| Internet-Draft | G-IKEv2 | November 2024 |
| Smyslov & Weis | Expires 6 May 2025 | [Page] |
This document presents an extension to the Internet Key Exchange version 2 (IKEv2) protocol for the purpose of a group key management. The protocol is in conformance with the Multicast Security (MSEC) key management architecture, which contains two components: member registration and group rekeying. Both components are required for a GCKS (Group Controller/Key Server) to provide authorized Group Members (GMs) with IPsec group security associations. The group members then exchange IP multicast or other group traffic as IPsec packets.¶
This document obsoletes RFC 6407. This documents also updates RFC 7296 by renaming a transform type 5 from "Extended Sequence Numbers (ESN)" to the "Anti-Replay Protection (ARP)" and by renaming IKEv2 authentication method 0 from "Reserved" to "NONE".¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 6 May 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
This document presents an extension to IKEv2 [RFC7296] called G-IKEv2, which allows performing a group key management. A group key management protocol provides IPsec keys and policy to a set of IPsec devices which are authorized to communicate using a Group Security Association (GSA) defined in Multicast Group Security Architecture [RFC3740]. The data communications within the group (e.g., IP multicast packets) are protected by a key pushed to the group members (GMs) by the Group Controller/Key Server (GCKS).¶
G-IKEv2 conforms to the Multicast Group Security Architecture [RFC3740], Multicast Extensions to the Security Architecture for the Internet Protocol [RFC5374] and the Multicast Security (MSEC) Group Key Management Architecture [RFC4046]. G-IKEv2 replaces GDOI [RFC6407], which defines a similar group key management protocol using IKEv1 [RFC2409] (since deprecated by IKEv2). When G-IKEv2 is used, group key management use cases can benefit from the simplicity, increased robustness and cryptographic improvements of IKEv2 (see Appendix A of [RFC7296]).¶
G-IKEv2 is composed of two phases: registration and rekeying. In the registration phase a GM contacts a GCKS to register to a group and to receive the necessary policy and the keying material to be able communicate with the other GMs in the group as well as with the GCKS. The rekeying phase allows the GCKS to periodically renew the keying material for both GM-to-GM communications as well as for communication between the GM and the GCKS.¶
G-IKEv2 defines two ways to perform registration. When a GM first contacts a GCKS it uses the GSA_AUTH exchange (Section 2.3.1) to register to a group. This exchange follows the IKE_SA_INIT exchange (similarly to the IKE_AUTH exchange in IKEv2) and results in establishing an IKE SA between the GM and the GCKS. During this exchange the GCKS authenticates and authorizes the GM, then pushes policy and keys used by the group to the GM. The other one is the GSA_REGISTRATION exchange (Section 2.3.2), which a GM can use within the already established IKE SA with the GCKS (e.g. for registering to another group).¶
Refreshing the group keys can be performed either in an unicast mode via the GSA_INBAND_REKEY exchange (Section 2.4.2) performed over an specific IKE SA between a GM and a GCKS or in an multicast mode with the GSA_REKEY pseudo exchange (Section 2.4.1), when new keys are being distributed to all GMs.¶
Large and small groups may use different sets of these mechanisms. When a large group of devices are communicating, the GCKS is likely to use the GSA_REKEY message for efficiency. This is shown in Figure 1, where multicast communications indicated with double line. (Note: For clarity, IKE_SA_INIT is omitted from Figure 1 and Figure 2).¶
+--------+
+----IKEv2---->| GCKS |<----IKEv2----+
| +--------+ |
| || ^ |
| || | |
| || GSA_AUTH |
| || or |
| || GSA_REGISTRATION |
| || | |
GSA_AUTH || IKEv2 GSA_AUTH
or || | or
GSA_REGISTRATION GSA_REKEY | GSA_REGISTRATION
| || | |
| *==========**================* |
| || || | || |
v \/ \/ v \/ v
+-------+ +--------+ +-------+
| GM | ... | GM | ... | GM |
+-------+ +--------+ +-------+
|| || ||
*=====ESP/AH=====**=====ESP/AH====*
Alternatively, a small group may simply use the GSA_AUTH or GSA_REGISTRATION as registration protocols, where the GCKS issues rekeys using the GSA_INBAND_REKEY within the same IKE SA.¶
GSA_AUTH or GSA_REGISTRATION, GSA_INBAND_REKEY
+--------------------IKEv2----------------------+
| |
| GSA_AUTH or GSA_REGISTRATION, |
| GSA_INBAND_REKEY |
| +-----------IKEv2-------------+ |
| | | |
| |GSA_AUTH or GSA_REGISTRATION,| |
| | GSA_INBAND_REKEY | |
| | +--IKEv2-+ | |
v v v v v v
+---------+ +----+ +----+ +----+
| GCKS/GM | | GM | | GM | | GM |
+---------+ +----+ +----+ +----+
|| || || ||
*==ESP/AH==**=====ESP/AH====**===ESP/AH===*
A combination of these approaches is also possible. For example, the GCKS may use more robust GSA_INBAND_REKEY to provide keys for some GMs (for example, those acting as senders in the group) and GSA_REKEY for the rest. Note also, that GCKS may also be a GM (as shown in Figure 2).¶
IKEv2 message semantics are preserved in that all communications consists of message request-response pairs. The exception to this rule is the GSA_REKEY pseudo-exchange, which is a single message delivering group updates to the GMs.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
It is assumed that readers are familiar with the IPsec architecture [RFC4301], and its extension for multicast [RFC5374]. This document defines an extension to the IKEv2 protocol [RFC7296], so it is assumed that readers have an understanding of this protocol. This document uses a notation and conventions from IKEv2 [RFC7296] for describing G-IKEv2 payloads and exchanges.¶
The following key terms are used throughout this document (mostly borrowed from Multicast Group Security Architecture [RFC3740], Multicast Extensions to the Security Architecture [RFC5374] and GDOI [RFC6407]).¶
G-IKEv2 is an extension to IKEv2 that provides group authorization, secure policy and keys download from the GCKS to GMs.¶
G-IKEv2 is compatible with most IKEv2 extensions defined so far. In particular, it is assumed that, if necessary, the IKE_INTERMEDIATE exchanges [RFC9242] may be utilized while establishing the registration SA. It is also believed that future IKEv2 extensions will be possible to use with G-IKEv2, however, some IKEv2 extensions require special handling when used with G-IKEv2. See Section 6 for more details.¶
It is assumed that readers are familiar with the IKEv2 protocol, so this document skips many details that are described in IKEv2 [RFC7296].¶
As IKEv2 extension, G-IKEv2 SHOULD use the IKEv2 ports (500, 4500). G-IKEv2 MAY also use TCP transport for registration (unicast) IKE SA, as defined in TCP Encapsulation of IKEv2 and IPsec [RFC9329]. G-IKEv2 MAY also use UDP port 848, the same as GDOI [RFC6407], because they serve a similar function. The version number in the IKE header distinguishes the G-IKEv2 protocol from GDOI protocol [RFC6407].¶
Section 2.23 of IKEv2 [RFC7296] describes how IKEv2 supports paths with NATs. G-IKEv2 registration SA doesn't create any unicast IPsec SAs, thus if a NAT is present between the GM and the GCKS, there is no unicast ESP traffic to encapsulate in UDP. However, the actions described in this section regarding the IKE SA MUST be honored. The behavior of GMs and GCKS MUST NOT depend on the port used to create the initial IKE SA. For example, if the GM and the GCKS used UDP port 848 for the IKE_SA_INIT exchange, they will operate the same as if they had used UDP port 500.¶
In the following descriptions, the payloads contained in the G-IKEv2 messages are indicated by names as listed below.¶
| Notation | Payload | Defined in |
|---|---|---|
| AUTH | Authentication | [RFC7296] |
| CERT | Certificate | [RFC7296] |
| CERTREQ | Certificate Request | [RFC7296] |
| D | Delete | [RFC7296] |
| GSA | Group Security Association | Section 4.4 |
| HDR | IKEv2 Header | [RFC7296] |
| IDg | Identification - Group | Section 4.2 |
| IDi | Identification - Initiator | [RFC7296] |
| IDr | Identification - Responder | [RFC7296] |
| KD | Key Download | Section 4.5 |
| KE | Key Exchange | [RFC7296] |
| Ni, Nr | Nonce | [RFC7296] |
| N | Notify | [RFC7296] |
| SA | Security Association | [RFC7296] |
| SAg | Security Association - GM Supported Transforms | Section 4.3 |
| SK | Encrypted | [RFC7296] |
Payloads defined as part of other IKEv2 extensions MAY also be included in these messages. Payloads that may optionally appear in G-IKEv2 messages will be shown in brackets, such as [CERTREQ].¶
G-IKEv2 defines several new payloads not used in IKEv2:¶
IDg (Group ID) -- The GM requests the GCKS for membership into the group by sending its IDg payload.¶
SAg (Security Association -- GM Supported Transforms) -- the GM optionally sends supported transforms, so that GCKS may select a policy appropriate for all members of the group (which is not negotiated, unlike SA parameters in IKEv2).¶
GSA (Group Security Association) -- The GCKS sends the group policy to the GM using this payload.¶
KD (Key Download) -- The GCKS sends the keys and the security parameters to the GMs using this payload.¶
The details of the contents of each payload are described in Section 4.¶
Registration consists of a minimum of two exchanges, IKE_SA_INIT and GSA_AUTH; member registration may have a few more messages exchanged if the EAP method, cookie challenge (for DoS protection), negotiation of key exchange method or IKEv2 extensions based on the IKEv2 Intermediate exchange [RFC9242] are used. Each exchange consists of request/response pairs. The first exchange IKE_SA_INIT is defined in IKEv2 [RFC7296]. This exchange negotiates cryptographic algorithms, exchanges nonces and computes a shared key between the GM and the GCKS. In addition to the cryptographic algorithms negotiated for use in IKEv2 SA, a key wrap algorithm is also negotiated in this exchange. This is achieved by augmenting each proposed Encryption Algorithm transform with a new "Key Wrap Algorithm" attribute. See Section 4.5.4 for details.¶
