the GO and the GC/KS to determine both their authorities within this group. After all these checks pass, the KEK can then be used to
decrypt and process the key material from the Key Download payload. If all is successful, the GM will create and send the Key Download - Ack/Failure message as described in Section 5.2.1.4. The Policy Token and Key Download Payloads are sent encrypted in the KEK generated by the Key Creation Payload information using the mechanisms defined in the group announcement. This guarantees that the sensitive policy and key data for the group and potential rekey data for this individual cannot be read by anyone but the intended recipient. If any error occurs during KeyDL message processing, regardless of whether the GM is in Terse or Verbose Mode, the registration session MUST be terminated, the GM MUST send a Key Download - Ack/Failure message, and all saved state information MUST be cleared. If in Terse Mode, the Notification Payload will be of type NACK to indicate termination. If in Verbose Mode, the Notification Payload will contain the type of error encountered.5.2.1.3. Request to Join Error
The exchange type for Request to Join Error is eleven (11). The components of the Request to Join Error Message are shown in Table 3: Table 3: Request to Join Error (RTJ-Err) Message Definition Message Name : Request to Join Error (RTJ-Err) Dissection : {HDR-GrpID, [Nonce_I], Notification, [VendorID]} Payload Types : GSAKMP Header, [Nonce] Notification, [Vendor ID] In response to an unacceptable RTJ, the GC/KS MAY send a Request to Join Error (RTJ-Err) message containing an appropriate Notification payload. Note that the RTJ-Err message is not a signed message for the following reasons: the lack of awareness on the GM's perspective of who is a valid GC/KS as well as the need to protect the GC/KS from signing messages and using valuable resources. Following the sending of an RTJ-Err, the GC/KS MUST terminate the session, and all saved state information MUST be cleared. Upon receipt of an RTJ-Err message, the GM will validate the following: the GroupID in the header belongs to a group to which the GM has sent an RTJ, and, if present, the Nonce_I matches a Nonce_I sent in an RTJ to that group. If the above checks are successful, the GM MAY terminate the state associated with that GroupID and
nonce. The GM SHOULD be capable of receiving a valid KeyDownload message for that GroupID and nonce after receiving an RTJ-Err for a locally configured amount of time.5.2.1.4. Key Download - Ack/Failure
The exchange type for Key Download - Ack/Failure is four (4). The components of the Key Download - Ack/Failure Message are shown in Table 4: Table 4: Key Download - Ack/Failure (KeyDL-A/F) Message Definition Message Name : Key Download - Ack/Failure (KeyDL-A/F) Dissection : {HDR-GrpID, [Nonce_C], Notif_Ack, [VendorID]}SigM Payload Types : GSAKMP Header, [Nonce], Notification, [Vendor ID], Signature SigM : Signature of Group Member {}SigX : Indicates fields used in Signature In response to a properly processed KeyDL message, the GM creates and sends the KeyDL-A/F message. As defined in the dissection of the message, this message MUST contain payloads to hold the following information: Notification payload of type Acknowledgement (ACK) and signature information. If synchronized time is not available, the Nonce payload MUST be present for freshness, and the nonce value transmitted MUST be the GM's generated Nonce_C value. If the GM does not receive a KeyDL message within a locally configured amount of time, the GM MAY send a new RTJ. If the GM receives a valid LOA (see Section 5.2.1.5) message from the GC/KS before receipt of a KeyDL message, the GM SHOULD send a KeyDL-A/F message of type NACK followed by a new RTJ. The GC/KS MUST be able to process the KeyDL-A/F message. <Process KeyDL-A/F> indicates the GC/KS actions that will determine how the KeyDL-A/F message will be acted upon. The following checks SHOULD be performed in the order presented. In this procedure, the GC/KS will verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, the GC/KS MUST check the message for freshness. If using nonces, the GC/KS MUST use its saved Nonce_C value and compare it for equality with the received Nonce_C value. If not using nonces, the GC/KS MUST check the timestamp in the Signature payload to determine if the message is new. After freshness is confirmed, the signature MUST be verified to ensure its authenticity. The GC/KS MUST use verified and trusted authentication material from a known root. If the message
signature verifies, the GC/KS processes the Notification payload. If the notification type is of type ACK, then the registration has completed successfully, and both parties SHOULD remove state information associated with this GM's registration. If the GC/KS does not receive a KeyDL-A/F message of proper form or is unable to correctly process the KeyDL-A/F message, the Notification payload type is any value except ACK; or if no KeyDL-A/F message is received within the locally configured timeout, the GC/KS MUST evict this GM from the group in the next policy-defined Rekey Event. The GC/KS MAY send the OPTIONAL Lack_of_Ack message if running in Verbose Mode as defined in Section 5.2.1.5.5.2.1.5. Lack of Ack
The exchange type for Lack of Ack is twelve (12). The components of a Lack of Ack Message are shown in Table 5: Table 5: Lack of Ack (LOA) Message Definition Message Name : Lack of Ack (LOA) Dissection : {HDR-GrpID, Member ID, [Nonce_R, Nonce_C], Notification, [VendorID]} SigC, [Cert] Payload Types : GSAKMP Header, Identification, [Nonce], Notification, [Vendor ID], Signature, [Certificate] SigC : Signature of Group Controller Key Server Cert : Necessary Certificates, zero or more {}SigX : Indicates fields used in Signature [] : Indicate an optional data item If the GC/KS's local timeout value expires prior to receiving a KeyDL-A/F from the GM, the GC/KS MAY create and send a LOA message to the GM. As defined in the dissection of the message, this message MUST contain payloads to hold the following information: GM identification, Notification of error, and signature information. If synchronized time is not available, the Nonce payloads MUST be present for freshness, and the nonce values transmitted MUST be the GC/KS's generated Nonce_R value and the combined Nonce_C value which was generated by using the GC/KS's Nonce_R value and the Nonce_I value received from the GM in the RTJ. These values were already generated during the Key Download message phase.