The second exchange GSA_AUTH is similar to the IKEv2 IKE_AUTH exchange [RFC7296]. It authenticates the previously exchanged messages, exchanges identities and certificates. The GSA_AUTH messages are encrypted and integrity protected with keys established through the previous exchanges, so the identities are hidden from eavesdroppers and all fields in all the messages are authenticated. The GCKS authorizes group members to be allowed into the group as part of the GSA_AUTH exchange. Once the GCKS accepts a GM to join a group it will provide the GM with the data-security keys (TEKs) and/or group key encrypting key (KEK) as part of the GSA_AUTH response message.¶
After the GM and GCKS complete the IKE_SA_INIT exchange, the GSA_AUTH exchange MUST complete before any other exchanges defined in this document can be done. GSA_AUTH is used to authenticate the previous exchanges, exchange identities and certificates. G-IKEv2 also uses this exchange for group member registration and authorization.¶
The GSA_AUTH exchange is similar to the IKE_AUTH exchange with the difference that its goal is to establish multicast Data-Security SA(s) and optionally provide GM with the keys for Rekey SA. The set of payloads in the GSA_AUTH exchange is slightly different, because policy is not negotiated between the group member and the GCKS, but instead provided by the GCKS for the GM. Note also, that GSA_AUTH has its own exchange type, which is different from the IKE_AUTH exchange type.¶
Note, that due to the similarities between IKE_AUTH and GSA_AUTH, most IKEv2 extensions to the IKE_AUTH exchange (like Secure Password authentication [RFC6467]) can also be used with the GSA_AUTH exchange.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
HDR, SK{IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, IDg, [SAg,] [N(SENDER),] [N]} -->
A group member initiates a GSA_AUTH request to join a group indicated by the IDg payload. The GM may include an SAg payload declaring which Transforms it is willing to accept. A GM that intends to act as Group Sender SHOULD include a Notify payload status type of SENDER, which enables the GCKS to provide any additional policy necessary by group senders.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
<-- HDR, SK{IDr, [CERT,]
AUTH, GSA, KD, [N]}
The GCKS responds with IDr, optional CERT, and AUTH payloads with the same meaning as in IKE_AUTH. It also informs the group member of the cryptographic policies of the group in the GSA payload and the key material in the KD payload.¶
In addition to the IKEv2 error handling, the GCKS can reject the registration request when the IDg is invalid or authorization fails, etc. In these cases, see Section 4.7, the GSA_AUTH response will not include the GSA and KD, but will include a Notify payload indicating errors. If a GM included an SAg payload, and the GCKS chooses to evaluate it, and the GCKS detects that the group member cannot support the security policy defined for the group, then the GCKS SHOULD return a NO_PROPOSAL_CHOSEN. Other types of error notifications can be INVALID_GROUP_ID, AUTHORIZATION_FAILED or REGISTRATION_FAILED.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
<-- HDR, SK{IDr, [CERT,] AUTH, N}
If the group member finds the policy sent by the GCKS is unacceptable, the member SHOULD initiate GSA_REGISTRATION exchange sending IDg and the Notify NO_PROPOSAL_CHOSEN (see Section 2.3.2)).¶
Once the IKE SA between the GM and the GCKS is established, the GM can use it for other registration requests, if this is needed. In this scenario the GM will use the GSA_REGISTRATION exchange. Payloads in the exchange are generated and processed as defined in Section 2.3.1.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
HDR, SK{IDg, [SAg,]
[N(SENDER),] [N]} -->
<-- HDR, SK{GSA, KD, [N]}
As with GSA_AUTH exchange, the GCKS can reject the registration request when the IDg is invalid or authorization fails, or GM cannot support the security policy defined for the group (which can be concluded by GCKS by evaluation of SAg payload). In this case the GCKS returns an appropriate error notification as described in Section 2.3.1.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
HDR, SK{IDg, [SAg,]
[N(SENDER),] [N]} -->
<-- HDR, SK{N}
This exchange can also be used if the group member finds the policy sent by the GCKS is unacceptable. The group member SHOULD notify the GCKS by sending IDg and the Notify type NO_PROPOSAL_CHOSEN, as shown below. The GCKS in this case MUST remove the GM from the group IDg.¶
Initiator (GM) Responder (GCKS)
-------------------- ------------------
HDR, SK{IDg, N} -->
<-- HDR, SK{}
A GM requesting registration contacts the GCKS using the IKE_SA_INIT exchange. This exchange is unchanged from IKE_SA_INIT in the IKEv2 protocol. The IKE_SA_INIT exchange may optionally be followed by one or more the IKE_INTERMEDIATE exchanges if the GM and the GCKS negotiated use of IKEv2 extensions based on this exchange.¶
Next the GM sends the GSA_AUTH request message with the IKEv2 payloads from IKE_AUTH (without the SAi2, TSi and TSr payloads) along with the Group ID informing the GCKS of the group the GM wishes to join. An GM intending to emit data traffic SHOULD send a SENDER Notify payload status. The SENDER notification not only signifies that it is a sender, but provides the GM the ability to request Sender-ID values, in case the Data-Security SA supports a counter mode cipher. Section 2.5 includes guidance on requesting Sender-ID values.¶
A GM may be limited in the Transforms IDs that it is able or willing to use, and may find it useful to inform the GCKS which Transform IDs it is willing to accept for different security protocols by including the SAg payload into the request message. Proposals for Rekey SA (with protocol GIKE_REKEY) and for Data-Security (AH [RFC4302] and/or ESP [RFC4303]) SAs may be included into SAg. Each Proposal contains a list of Transforms that the GM is able and willing to support for that protocol. Valid transform types depend on the protocol (AH, ESP, GIKE_REKEY) and are defined in Figure 16. Other transform types SHOULD NOT be included. The SPI length of each Proposal in an SAg is set to zero, and thus the SPI field is empty. The GCKS MUST ignore SPI length and SPI fields in the SAg payload.¶
Generally, a single Proposal for each protocol (GIKE_REKEY, AH/ESP) will suffice, because the transforms are not negotiated, the GM simply alerts the GCKS to restrictions it may have. In particular, the restriction from Section 3.3 of IKEv2 [RFC7296] that AEAD and non-AEAD transforms not be combined in a single proposal doesn't hold when the SAg payload is being formed. However if the GM has restrictions on combination of algorithms, this can be expressed by sending several proposals.¶
Proposal Num field in Proposal substructure is treated specially in SAg payload: it allows a GM to indicate that algorithms used in Rekey SA and in Data-Security (AH and/or ESP) SAs are dependent. In particular, Proposals for different protocols having the same value in Proposal Num field are treated as a set, so that if GCKS uses transforms from one of such Proposal for one protocol, then it MUST only use transforms from one of the Proposals with the same value in Proposal Num field for other protocols. For example, a GM may support algorithms X and Y for both Rekey and Data-Security SAs, but with a restriction that if X is used in Rekey SA, then only X can be used in Data-Security SAs, and the same for Y. Use of the same value in the Proposal Num field of different proposals indicates that the GM expects these proposals to be used in conjunction with each other. In the simplest case when no dependency between transforms exists, all Proposals in SAg payload will have the same value in Proposal Num field.¶
Although the SAg payload is optional, it is RECOMMENDED for the GM to include this payload into the GSA_AUTH request to allow the GCKS to select an appropriate policy.¶
A GM MAY also indicate the support for IPcomp by inclusion one or more the IPCOMP_SUPPORTED notifications along with the SAg payload. The Compression Parameter Index (CPI) in these notifications is set to zero and MUST be ignored by the GCKS.¶
Upon receiving the GSA_AUTH response, the GM parses the response from the GCKS authenticating the exchange using the IKEv2 method, then processes the GSA and KD payloads.¶
The GSA payload contains the security policy and cryptographic protocols used by the group. This policy describes the optional Rekey SA (KEK), Data-Security SAs (TEK), and optional Group-Wise (GW) policy. If the policy in the GSA payload is not acceptable to the GM, it SHOULD notify the GCKS by initiating a GSA_REGISTRATION exchange with a NO_PROPOSAL_CHOSEN Notify payload (see Section 2.3.2). Note, that this should normally not happen if the GM includes SAg payload in the GSA_AUTH request and the GCKS takes it into account. Finally the KD payload is parsed providing the keying material for the TEK and/or KEK. The KD payload contains a list of key bags, where each key bag includes the keying material for SAs distributed in the GSA payload. Keying material is matched by comparing the SPIs in the key bags to SPIs previously included in the GSA payloads. Once TEK keys and policy are matched, the GM provides them to the data-security subsystem, and it is ready to send or receive packets matching the TEK policy.¶
If the group member is not a sender for a received Data-Security SA, then it MUST install this SA only in the inbound direction. If the group member is a sender for a received Data-Security SA, and it is not going to receive back the data it sends, then it MUST install this SA only in the outgoing direction.¶
The GSA KEK policy MUST include the attribute GSA_INITIAL_MESSAGE_ID with a first Message ID the GM should expect to receive if it is non-zero. The value of the attribute MUST be checked by a GM against any previously received Message ID for this group. If it is less than the previously received number, it should be considered stale and MUST be ignored. This could happen if two GSA_AUTH exchanges happened in parallel, and the Message ID changed. This attribute is used by the GM to prevent GSA_REKEY message replay attacks. The first GSA_REKEY message that the GM receives from the GCKS will have a Message ID greater or equal to the Message ID received in the GSA_INITIAL_MESSAGE_ID attribute.¶
Group members MUST install the GIKE_REKEY SA only in the inbound direction.¶
Once a GM successfully registers to the group it MUST replace any information related to this group (policy, keys) that it might have as a result of a previous registration with a new one.¶
Once a GM has received GIKE_REKEY policy during a registration, the IKE SA MAY be closed. By convention, the GCKS closes the IKE SA. The GKCS MAY choose to keep the IKE SA open for inband rekey, especially for small groups. If inband rekey is used, then the initial IKE SA can be rekeyed with the standard IKEv2 mechanism described in Section 1.3.2 of IKEv2 [RFC7296]. If for some reason the IKE SA is closed and no GIKE_REKEY policy is received during the registration process, the GM MUST consider itself excluded from the group. To continue participating in the group, the GM needs to re-register.¶
A G-IKEv2 GCKS passively listens for incoming requests from group members. When the GCKS receives an IKE_SA_INIT request, it selects an IKE proposal and generates a nonce and DH to include them in the IKE_SA_INIT response.¶
Upon receiving the GSA_AUTH request, the GCKS authenticates the group member via the GSA_AUTH exchange. The GCKS then authorizes the group member according to group policy before preparing to send the GSA_AUTH response. If the GCKS fails to authorize the GM, it SHOULD respond with an AUTHORIZATION_FAILED notify message. The GCKS SHOULD also respond with an INVALID_GROUP_ID notify message if the requested group is unknown to the GCKS or with an REGISTRATION_FAILED notify message if there is a problem with the requested group (for example the capacity of the group is exceeded).¶
The GSA_AUTH response will include the group policy in the GSA payload and keys in the KD payload. If the GCKS policy includes a group rekey option, it MUST include the GSA_INITIAL_MESSAGE_ID attribute, specifying the starting Message ID the GCKS will use when sending the GSA_REKEY message to the group members if this Message ID is non-zero. This Message ID is used to prevent GSA_REKEY message replay attacks and will be increased each time a GSA_REKEY message is sent to the group. The GCKS data traffic policy is included in the GSA TEK and keys are included in the KD TEK. The GW policy MAY also be included to provide the ATD and/or DTD (Section 4.4.3.1.1) specifying activation and deactivation delays for SAs generated from the TEKs. If the group member has indicated that it is a sender of data traffic and one or more Data Security SAs distributed in the GSA payload included a counter mode of operation, the GCKS responds with one or more Sender-ID values (see Section 2.5).¶
Multicast Extensions to the Security Architecture [RFC5374] defines two modes of operation for multicast Data-Security SAs: transport mode and tunnel mode with address preservation. In the latter case outer source and destination addresses are taken from the inner IP packet. The mode of operation for the Data-Security SAs is determined by the presence of the USE_TRANSPORT_MODE notification in the GCKS's response message of the registration exchange: if it is present, then SAs are created in transport mode; otherwise - in tunnel mode. If multiple Data-Security SAs are being created in a single registration exchange, then all of them will have the same mode of operation.¶
If the GCKS receives a GSA_REGISTRATION exchange with a request to register a GM to a group, the GCKS will need to authorize the GM with the new group (IDg) and respond with the corresponding group policy and keys. If the GCKS fails to authorize the GM, it will respond with the AUTHORIZATION_FAILED notification. The GCKS may also respond with an INVALID_GROUP_ID or REGISTRATION_FAILED notify messages for the reasons described above.¶
If a group member includes an SAg in its GSA_AUTH or GSA_REGISTRATION request, the GCKS may evaluate it according to an implementation specific policy.¶
The GCKS could evaluate the list of Transforms and compare it to its current policy for the group. If the group member did not include all of the ESP, AH or GIKE_REKEY Transforms that match the current group policy or the capabilities of all other currently active GMs, then the GCKS SHOULD return a NO_PROPOSAL_CHOSEN Notification.¶
The GCKS could store the list of Transforms, with the goal of migrating the group policy to a different Transforms when all of the group members indicate that they can support that Transforms.¶
The GCKS could store the list of Transforms and adjust the current group policy based on the capabilities of the devices as long as they fall within the acceptable security policy of the GCKS.¶
Depending on its policy, the GCKS may have no further need for the IKE SA (e.g., it does not plan to initiate an GSA_INBAND_REKEY exchange). If the GM does not initiate another registration exchange or Notify (e.g., NO_PROPOSAL_CHOSEN), and also does not close the IKE SA and the GCKS is not intended to use the SA, then after a short period of time the GCKS SHOULD close the IKE SA.¶
The GCKS is responsible for rekeying the secure group per the group policy. Rekeying is an operation whereby the GCKS provides replacement TEKs and KEK, deleting TEKs, and/or excluding group members. The GCKS may initiate a rekey message if group membership and/or policy has changed, or if the keys are about to expire. Two forms of group maintenance channels are provided in G-IKEv2 to push new policy to group members.¶
Depending on its policy the GCKS MAY combine these two methods. For example, it may use the GSA_INBAND_REKEY to deliver key to the GMs in the group acting as senders (as this would provide reliable keys delivery), and the GSA_REKEY for the rest GMs.¶
The GCKS initiates the G-IKEv2 Rekey by sending a protected message to the GMs, usually using IP multicast. Since the Rekey messages do not require responses and they are sent to multiple GMs, the windowing mechanism described in Section 2.3 of IKEv2 [RFC7296] MUST NOT be used for the Rekey messages. The GCKS rekey message replaces the current rekey GSA KEK or KEK array (e.g. in case of LKH), and/or creates new Data-Security GSA TEKs. The GM_SENDER_ID attribute in the Key Download payload (defined in Section 4.5.3.3) MUST NOT be part of the Rekey Exchange as this is sender specific information and the Rekey Exchange is group specific. The GCKS initiates the GSA_REKEY pseudo-exchange as following:¶
GMs (Receivers) GCKS (Sender)
----------------- ---------------
<-- HDR, SK{GSA, KD, [N] [AUTH]}
HDR is defined in Section 4.1. The Message ID in this message will start with the value the GCKS sent to the group members in the attribute GSA_INITIAL_MESSAGE_ID or from zero if this attribute wasn't sent. The Message ID will be incremented each time a new GSA_REKEY message is sent to the group members.¶
The GSA payload contains the current policy for rekey and Data-Security SAs. The GSA may contain a new Rekey SA and/or a new Data-Security SAs Section 4.4.¶
The KD payload contains the keys for the policy included in the GSA. If one or more Data-Security SAs are being refreshed in this rekey message, the IPsec keys are updated in the KD, and/or if the rekey SA is being refreshed in this rekey message, the rekey Key or the LKH KEK array (e.g. in case of LKH) is updated in the KD payload.¶
A Delete payload MAY be included to instruct the GM to delete existing SAs. See Section 4.6 for more detail.¶
The AUTH payload MUST be included to authenticate the GSA_REKEY message if the authentication method is based on public key signatures and MUST NOT be included if authentication is implicit. In the latter case, the fact that a GM can decrypt the GSA_REKEY message and verify its ICV proves that the sender of this message knows the current KEK, thus authenticating the sender as a member of the group. Note, that implicit authentication doesn't provide source origin authentication. For this reason using implicit authentication for GSA_REKEY is NOT RECOMMENDED unless source origin authentication is not required (for example, in a small group of highly trusted GMs). See more about authentication methods in Section 4.4.2.1.1.¶
During group member registration, the GCKS sends the authentication key in the KD payload, AUTH_KEY attribute, which the group member uses to authenticate the key server. Before the current authentication key expires, the GCKS will send a new AUTH_KEY to the group members in a GSA_REKEY message. The authentication key that is sent in the rekey message may be not the same as the authentication key sent during the GM registration. If implicit authentication is used, then AUTH_KEY MUST NOT be sent to GMs.¶
The content of the AUTH payload generally depends on the authentication method from the Authentication Method transform (Section 4.4.2.1.1). This specification defines the use of only one authentication method - Digital Signature, and the AUTH payload contains digital signature calculated over the content of the not yet encrypted GSA_REKEY message.¶
The digital signing is applied to the concatenation of two chunks: A and P. The chunk A starts with the first octet of the G-IKEv2 header (not including prepended four octets of zeros, if port 4500 is used) and continues to the last octet of the Encrypted Payload header. The chunk P consists of the not yet encrypted content of the Encrypted payload, excluding the Initialization Vector, the Padding, the Pad Length and the Integrity Checksum Data fields (see 3.14 of IKEv2 [RFC7296] for description of the Encrypted payload). In other words, the P chunk is the inner payloads of the Encrypted payload in plaintext form. Figure 10 illustrates the layout of the P and A chunks in the GSA_REKEY message.¶
Before the calculation of the AUTH payload the inner payloads of Encrypted payload must be fully formed and ready for encryption, except for the content of the AUTH payload. The AUTH payload must have correct values in the Payload Header, the Auth Method and the RESERVED fields. The Authentication Data field is zeroed, but the ASN.1 Length and the AlgorithmIdentifier fields must be properly filled in, see Signature Authentication in IKEv2 [RFC7427].¶
For the purpose of the AUTH payload calculation the Length field in the IKE header and the Payload Length field in the Encrypted Payload header are adjusted so that they don't count the lengths of Initialization Vector, Integrity Checksum Data and Padding (along with Pad Length field). In other words, the Length field in the IKE header (denoted as AdjustedLen in Figure 10) is set to the sum of the lengths of A and P, and the Payload Length field in the Encrypted Payload header (denoted as AdjustedPldLen in Figure 10) is set to the length of P plus the size of the Payload header (four octets).¶