The GM MAY be able to process the LOA message based upon local configuration. <Process LOA> indicates the GM actions that will determine how the LOA message will be acted upon. The following checks SHOULD be performed in the order presented. In this procedure, the GM MUST verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, the GM MUST confirm that this message was intended for itself by comparing the Member ID in the Identification payload to its identity. After identification confirmation, the freshness values are checked. If using nonces, the GM MUST use its save Nonce_I value, extract the received GC/KS Nonce_R value, compute the combined Nonce_C value, and compare it to the received Nonce_C value. If not using nonces, the GM MUST check the timestamp in the Signature payload to determine if the message is new. After freshness is confirmed, access control checks MUST be performed on the GC/KS to determine its authority within this group. Then signature MUST be verified to ensure its authenticity, The GM MUST use verified and trusted authentication material from a known root. If the checks succeed, the GM SHOULD resend a KeyDL-A/F for that session.5.2.2. Cookies: Group Establishment with Denial of Service Protection
This section defines an OPTIONAL capability that MAY be implemented into GSAKMP when using IP-based groups. The information in this section borrows heavily from [IKEv2] as this protocol has already worked through this issue and GSAKMP is copying this concept. This section will contain paraphrased sections of [IKEv2] modified for GSAKMP to define the purpose of Cookies. An optional Cookie mode is being defined for the GSAKMP to help against DoS attacks. The term "cookies" originates with Karn and Simpson [RFC2522] in Photuris, an early proposal for key management with IPSec. The ISAKMP fixed message header includes two eight-octet fields titled "cookies". Instead of placing this cookie data in the header, in GSAKMP this data is moved into a Notification payload. An expected attack against GSAKMP is state and CPU exhaustion, where the target GC/KS is flooded with Request to Join requests from forged IP addresses. This attack can be made less effective if a GC/KS implementation uses minimal CPU and commits no state to the communication until it knows the initiator potential GM can receive packets at the address from which it claims to be sending them. To
accomplish this, the GC/KS (when operating in Cookie mode) SHOULD reject initial Request to Join messages unless they contain a Notification payload of type "cookie". It SHOULD instead send a Cookie Download message as a response to the RTJ and include a cookie in a notify payload of type Cookie_Required. Potential GMs who receive such responses MUST retry the Request to Join message with the responder-GC/KS-supplied cookie in its notification payload of type Cookie, as defined by the optional Notification payload of the Request to Join Msg in Section 5.2.1.1. This initial exchange will then be as shown in Figure 2 with the components of the new message Cookie Download shown in Table 6. The exchange type for Cookie Download is ten (10). CONTROLLER MESSAGE MEMBER in Cookie Mode !<--Request to Join without Cookie Info---! <Gen Cookie>! ! <Response >! ! !----------Cookie Download--------------->! ! ! <Process CD> !<----Request to Join with Cookie Info----! <Process> ! ! <RTJ > ! ! !-------------Key Download--------------->! ! ! <Proc KeyDL> !<-----Key Download - Ack/Failure--------! <Process >! ! <KeyDL-A/F>! ! !<=======SHARED KEYED GROUP SESSION======>! Figure 2: GSAKMP Ladder Diagram with Cookies Table 6: Cookie Download Message Definition Message Name : Cookie Download Dissection : {HDR-GrpID, Notif_COOKIE_REQUIRED, [VendorID]} Payload Types : GSAKMP Header, Notification, [Vendor ID] The first two messages do not affect any GM or GC/KS state except for communicating the cookie. A GSAKMP implementation SHOULD implement its GC/KS cookie generation in such a way as not to require any saved state to recognize its valid cookie when the second Request to Join message arrives. The exact algorithms and syntax they use to generate cookies does not affect interoperability and hence is not specified here.