The input to the digital signature algorithm that computes the content of the AUTH payload can be described as:¶
DataToAuthenticate = A | P GsaRekeyMessage = GenIKEHDR | EncPayload GenIKEHDR = [ four octets 0 if using port 4500 ] | AdjustedIKEHDR AdjustedIKEHDR = SPIi | SPIr | . . . | AdjustedLen EncPayload = AdjustedEncPldHdr | IV | InnerPlds | Pad | PadLen | ICV AdjustedEncPldHdr = NextPld | C | RESERVED | AdjustedPldLen A = AdjustedIKEHDR | AdjustedEncPldHdr P = InnerPlds¶
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^
| G-IKEv2 SA Initiator's SPI | | |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I |
| G-IKEv2 SA Responder's SPI | K |
| | E |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Next Payload | MjVer | MnVer | Exchange Type | Flags | H A
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ d |
| Message ID | r |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| AdjustedLen | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ x |
| Next Payload |C| RESERVED | AdjustedPldLen | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | v
| | |
~ Initialization Vector ~ E
| | n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ c ^
| | r |
~ Inner payloads (not yet encrypted) ~ P
| | P |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ l v
~ Padding (0-255 octets) | Pad Length | d
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| | |
~ Integrity Checksum Data ~ |
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ v
The authentication data is calculated using the authentication algorithm from the Authentication Method transform (Section 4.4.2.1.1) and the current authentication key provided in the AUTH_KEY attribute (Section 4.5.3.2). The calculated authentication data is placed into the AUTH payload, the Length fields in the IKE Header and the Encryption Payload header are restored, the content of the Encrypted payload is encrypted and the ICV is computed using the current KEK keys.¶
IKEv2 fragmentation [RFC7383] can be used to perform fragmentation of large GSA_REKEY messages; however, when the GSA_REKEY message is emitted as an IP multicast packet there is a lack of response from the GMs. This has the following implications.¶
Policy regarding the use of IKE fragmentation is implicit. If a GCKS detects that all GMs have negotiated support of IKE fragmentation in IKE_SA_INIT, then it MAY use IKE fragmentation on large GSA_REKEY messages.¶
The GCKS must always use IKE fragmentation based on a pre-configured fragmentation threshold, as there is no way to check if fragmentation is needed by first sending unfragmented messages and waiting for response. Section 2.5.1 of IKEv2 Fragmentation [RFC7383] contains recommendation on selecting the fragmentation threshold.¶
PMTU mechanism, defined in Section 2.5.2 of IKEv2 Fragmentation [RFC7383], cannot be used due to lack of GSA_REKEY response messages.¶
The calculation of authentication data MUST be applied to whole messages only, before possible IKE Fragmentation. If the message was received in fragmented form, it should be reconstructed before verifying its authenticity as if it were received unfragmented. The RESERVED field in the reconstructed Encrypted Payload header MUST be set to the value of the RESERVED field in the Encrypted Fragment payload header from the first fragment (that with Fragment Number equal to 1).¶
The GCKS builds the rekey message with a Message ID value that is one greater than the value included in the previous rekey message. The first message sent over a new Rekey SA MUST use Message ID of 0. The GSA, KD, N and D payloads follow with the same characteristics as in the GSA Registration exchange. The AUTH payload (if present) is created as defined in Section 2.4.1.1.¶
Because GSA_REKEY messages are not acknowledged and could be discarded by the network, one or more GMs may not receive the new policy. To mitigate such lost messages, during a rekey event the GCKS may transmit several copies of an encrypted GSA_REKEY message with the new policy. The (encrypted) retransmitted messages MUST be bitwise identical and SHOULD be sent within a short time interval (a few seconds) to ensure that time-to-live would not be substantially skewed for the GMs that would receive different copies of the messages.¶
GCKS may also include one or several GSA_NEXT_SPI attributes specifying SPIs for the prospected rekeys, so that listening GMs are able to detect lost rekey messages and recover from this situation. See Sections Section 4.4.2.2.3 for more detail.¶
When a group member receives the Rekey message from the GCKS it decrypts the message and verifies its integrity using the current KEK. If the AUTH payload is present in the decrypted message, then the GM validates authenticity of the message using the key retrieved in a previous G-IKEv2 exchange. Then the GM verifies the Message ID, and processes the GSA and KD payloads. The group member then downloads the new Data-Security SA(s) and/or new Rekey SA. The parsing of the payloads is identical to the parsing done in the registration exchange.¶
Anti-replay protection is achieved by a group member rejecting a GSA_REKEY message which has a Message ID smaller than the current Message ID that the GM is expecting. The GM expects the Message ID in the first GSA_REKEY message it receives to be equal or greater than the Message ID it receives in the GSA_INITIAL_MESSAGE_ID attribute. Note, that if the GSA_INITIAL_MESSAGE_ID attribute is not received for the Rekey SA, the GM MUST assume zero as the first expected Message ID. The GM expects the Message ID in subsequent GSA_REKEY messages to be greater than the last valid GSA_REKEY message ID it received.¶
GSA_REKEY messages are sent infrequently (typically one per several hours or, in extreme cases, several minutes), which is much greater than typical network packet reordering intervals.¶
If the GSA payload includes a Data-Security SA using cipher in a counter-modes of operation and the receiving group member is a sender for that SA, the group member uses its current Sender-ID value with the Data-Security SAs to create counter-mode nonces. If it is a sender and does not hold a current Sender-ID value, it MUST NOT install the Data-Security SAs. It MUST initiate a re-registration to the GCKS in order to obtain an Sender-ID value (along with the current group policy).¶
Once a new Rekey SA is installed as a result of GSA_REKEY message, the current Rekey SA (over which the message was received) MUST be silently deleted after waiting DEACTIVATION_TIME_DELAY interval regardless of its expiration time. If the message includes Delete payload for existing Data-Security SA, then after installing a new Data-Security SA the old one, identified by the Protocol and SPI fields in the Delete payload, MUST be silently deleted after waiting DEACTIVATION_TIME_DELAY interval regardless of its expiration time.¶
If a Data-Security SA is not rekeyed yet and is about to expire (a "soft lifetime" expiration is described in Section 4.4.2.1 of [RFC4301]), the GM SHOULD initiate a registration to the GCKS. This registration serves as a request for current SAs, and will result in the download of replacement SAs, assuming the GCKS policy has created them. A GM SHOULD also initiate a registration request if a Rekey SA is about to expire and not yet replaced with a new one.¶
When the IKE SA protecting the member registration exchange is maintained while group member participates in the group, the GCKS can use the GSA_INBAND_REKEY exchange to individually provide policy updates to the group member.¶
GM (Responder) GCKS (Initiator)
---------------- ------------------
<-- HDR, SK{GSA, KD, [N]}
HDR, SK{} -->
Because this is a normal IKEv2 exchange, the HDR is treated as defined in IKEv2 [RFC7296].¶
The GSA, KD, N and D payloads are built in the same manner as in a registration exchange.¶
The GM processes the GSA, KD, N and D payloads in the same manner as if they were received in a registration exchange.¶
There are occasions when the GCKS may want to signal to group members to delete policy when the application sending data traffic has ended, or if group policy has changed. Deletion of SAs is accomplished by sending the Delete Payload described in Section 3.11 of IKEv2 [RFC7296] as part of the GSA_REKEY pseudo-exchange as shown below.¶
GMs (Receivers) GCKS (Sender)
---------------- ---------------
<-- HDR, SK{D, [N,] [AUTH]}
If GCKS has a unicast SA with group member then it can use the GSA_INBAND_REKEY exchange to delete SAs.¶
GM (Responder) GCKS (Initiator)
--------------- ------------------
<-- HDR, SK{D, [N,]}
HDR, SK{} -->
There may be circumstances where the GCKS may want to start over with a clean state, for example in case it runs out of available Sender-IDs. The GCKS can signal deletion of all the Data-Security SAs by sending a Delete payload with an SPI value equal to zero. For example, if the GCKS wishes to remove the Rekey SA and all the Data-Security SAs, the GCKS sends a Delete payload with an SPI of zero and Protocol ID of AH or ESP, followed by another Delete payload with a SPI of zero and Protocol ID of GIKE_REKEY.¶
If a group member receives a Delete payload with zero SPI and protocol ID of GIKE_REKEY either via multicast Rekey SA or via unicast SA using the GSA_INBAND_REKEY exchange, it means that the group member is excluded from the group. The group member MUST re-register if it wants to continue participating in this group. The registration is performed as described in Section 2.3. Note, that if the GSA_INBAND_REKEY exchange is used to exclude a group member from the group, and thus the unicast SA between the group member and the GCKS exists, then this SA persists after this exchange and the group member may use the GSA_REGISTRATION exchange to re-register. However, after excluding an GM from the group the GCKS MAY immediately delete the unicast SA with this GM if its credentials are revoked.¶
Several counter-based modes of operation have been specified for ESP (e.g., AES-CTR [RFC3686], AES-GCM [RFC4106], AES-CCM [RFC4309], ChaCha20-Poly1305 [RFC7634], AES-GMAC [RFC4543]) and AH (e.g., AES-GMAC [RFC4543]). These counter-based modes require that no two senders in the group ever send a packet with the same Initialization Vector (IV) using the same cipher key and mode. This requirement is met in G-IKEv2 when the following measures are taken:¶
The GCKS distributes a unique key for each Data-Security SA.¶
The GCKS uses the method described in Using Counter Modes with ESP and AH to Protect Group Traffic [RFC6054], which assigns each sender a portion of the IV space by provisioning each sender with one or more unique Sender-ID values.¶
When at least one Data-Security SA included in the group policy includes a counter-based mode of operation, the GCKS automatically allocates and distributes one Sender-ID to each group member acting in the role of sender on the Data-Security SA. The Sender-ID value is used exclusively by the group sender to which it was allocated. The group sender uses the same Sender-ID for each Data-Security SA specifying the use of a counter-based mode of operation. A GCKS MUST distribute unique keys for each Data-Security SA including a counter-based mode of operation in order to maintain unique key and nonce usage.¶
During registration, the group sender can choose to request one or more Sender-ID values. Requesting a value of 1 is not necessary since the GCKS will automatically allocate exactly one to the group sender. A group sender MUST request as many Sender-ID values matching the number of encryption modules in which it will be installing the TEKs in the outbound direction. Alternatively, a group sender MAY request more than one Sender-ID and use them serially. This could be useful when it is anticipated that the group sender will exhaust their range of Data- Security SA nonces using a single Sender-ID too quickly (e.g., before the time-based policy in the TEK expires).¶
When the group policy includes a counter-based mode of operation, a GCKS SHOULD use the following method to allocate Sender-ID values, which ensures that each Sender-ID will be allocated to just one group sender.¶
A GCKS maintains an Sender-ID counter, which records the Sender-IDs that have been allocated. Sender-IDs are allocated sequentially, with zero as the first allocated value.¶
Each time an Sender-ID is allocated, the current value of the counter is saved and allocated to the group sender. The Sender-ID counter is then incremented in preparation for the next allocation.¶
When the GCKS specifies a counter-based mode of operation in the Data-Security SA a group sender may request a count of Sender-IDs during registration in a Notify payload information of type SENDER. When the GCKS receives this request, it increments the Sender-ID counter once for each requested Sender-ID, and distributes each Sender-ID value to the group sender. The GCKS SHOULD have a policy-defined upper bound for the number of Sender-ID values that it will return irrespective of the number requested by the GM.¶
A GCKS allocates new Sender-ID values for each registration operation by a group sender, regardless of whether the group sender had previously contacted the GCKS. In this way, the GCKS is not required to maintaining a record of which Sender-ID values it had previously allocated to each group sender. More importantly, since the GCKS cannot reliably detect whether the group sender had sent data on the current group Data-Security SAs it does not know what Data-Security counter-mode nonce values that a group sender has used. By distributing new Sender-ID values, the key server ensures that each time a conforming group sender installs a Data-Security SA it will use a unique set of counter-based mode nonces.¶
When the Sender-ID counter maintained by the GCKS reaches its final Sender-ID value, no more Sender-ID values can be distributed. Before distributing any new Sender-ID values, the GCKS MUST exclude all group members from the group as described in Section 2.4.3. This will result in the group members performing re-registration, during which they will receive new Data-Security SAs and group senders will additionally receive new Sender-ID values. The new Sender-ID values can safely be used because they are only used with the new Data-Security SAs.¶
A GM applies the Sender-ID to Data-Security SAs as follows.¶
The most significant bits of the IV indicated in the GWP_SENDER_ID_BITS attribute (Section 4.4.3.1.2) are taken to be the Sender-ID field of the IV.¶
The Sender-ID is placed in the least significant bits of the Sender-ID field, where any unused most significant bits are set to zero. If the Sender-ID value doesn't fit into the number of bits from the GWP_SENDER_ID_BITS attributes, then the GM MUST treat this as a fatal error and re-register to the group.¶
IPsec provides anti-replay protection as part of its security services. With multicast extension for IPsec anti-replay protection is not always possible to achieve (see Section 6.1 of Multicast Group Security Architecture [RFC3740]). In particular, if there are many group senders for a Data-Security SA, then each of them will independently increment the Sequence Number field in the ESP header (see Section 2.2 of ESP [RFC4303] and Section 2.5 of AH [RFC4302]) thus making it impossible for the group receivers to filter out replayed packets. However, if there is only one group sender for a Data-Security SA, then it is possible to achieve anti-replay protection with some restrictions (see Section 4.4.2.1.3). The GCKS MAY create several Data-Security SAs with the same traffic selectors allowing only a single group sender in each SA if it is desirable to get anti-replay protection with multiple (but still limited number) of group senders.¶
IPsec architecture assumes that it is a local matter for an IPsec receiver whether anti-replay protection is active or not. In other words, an IPsec sender always increments the Sequence Number field in the ESP/AH header and a receiver decides whether to check for replayed packets or not. With multicast extension for IPsec this approach doesn't work in case when multiple group senders exist for a Data-Security SA. For this reason it is desirable that the GCKS informs the GMs whether there are multiple group senders in the group or not.¶
For this purpose this specification re-uses the "Extended Sequence Numbers" transform, defined in Section 3.3.2 of IKEv2 [RFC7296]. This specification renames this transform to "Anti-Replay Protection" and adds a new value for possible Transform IDs: "Not Used" (<TBA by IANA>), which means that anti-replay protection is not applicable for the SA. The decision whether to enable anti-replay protection is still a local matter of a GM (in accordance with IPsec architecture), but the GCKS should inform the GMs when this protection is not feasable (e.g. for a multi-sender SA).¶
The GCKS SHOULD include this transform in the GSA payload for every Data-Security SA. For a single-sender Data-Security SA the GCKS MAY omit this transform. See Section 4.4.2.1.3 for more details.¶
When a Data-Security SA has a single sender, the GCKS MUST be configured to rekey the SA frequently enough so that the Sequence Number (SN) does not wrap.¶
IKEv2 IANA registry for Encryption Algorithm Transform IDs [IKEV2-IANA] defines several transforms with implicit IV. These transforms rely on ESP Sequence Number for constructing IV (see Implicit IV for Counter-Based Ciphers in ESP [RFC8750] for details). It requires anti-replay protection to be enabled for an ESP SA using these encryption transforms. Unless anti-replay protection is active for a multicast ESP SA (see Section 2.6, encryption transforms that rely on Sequence Number for IV construction MUST NOT be used. In any case, such transforms MUST NOT be used for any G-IKEv2 SA (both unicast and multicast).¶
Through the G-IKEv2 rekey, G-IKEv2 supports algorithms such as Logical Key Hierarchy (LKH) that have the property of denying access to a new group key by a member removed from the group (forward access control) and to an old group key by a member added to the group (backward access control). This is unrelated to PFS (Perfect Forward Secrecy) property as defined in Section 2.12 of IKEv2 [RFC7296].¶
Group management algorithms providing forward and backward access control other than LKH have been proposed in the literature, including OFT [OFT] and Subset Difference [NNL]. These algorithms could be used with G-IKEv2, but are not specified as a part of this document.¶
The Group Key Management Method transform from the GSA policy specifies how members of the group obtain group keys. This document specifies a single method for the group key management -- Wrapped Key Download. This method assumes that all group keys are sent to the GMs by the GCKS encrypted with some other keys, called Key Wrap Keys (KWK).¶
Every GM always knows at least one KWK -- the KWK that is associated with the IKE SA or multicast Rekey SA the wrapped keys are sent over. In this document it is called default KWK and is denoted as GSK_w.¶
For the purpose of forward access control the GCKS may provide each GM with its personal KWK at the time of registration. Besides, several intermediate KWKs that form a key hierarchy and are shared among several GMs may be provided by the GCKS.¶
Each KWK is associated with a key wrap algorithm, specified in the Key Wrap Algorithm attribute in the Encryption Algorithm transform used for the SA the KWK is sent over. The size of these KWKs is determined by the used key wrap algorithm requirements, but it SHOULD NOT be less than the size of the key for the Encryption Algorithm transform for the Rekey SA and for all Data-Security SAs in the group (taking into consideration the Key Length attribute if present).¶
The default KWK (GSK_w) is only used in the context of a single IKE SA. Every IKE SA (unicast IKE SA or multicast Rekey SA) will have its own GSK_w.¶
For the unicast IKE SA (used for the GM registration and for the GSA_INBAND_REKEY exchanges, if they are take place) the GSK_w is computed as follows:¶
GSK_w = prf+(SK_d, "Key Wrap for G-IKEv2")¶
where the string "Key Wrap for G-IKEv2" is 20 ASCII characters without null termination.¶
For the multicast Rekey SA the GSK_w is provided along with other SA keys as defined in Section 3.4.¶
Wrapped Key Download method allows the GCKS to employ various key management methods¶
A simple key management methods -- when the GCKS always sends group SA keys encrypted with the GSK_w.¶
An LKH key management method -- when the GCKS provides each GM with an individual key at the time of the GM registration (encrypted with GSK_w). Then the GCKS forms an hierarchy of keys so that the group SA keys are encrypted with other keys which are encrypted with other keys and so on, tracing back to the keys for each GM.¶