The following is an example of how an endpoint could use cookies to implement limited DoS protection. A good way to do this is to set the cookie to be: Cookie = <SecretVersionNumber> | Hash(Ni | IPi | <secret>) where <secret> is a randomly generated secret known only to the responder GC/KS and periodically changed, Ni is the nonce value taken from the initiator potential GM, and IPi is the asserted IP address of the candidate GM. The IP address is either the IP header's source IP address or else the IP address contained in the optional Notification "IPvalue" payload (if it is present). <SecretVersionNumber> should be changed whenever <secret> is regenerated. The cookie can be recomputed when the "Request to Join with Cookie Info" arrives and compared to the cookie in the received message. If it matches, the responder GC/KS knows that all values have been computed since the last change to <secret> and that IPi MUST be the same as the source address it saw the first time. Incorporating Ni into the hash assures that an attacker who sees only the Cookie_Download message cannot successfully forge a "Request to Join with Cookie Info" message. This Ni value MUST be the same Ni value from the original "Request to Join" message for the calculation to be successful. If a new value for <secret> is chosen while connections are in the process of being initialized, a "Request to Join with Cookie Info" might be returned with a <SecretVersionNumber> other than the current one. The responder GC/KS in that case MAY reject the message by sending another response with a new cookie, or it MAY keep the old value of <secret> around for a short time and accept cookies computed from either one. The responder GC/KS SHOULD NOT accept cookies indefinitely after <secret> is changed, since that would defeat part of the denial of service protection. The responder GC/KS SHOULD change the value of <secret> frequently, especially if under attack. An alternative example for Cookie value generation in a NAT environment is to substitute the IPi value with the IPValue received in the Notification payload in the RTJ message. This scenario is indicated by the presence of the Notification payload of type IPValue. With this substitution, a calculation similar to that described above can be used.
5.2.3. Group Establishment for Receive-Only Members
This section describes an OPTIONAL capability that may be implemented in a structured system where the authorized (S-)GC/KS is known in advance through out-of-band means and where synchronized time is available. Unlike Standard Group Establishment, in the Receive-Only system, the GMs and (S-)GC/KSes operate in Terse Mode and exchange one message only: the Key Download. Potential new GMs do not send an RTJ. (S-)GC/KSes do not expect Key Download - ACK/Failure messages and do not remove GMs for lack or receipt of the message. Operation is as follows: upon notification via an authorized out-of- band event, the (S-)GC/KS forms and sends a Key Download message to the new member with the Nonce payloads ABSENT. The GM verifies - the ID payload identifies that GM - the timestamp in the message is fresh - the message is signed by an authorized (S-)GC/KS - the signature on the message verifies When using a Diffie-Hellman Key Creation Type for receive-only members, a static-ephemeral model is assumed: the Key Creation payload in the Key Download message contains the (S-)GC/KS's public component. The member's public component is assumed to be obtained through secure out-of-band means.5.3. Group Maintenance
The Group Maintenance phase includes member joins and leaves, group rekey activities, policy updates, and group destruction. These activities are presented in the following sections.5.3.1. Group Management
5.3.1.1. Rekey Events
A Rekey Event is any action, including a compromise report or key expiration, that requires the creation of a new group key and/or rekey information. Once an event has been identified (as defined in the group security policy token), the GC/KS MUST create and provide a signed message containing the GTPK and rekey information to the group.
Each GM who receives this message MUST verify the signature on the
message to ensure its authenticity. If the message signature does
not verify, the message MUST be discarded. Upon verification, the GM
will find the appropriate rekey download packet and decrypt the
information with a stored rekey key(s). If a new Policy Token is
distributed with the message, it MUST be encrypted in the old GTPK.
The exchange type for Rekey Event is five (5).
The components of a Rekey Event message are shown in Table 7:
Table 7: Rekey Event Message Definition
Message Name : Rekey Event
Dissection : {HDR-GrpID, ([Policy Token])*, Rekey Array,
[VendorID]}SigC, [Cert]
Payload Types : GSAKMP Header, [Policy Token], Rekey Event,
[Vendor ID], Signature, [Certificate],
SigC : Signature of Group Controller Key Server
Cert : Necessary Certificates, zero or more
{}SigX : Indicates fields used in Signature
(data)* : Indicates encrypted information
[] : Indicate an optional data item
5.3.1.2. Policy Updates
New policy tokens are sent via the Rekey Event message. These policy
updates may be coupled with an existing rekey event or may be sent in
a message with the Rekey Event Type of the Rekey Event Payload set to
None(0) (see Section 7.5.1).
A policy token MUST NOT be processed if the processing of the Rekey
Event message carrying it fails. Policy token processing is type
dependent and is beyond the scope of this document.
5.3.1.3. Group Destruction
Group destruction is also accomplished via the Rekey Event message.
In a Rekey Event message for group destruction, the Sequence ID is
set to 0xFFFFFFFF. Upon receipt of this authenticated Rekey Event
message, group components MUST terminate processing of information
associated with the indicated group.
5.3.2. Leaving a Group
There are several conditions under which a member will leave a group: eviction, voluntary departure without notice, and voluntary departure with notice (de-registration). Each of these is discussed in this section.5.3.2.1. Eviction
At some point in the group's lifetime, it may be desirable to evict one or more members from a group. From a key management viewpoint, this involves revoking access to the group's protected data by "disabling" the departing members' keys. This is accomplished with a Rekey Event, which is discussed in more detail in Section 5.3.1.1. If future access to the group is also to be denied, the members MUST be added to a denied access control list, and the policy token's authorization rules MUST be appropriately updated so that they will exclude the expelled GM(s). After receipt of a new PT, GMs SHOULD evaluate the trustworthiness of any recent application data originating from the expelled GM(s).5.3.2.2. Voluntary Departure without Notice
If a member wishes to leave a group for which membership imposes no cost or responsibility to that member, then the member MAY merely delete local copies of group keys and cease group activities.5.3.2.3. De-Registration
If the membership in the group does impose cost or responsibility to the departing member, then the member SHOULD de-register from the group when that member wishes to leave. De-registration consists of a three-message exchange between the GM and the member's GC/KS: the Request_to_Depart, Departure_Response, and the Departure_Ack. Compliant GSAKMP implementations for GMs SHOULD support the de- registration messages. Compliant GSAKMP implementations for GC/KSes MUST support the de-registration messages.5.3.2.3.1. Request to Depart
The Exchange Type for a Request_to_Depart Message is thirteen (13). The components of a Request_to_Depart Message are shown in Table 8. Any GM desiring to initiate the de-registration process MUST generate and send an RTD message to notify the GC/KS of its intent. As defined in the dissection of the RTD message, this message MUST contain payloads to hold the following information: the GC/KS identification and Notification of the desire to leave the group.
When synchronization time is not available to the system as defined
by the Policy Token, a Nonce payload MUST be included for freshness,
and the Nonce_I value MUST be saved for later use. This message MUST
then be signed by the GM.
Table 8: Request_to_Depart (RTD) Message Definition
Message Name : Request_to_Depart (RTD)
Dissection : {HDR-GrpID, GC/KS_ID, [Nonce_I], Notif_Leave_Group,
[VendorID]} SigM, [Cert]
Payload Types : GSAKMP Header, Identification, [Nonce],
Notification, [Vendor ID], Signature,
[Certificate]
SigM : Signature of Group Member
Cert : Necessary Certificates, zero or more
{}SigX : Indicates fields used in Signature
[] : Indicate an optional data item
Upon receipt of the RTD message, the GC/KS MUST verify that the
message header is properly formed and confirm that this message is
for this group by checking the value of the GroupID. If the header
checks pass, then the identifier value in Identification payload is
compared to its own, the GC/KS's identity, to confirm that the GM
intended to converse with this GC/KS, the GC/KS who registered this
member into the group. Then the identity of the sender is extracted
from the Signature payload. This identity MUST be used to confirm
that this GM is a member of the group serviced by this GC/KS. Then
the GC/KS will confirm from the Notification payload that the GM is
requesting to leave the group. Then the GC/KS will verify the
signature on the message to ensure its authenticity. The GC/KS MUST
use verified and trusted authentication material from a known root.
If all checks pass and the message is successfully processed, then
the GC/KS MUST form a Departure_Response message as defined in
Section 5.3.2.3.2.
If the processing of the message fails, the de-registration session
MUST be terminated, and all state associated with this session is
removed. If the GC/KS is operating in Terse Mode, then no error
message is sent to the GM. If the GC/KS is operating in Verbose
Mode, then the GC/KS sends a Departure_Response Message with a
Notification Payload of type Request_to_Depart_Error.