Other key policies may also be employed by the GCKS.¶
When group membership is altered using a group management algorithm new Data-Security SAs and their associated keys are usually also needed. New Data-Security SAs and keys ensure that members who were denied access can no longer participate in the group.¶
If forward access control is a desired property of the group, new TEK policy and the associated keys MUST NOT be included in a G-IKEv2 rekey message which changes group membership. This is required because the GSA TEK policy and the associated keys are not protected with the new KEK. A second G-IKEv2 rekey message can deliver the new GSA TEK policies and their associated keys because it will be protected with the new KEK, and thus will not be visible to the members who were denied access.¶
If forward access control policy for the group includes keeping group policy changes from members that are denied access to the group, then two sequential G-IKEv2 rekey messages changing the group KEK MUST be sent by the GCKS. The first G-IKEv2 rekey message creates a new KEK for the group. Group members, which are denied access, will not be able to access the new KEK, but will see the group policy since the G-IKEv2 rekey message is protected under the current KEK. A subsequent G-IKEv2 rekey message containing the changed group policy and again changing the KEK allows complete forward access control. A G-IKEv2 rekey message MUST NOT change the policy without creating a new KEK.¶
If other methods of using LKH or other group management algorithms are added to G-IKEv2, those methods MAY remove the above restrictions requiring multiple G-IKEv2 rekey messages, providing those methods specify how the forward access control policy is maintained within a single G-IKEv2 rekey message.¶
This specification defines a GM Key Management semantics in such a way, that it doesn't depend on the key management method employed by the GCKS. This allows having all the complexity of key management in the GCKS, which is free to implement various key management methods, such as direct transmitting of group SA keys or using some kind of key hierarchy (e.g. LKH). For all these policies the GM behavior is the same.¶
All keys in G-IKEv2 are transmitted in encrypted form, as specified in Section 4.5.4. This format includes a 32-bit Key ID (ID of a key that is encrypted) and a 32-bit KWK ID (ID of a key that was used to encrypt this key). Keys may be encrypted either with default KWK (GSK_w) or with other keys, which the GM has received in the WRAP_KEY attributes. If a key was encrypted with GSK_w, then the KWK ID field is set to zero, otherwise the KWK ID field identifies the key used for encryption. Zero Key ID always identifies the key from which the keys for protecting Data-Security SAs and Rekey SA are taken.¶
When a GM receives a message from the GCKS installing new Data-Security or Rekey SA, it will contain a KD payload with an SA_KEY attribute containing keying material for this SA. For a Data-Security SA exactly one SA_KEY attribute will be present with both Key ID and KWK ID fields set to zero. This means that the default KWK (GSK_w) should be used to extract this keying material.¶
For a multicast Rekey SA multiple SA_KEY attributes may be present depending on the key management method employed by the GCKS. If multiple SA_KEY attributes are present then all of them MUST contain the same keying material encrypted using different KWKs. The GM in general is unaware of the key management method used by the GCKS and can always use the same procedure to get the keys. The GM tries to decrypt at least one of the SA_KEY attributes using either the GSK_w or the keys from the WRAP_KEY attributes that are present in the same message or were receives in previous messages.¶
We will use the term "Key Path" to describe an ordered sequence of keys where each subsequent key was used to encrypt the previous one. The GM keeps its own Key Path (called Working Key Path) in the memory associated with each group it is registered to and updates it when needed. When the GSA_REKEY message is received the GM processes the received SA_KEY attributes one by one trying to construct a new key path that starts from one of these attributes and ends with any key in the Working Key Path or with the default KWK (GSK_w).¶
In the simplest case the SA_KEY attribute is encrypted with GSK_w so that the new Key Path is empty. If more complex key management methods are used then a Key Path will contain intermediate keys from the WRAP_KEY attributes received by a GM so far starting from its registration to the group. If the GM is able to construct a new Key Path using intermediate keys it has, then it is able to decrypt the SA_KEY attribute and use its content to form new SA keys. If it is unable to build a new Key Path, then in means that the GM is excluded from the group.¶
Depending on the new Key Path the GM should do the following actions to be prepared for future key updates:¶
If the new Key Path is empty then no actions are needed. This may happen if no WRAP_KEY attributes from the received message were used.¶
If the new Key Path is non-empty and it ends with the default KWK (GSK_w), then the whole new Key Path is stored by the GM as the GM's Working Key Path. This situation may only happen at the time the GM is registering to the group, when the GCKS is providing it with its personal key and the other keys from the key tree that are needed for this GM. These keys form an initial Working Key Path for this GM.¶
In all other cases the new Key Path will end at some intermediate key from the GM's current Working Key Path. In this case the new Key Path is constructed by replacing a part of the GM's current Working Key Path from the beginning and up to (but not including) the key that the GM has used to decrypt the last key in the new Key Path.¶
Appendix A contains an example of how this algorithm works in case of LKH key management method.¶
The keys that are used for Data-Security SAs or Rekey SA (called here SA keys) are downloaded to GMs in the form of keying material from which, according to policy, a set of keys are deterministically extracted.¶
For a Data-Security SA the keys are taken in accordance to the third bullet from Section 2.17 of [RFC7296]. In particular, for the ESP and AH SAs the encryption key (if any) MUST be taken from the leftmost bits of the keying material and the integrity key (if any) MUST be taken from the remaining bits.¶
For a Rekey SA the following keys are taken from the keying material:¶
GSK_e | GSK_a | GSK_w = KEYMAT¶
where GSK_e and GSK_a are the keys used for the Encryption Algorithm and the Integrity Algorithm transforms for the corresponding SA and GSK_w is a default KWK for this SA. Note, that GSK_w is used with the key wrap algorithm specified in the Key Wrap Algorithm attribute in the Encryption Algorithm transform. If an AEAD algorithm is used for encryption, then GSK_a key will not be used (GM can use the formula above assuming the length of GSK_a is zero).¶
The G-IKEv2 is an IKEv2 extension and thus inherits its wire format for data structures. However, the processing of some payloads are different. Several new payloads are defined: Group Identification (IDg, Section 4.2), Security Association - GM Supported Transforms (SAg, Section 4.3), Group Security Association (GSA, Section 4.4), and Key Download (KD, Section 4.5). G-IKEv2 header (Section 4.1), IDg payload and SAg payload reuse IKEv2 format for the IKEv2 header, IDi/IDr payloads and SA payload respectively. New exchange types GSA_AUTH, GSA_REGISTRATION, GSA_REKEY and GSA_INBAND_REKEY are also added.¶
This section describes new payloads and the differences in processing of existing IKEv2 payloads.¶
G-IKEv2 uses the same IKE header format as specified in [RFC7296] section 3.1. Major Version is 2 and Minor Version is 0 as in IKEv2. IKE SA Initiator's SPI, IKE SA Responder's SPI, Flags, Message ID, and Length are as specified in [RFC7296].¶
The Group Identification (IDg) payload allows the group member to indicate which group it wants to join. The payload is constructed by using the IKEv2 Identification Payload (section 3.5 of [RFC7296]). ID type ID_KEY_ID MUST be supported. ID types ID_IPV4_ADDR, ID_FQDN, ID_RFC822_ADDR, ID_IPV6_ADDR SHOULD be supported. ID types ID_DER_ASN1_DN and ID_DER_ASN1_GN are not expected to be used. The Payload Type for the Group Identification payload is fifty (50).¶
The Security Association - GM Supported Transforms Payload (SAg) payload declares which Transforms a GM is willing to accept. The payload is constructed using the format of the IKEv2 Security Association payload (section 3.3 of [RFC7296]). The Payload Type for SAg is identical to the SA Payload Type -- thirty-three (33).¶
The Group Security Association (GSA) payload is used by the GCKS to assert security attributes for both Rekey SA and Data-Security SAs. The Payload Type for the Group Security Association payload is fifty-one (51).¶
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Group Policies> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Security Association Payload fields are defined as follows:¶
Next Payload, C, RESERVED, Payload Length fields comprise the IKEv2 Generic Payload Header and are defined in Section 3.2. of [RFC7296].¶
Group Policies (variable) -- A set of group policies for the group.¶
Group policies are comprised of two types of policy -- Group SA (GSA) policy and Group-Wise (GW) policy. GSA policy defines parameters for the Security Association for the group. Depending on the employed security protocol GSA policies may further be classified as Rekey SA policy (GSA KEK) and Data-Security SA policy (GSA TEK). GSA payload may contain zero or one GSA KEK policy, zero or more GSA TEK policies, and zero or one GW policy, where either one GSA KEK or GSA TEK policy MUST be present.¶
This latitude allows various group policies to be accommodated. For example if the group policy does not require the use of a Rekey SA, the GCKS would not need to send a GSA KEK policy to the group member since all SA updates would be performed using the GSA_INBAND_REKEY exchange via the unicast IKE SA. Alternatively, group policy might use a Rekey SA but choose to download a KEK to the group member only as part of the unicast IKE SA. Therefore, the GSA KEK policy would not be necessary as part of the GSA_REKEY message.¶