5.3.2.3.2. Departure_Response
The Exchange Type for a Departure_Response Message is fourteen (14). The components of a Departure_Response Message are shown in Table 9. In response to a properly formed and verified RTD message, the GC/KS MUST create and send the DR message. As defined in the dissection of the message, this message MUST contain payloads to hold the following information: GM identification, Notification for acceptance of departure, and signature information. If synchronization time is not available, the Nonce payloads MUST be included in the message for freshness. Table 9: Departure_Response (DR) Message Definition Message Name : Departure_Response (DR) Dissection : {HDR-GrpID, Member_ID, [Nonce_R, Nonce_C], Notification, [VendorID]} SigC, [Cert] Payload Types : GSAKMP Header, Identification, [Nonce], Notification, [Vendor ID], Signature, [Certificate] SigC : Signature of Group Member Cert : Necessary Certificates, zero or more {}SigX : Indicates fields used in Signature [] : Indicate an optional data item If present, the nonce values transmitted MUST be the GC/KS's generated Nonce_R value and the combined Nonce_C value that was generated by using the GC/KS's Nonce_R value and the Nonce_I value received from the GM in the RTD. This Nonce_C value MUST be saved relative to this departing GM's ID. The GM MUST be able to process the Departure_Response message. The following checks SHOULD be performed in the order presented. The GM MUST verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, the GM MUST confirm that this message was intended for itself by comparing the Member ID in the Identification payload to its identity. After identification confirmation, the freshness values are checked. If using nonces, the GM MUST use its saved Nonce_I value, extract the received GC/KS Nonce_R value, compute the combined Nonce_C value, and compare it for equality with the received Nonce_C value. If not using nonces, the GM MUST check the timestamp in the signature payload to determine if the message is new. After freshness is confirmed, confirmation of the identity of the signer of the DR message is the GMs authorized
GC/KS is performed. Then, the signature MUST be verified to ensure its authenticity. The GM MUST use verified and trusted authentication material from a known root. If the message signature verifies, then the GM MUST verify that the Notification is of Type Departure_Accepted or Request_to_Depart_Error. If the processing is successful, and the Notification payload is of type Departure_Accepted, the member MUST form the Departure_ACK message as defined in Section 5.3.2.3.3. If the processing is successful, and the Notification payload is of type Request_to_Depart_Error, the member MUST remove all state associated with the de-registration session. If the member still desires to De-Register from the group, the member MUST restart the de- registration process. If the processing of the message fails, the de-registration session MUST be terminated, and all state associated with this session is removed. If the GM is operating in Terse Mode, then a Departure_Ack Message with Notification Payload of type NACK is sent to the GC/KS. If the GM is operating in Verbose Mode, then the GM sends a Departure_Ack Message with a Notification Payload of the appropriate failure type.5.3.2.3.3. Departure_ACK
The Exchange Type for a Departure_ACK Message is fifteen (15). The components of the Departure_ACK Message are shown in Table 10: Table 10: Departure_ACK (DA) Message Definition Message Name : Departure_ACK (DA) Dissection : {HDR-GrpID, [Nonce_C], Notif_Ack, [VendorID]}SigM Payload Types : GSAKMP Header, [Nonce], Notification, [Vendor ID], Signature SigM : Signature of Group Member {}SigX : Indicates fields used in Signature In response to a properly processed Departure_Response message, the GM MUST create and send the Departure_ACK message. As defined in the dissection of the message, this message MUST contain payloads to hold the following information: Notification payload of type Acknowledgement (ACK) and signature information. If synchronization time is not available, the Nonce payload MUST be present for freshness, and the nonce value transmitted MUST be the GM's generated Nonce_C value.
Upon receipt of the Departure_ACK, the GC/KS MUST perform the following checks. These checks SHOULD be performed in the order presented. In this procedure, the GC/KS MUST verify that the message header is properly formed and confirm that this message is for this group by checking the value of the GroupID. If the header checks pass, the GC/KS MUST check the message for freshness. If using nonces, the GC/KS MUST use its saved Nonce_C value and compare it to the received Nonce_C value. If not using nonces, the GC/KS MUST check the timestamp in the signature payload to determine if the message is new. After freshness is confirmed, the signature MUST be verified to ensure its authenticity. The GC/KS MUST use verified and trusted authentication material from a known root. If the message signature verifies, the GC/KS processes the Notification payload. If the notification type is of type ACK, this is considered a successful processing of this message. If the processing of the message is successful, the GC/KS MUST remove the member from the group. This MAY involve initiating a Rekey Event for the group. If the processing of the message fails or if no Departure_Ack is received, the GC/KS MAY issue a LOA message.
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