Specifying multiple GSA TEKs allows multiple related data streams (e.g., video, audio, and text) to be associated with a session, but each protected with an individual security association policy.¶
A GW policy allows for the distribution of group-wise policy, such as instructions for when to activate and de-activate SAs.¶
Policies are distributed in substructures to the GSA payload. The format of the substructures is defined below in Section 4.4.2 (for GSA policy) and in Section 4.4.3 (for GW policy). The first octet of the substructure unambiguously determines its type -- it is zero for GW policy and non-zero (actually, it is a security protocol ID) for GSA policies.¶
The GSA policy substructure contains parameters for the SA used with this group. Depending on the security protocol the SA is either a Rekey SA or a Data-Security SA (ESP and AH). It is NOT RECOMMENDED that the GCKS distribute both ESP and AH policies for the same set of Traffic Selectors.¶
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | SPI Size | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ SPI ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Source Traffic Selector ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Destination Traffic Selector ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <GSA Transforms> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <GSA Attributes> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The GSA policy fields are defined as follows:¶
Protocol (1 octet) -- Identifies the security protocol for this group SA. The values are defined in the IKEv2 Security Protocol Identifiers in [IKEV2-IANA]. The valid values for this field are: <TBA> (GIKE_REKEY) for Rekey SA and 2 (AH) or 3 (ESP) for Data-Security SAs.¶
SPI Size (1 octet) -- Size of Security Parameter Index (SPI) for the SA. SPI size depends on the SA protocol. For GIKE_REKEY it is 16 octets, while for AH and ESP it is 4 octets.¶
Length (2 octets, unsigned integer) -- Length of this substructure including the header.¶
SPI (variable) -- Security Parameter Index for the group SA. The size of this field is determined by the SPI Size field. As described above, these SPIs are assigned by the GCKS. In case of GIKE_REKEY the SPI is the IKEv2 Header SPI pair where the first 8 octets become the "Initiator's SPI" field in the G-IKEv2 rekey message IKEv2 HDR, and the second 8 octets become the "Responder's SPI" in the same HDR.¶
Source & Destination Traffic Selectors (variable) -- Substructures describing the source and destination of the network identities. The format for these substructures is defined in IKEv2 [RFC7296], section 3.13.1. For the Rekey SA (with the GIKE_REKEY protocol) the destination traffic selectors MUST define a single multicast IP address, an IP protocol (assumed to be UDP) and a single port the GSA_REKEY messages will be destined to. The source traffic selector in this case MUST either define a single IP address, an IP protocol (assumed to be UDP) and a single port the GSA_REKEY messages will be originated from or be a wildcard selector. For the Data-Security (AH and ESP) SAs the destination traffic selectors SHOULD define a single multicast IP address. The source traffic selector in this case SHOULD define a single IP address or be a wildcard selector. IP protocol and ports define the characteristics of traffic protected by this Data-Security SA. If the Data-Security SAs are created in tunnel mode, then it MUST be tunnel mode with address preservation (see Multicast Extensions to the Security Architecture [RFC5374]. UDP encapsulation of ESP packets [RFC3948] cannot be specified in G-IKEv2 and thus it is not used for the multicast Data-Security SAs.¶
GSA Transforms (variable) -- A list of Transform Substructures specifies the policy information for the SA. The format is defined in IKEv2 [RFC7296], section 3.3.2. The "Last Substruc" field in each Transform Substructure is set to 3 except for the last Transform Substructure, where it is set to 0. Section 4.4.2.1 describes using IKEv2 transforms in GSA policy substructure.¶
GSA Attributes (variable) -- Contains policy attributes associated with the group SA. The following sections describe the possible attributes. Any or all attributes may be optional, depending on the protocol and the group policy. Section 4.4.2.2 defines attributes used in GSA policy substructure.¶
GSA policy is defined by means of transforms in the GSA policy substructure. For this purpose the transforms defined in [RFC7296] are used. In addition, new transform types are defined for using in G-IKEv2: Authentication Method (AUTHMETH) and Group Key Management Method (GKM), see Section 9.¶
Valid Transform Types depend on the SA protocol and are summarized in the table below.¶
Protocol Mandatory Types Optional Types ---------------------------------------------------------------- GIKE_REKEY ENCR, INTEG*, PRF, AUTHMETH**, GKM** ESP ENCR INTEG, ARP AH INTEG ARP
(*) If AEAD encryption algorithm is used, then INTEG transform MUST NOT be specified, otherwise it MUST be specified.¶
(**) May only appear at the time of a GM registration, (in the GSA_AUTH and GSA_REGISTRATION exchanges).¶
The Authentication Method (AUTHMETH) transform is used in the GIKE_REKEY policy to convey information of how GCKS will authenticate the GSA_REKEY messages. This values are from the IKEv2 Authentication Method registry [IKEV2-IANA]. Note, that this registry defines only values in a range 0-255, so even that Transform ID field in the Transform substructure allows for 65536 possible values, in case of the Authentication Method transform the values 256-65535 MUST NOT appear. This document renames the "Reserved" (0) value in the "IKEv2 Authentication Method" registry [IKEV2-IANA] to "NONE".¶
Among the currently defined authentication methods in the IKEv2 Authentication Method registry, only the following are allowed to be used in the Authentication Method transform: NONE (0) and Digital Signature (14). Other currently defined authentication methods MUST NOT be used. The following semantics is associated with each of the allowed methods.¶
The authentication method MUST NOT change as a result of rekey operations. This means that the Authentication Method transform may not appear in the rekey messages, it may only appear in the registration exchange (either GSA_AUTH or GSA_REGISTRATION).¶
The type of the Authentication Method Transform is <TBA by IANA>.¶
The Group Key Management Method (GKM) transform is used in the GIKE_REKEY policy to convey information of how GCKS will manage the group keys to provide forward and backward access control (i.e., used to exclude group members). Possible key management methods are defined in a new IKEv2 registry "Transform Type <TBA> -- Group Key Management Methods" (see Section 9). This document defines one values for this registry:¶
The group key management method MUST NOT change as a result of rekey operations. This means that the Group Key Management Method transform may not appear in the rekey messages, it may only appear in the registration exchange (either GSA_AUTH or GSA_REGISTRATION).¶
The type of the Group Key Management Method transform is <TBA by IANA>.¶
The "Extended Sequence Number (ESN)" Transform is defined in [RFC7296]. This specification renames this transform to "Anti-Replay Protection (ARP)". This transform allows to specify whether the 64-bit Extended Sequence Numbers (ESN) are to be used in ESP and AH.¶
Since both AH [RFC4302] and ESP [RFC4303] are defined in such a way, that high-order 32 bits of extended sequence numbers are never transmitted, it makes using ESN in multicast Data-Security SAs problematic, because GMs that join group long after it is created will have to somehow learn the current high order 32 bits of ESN for each sender in the group. The algorithm for doing this described in AH [RFC4302] and ESP [RFC4303] is resource-consuming and is only suitable when a receiver is able to guess the high-order 32 bits close enough to its real value, which is not the case for multicast SAs. For this reason the value "Extended Sequence Numbers" (1) for the Anti-Replay Protection transform type MUST NOT be used in the GSA Payload for Data-Security SAs utilizing protocols ESP or AH.¶
GSA attributes are generally used to provide GMs with additional parameters for the GSA policy. Unlike security parameters distributed via transforms, which are expected not to change over time (unless policy changes), the parameters distributed via GSA attributes may depend on the time the provision takes place, on the existence of others group SAs or on other conditions.¶
This document creates a new IKEv2 IANA registry for the types of the GSA attributes which is initially filled as described in Section 9. In particular, the following attributes are initially added.¶
GSA Attributes Value Format Multi-Valued Protocol --------------------------------------------------------------------- Reserved 0 GSA_KEY_LIFETIME 1 TLV NO GIKE_REKEY, AH, ESP GSA_INITIAL_MESSAGE_ID 2 TLV NO GIKE_REKEY GSA_NEXT_SPI 3 TLV YES GIKE_REKEY, AH, ESP¶
The attributes follow the format defined in the IKEv2 [RFC7296] section 3.3.5.¶
The GSA_KEY_LIFETIME attribute (1) specifies the maximum time for which the SA is valid. The value is a 4 octet unsigned integer in a network byte order, specifying a valid time period in seconds. When the lifetime expires, the group security association and all associated keys MUST be deleted. The GCKS may delete the SA at any time before the end of the validity period.¶
A single attribute of this type MUST be included into any GSA policy substructure if multicast rekey is employed by the GCKS. This attribute SHOULD NOT be used if inband rekey (via the GSA_INBAND_REKEY exchange) is employed by the GCKS for the GM.¶
The GSA_INITIAL_MESSAGE_ID attribute (2) defines the initial Message ID to be used by the GCKS in the GSA_REKEY messages. The Message ID is a 4 octet unsigned integer in network byte order.¶
A single attribute of this type MUST be included into the GSA KEK policy substructure if the initial Message ID of the Rekey SA is non-zero. Note, that it is always the case if GMs join the group after some multicast rekey operations have already taken place, so in these cases this attribute will be included into the GSA policy at the time of GMs' registration.¶
This attribute MUST NOT be used if inband rekey (via the GSA_INBAND_REKEY exchange) is employed by the GCKS for the GM.¶
The optional GSA_NEXT_SPI attribute (3) contains SPI that the GCKS reserved for the next Rekey SA or Data-Security SAs replacing the current ones. The length of the attribute data is determined by the SPI Size field in the GSA Policy substructure the attribute resides in (see Section 4.4.2), and the attribute data contains SPI as it would appear on the network. Multiple attributes of this type MAY be included, meaning that any of the supplied SPIs can be used in the replacement group SA.¶
The GM MAY store these values and if later the GM starts receiving messages with one of these SPIs without seeing a rekey message over the current Rekey SA, this may be used as an indication, that the rekey message got lost on its way to this GM. In this case the GM SHOULD re-register to the group.¶
Note, that this method of detecting lost rekey messages can only be used by group receivers. Additionally there is no point to include this attribute in the GSA_INBAND_REKEY messages, since they use reliable transport. Note also, that the GCKS is free to forget its promises and not to use the SPIs it sent in the GSA_NEXT_SPI attributes before (e.g. in case of the GCKS is rebooted), so the GM must only treat these information as a "best effort" made by the GCKS to prepare for future rekeys.¶
This attribute MUST NOT be used if inband rekey (via the GSA_INBAND_REKEY exchange) is employed by the GCKS for the GM.¶
Group specific policy that does not belong to any SA policy can be distributed to all group member using Group-Wise (GW) policy substructure.¶
The GW policy substructure is defined as follows:¶
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RESERVED | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <GW Policy Attributes> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The GW policy substructure fields are defined as follows:¶
Protocol (1 octet) -- MUST be zero. This value is reserved in Section 9 and is never used for any security protocol, so it is used here to indicate that this substructure contains policy not related to any specific protocol.¶
RESERVED ( octet) -- MUST be zero on transmission, MUST be ignored on receipt.¶
Length (2 octets, unsigned integer) -- Length of this substructure including the header.¶
GW Policy Attributes (variable) -- Contains policy attributes associated with no specific SA. The following sections describe possible attributes. Any or all attributes may be optional, depending on the group policy.¶
This document creates a new IKEv2 IANA registry for the types of the Group-Wise Policy attributes which is initially filled as described in Section 9. In particular, the following attributes are initially added.¶
GW Policy Attributes Value Format Multi-Valued -------------------------------------------------------- Reserved 0 GWP_ATD 1 TV NO GWP_DTD 2 TV NO GWP_SENDER_ID_BITS 3 TV NO¶
The attributes follow the format defined in the IKEv2 [RFC7296] section 3.3.5.¶
Section 4.2.1 of Multicast Extensions to the Security Architecture [RFC5374] specifies a key rollover method that requires two values be provided to group members -- Activation Time Delay (ATD) and Deactivation Time Delay (DTD).¶
The GWP_ATD attribute (1) allows a GCKS to set the Activation Time Delay for Data-Security SAs of the group. The ATD defines how long active members of the group (those who sends traffic) should wait after receiving new SAs before staring sending traffic over them. Note, that to achieve smooth rollover passive members of the group should activate the SAs immediately once they receive them.¶
The GWP_DTD attribute (2) allows the GCKS to set the Deactivation Time Delay for previously distributed SAs. The DTD defines how long after receiving a request to delete Data-Security SAs passive group members should wait before actually deleting them. Note that active members of the group should stop sending traffic over these old SAs once new replacement SAs are activated (after time specified in the GWP_ATD attribute).¶
The GWP_ATD and GWP_DTD attributes contain 16 bit unsigned integer in a network byte order, specifying the delay in seconds. These attributes are OPTIONAL. If one of them or both are not sent by the GCKS, then no corresponding delay should be employed.¶
The GWP_SENDER_ID_BITS attribute (3) declares how many bits of the cipher nonce are taken to represent a Sender-ID value. The bits are applied as the most significant bits of the IV, as shown in Figure 1 of Using Counter Modes with ESP and AH to Protect Group Traffic [RFC6054] and specified in Section 2.5.2. Guidance for a GCKS choosing the value is provided in Section 3 of Using Counter Modes with ESP and AH to Protect Group Traffic [RFC6054]. This value is applied to each Sender-ID value distributed in the KD payload.¶
The GCKS MUST include this attribute if there are more than one sender in the group and any of the Data-Security SAs use counter-based cipher mode. The number of Sender-ID bits is represented as 16 bit unsigned integer in network byte order.¶
The Key Download (KD) payload contains the group keys for the SAs specified in the GSA Payload. The Payload Type for the Key Download payload is fifty-two (52).¶
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Key Bags> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Key Download payload fields are defined as follows:¶
Next Payload, C, RESERVED, Payload Length fields comprise the IKEv2 Generic Payload Header and are defined in Section 3.2. of [RFC7296].¶
Key Bags (variable) -- A set of Key Bag substructures.¶
Keys are distributed in a substructures called key bags. Each key bag contains one or more keys that are logically related -- either these are keys for a single SA (Data-Security SA or Rekey SA) or these are keys for a single group member (in the latter case besides keys the key bag may also contain security parameters for this group member).¶
For this reason two types of key bags are defined -- Group Key Bag and Member Key Bag. The type is unambiguously determined by the first byte of the key bag substructure -- for member key bag it is zero and for group key bag it represents the protocol number, which along with the following SPI, identify the SA the keys in the bag are for.¶
The Group Key Bag substructure contains SA key information. This key information is associated with some group SAs: either with Data-Security SAs or with group Rekey SA.¶
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | SPI Size | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ SPI ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Group Key Bag Attributes> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Protocol (1 octet) -- Identifies the security protocol for this key bag. The values are defined in the IKEv2 Security Protocol Identifiers in [IKEV2-IANA]. The valid values for this field are: <TBA> (GIKE_REKEY) for KEK Key packet and 2 (AH) or 3 (ESP) for TEK key bag.¶
SPI Size (1 octet) -- Size of Security Parameter Index (SPI) for the corresponding SA. SPI size depends on the security protocol. For GIKE_REKEY it is 16 octets, while for AH and ESP it is 4 octets.¶
Length (2 octets, unsigned integer) -- Length of this substructure including the header.¶
SPI (variable) -- Security Parameter Index for the corresponding SA. The size of this field is determined by the SPI Size field. In case of GIKE_REKEY the SPI is the IKEv2 Header SPI pair where the first 8 octets become the "Initiator's SPI" field in the G-IKEv2 rekey message IKEv2 HDR, and the second 8 octets become the "Responder's SPI" in the same HDR.¶
Group Key Bag Attributes (variable) -- Contains Key information for the corresponding SA.¶
This document creates a new IKEv2 IANA registry for the types of the Group Key Bag attributes which is initially filled as described in Section 9. In particular, the following attributes are initially added.¶
Group Key Bag
Attributes Value Format Multi-Valued Protocol
------------------------------------------------------------
Reserved 0
SA_KEY 1 TLV NO/YES* GIKE_REKEY,
NO AH, ESP
¶
(*) Multiple SA_KEY attributes may only appear for the GIKE_REKEY protocol in the GSA_REKEY exchange if the GCKS uses the Group Key Management method that allows excluding GMs from the group (like LKH).¶
The attributes follow the format defined in the IKEv2 [RFC7296] section 3.3.5.¶
The SA_KEY attribute (1) contains a keying material for the corresponding SA. The content of the attribute is formatted according to Section 4.5.4 with a precondition that the Key ID field MUST always be zero. The size of the keying material MUST be equal to the total size of the keys needed to be taken from this keying material (see Section 3.4) for the corresponding SA.¶
If the key bag is for a Data-Security SA (AH or ESP protocols), then exactly one SA_KEY attribute MUST be present with both Key ID and KWK ID fields set to zero.¶
If the key bag is for a Rekey SA (GIKE_REKEY protocol), then in the GSA_AUTH, GSA_REGISTRATION and GSA_INBAND_REKEY exchanges exactly one SA_KEY attribute MUST be present. In the GSA_REKEY exchange at least one SA_KEY attribute MUST be present, and more attributes MAY be present (depending on the key management method employed by the GCKS).¶
The Member Key Bag substructure contains keys and other parameters that are specific for a member of the group and are not associated with any particular group SA.¶
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RESERVED | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Member Key Bag Attributes> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Member Key Bag substructure fields are defined as follows:¶
Protocol (1 octet) -- MUST be zero. This value is reserved in Section 9 and is never used for any security protocol, so it is used here to indicate that this key bag is not associated with any particular SA.¶
RESERVED ( octet) -- MUST be zero on transmission, MUST be ignored on receipt.¶
Length (2 octets, unsigned integer) -- Length of this substructure including the header.¶
Member Key Bag Attributes (variable) -- Contains Key information and other parameters exclusively for a particular member of the group.¶
The member Key Bag substructure contains sensitive information for a single GM, for this reason it MUST NOT be sent in GSA_REKEY messages and MUST only be sent via unicast SA at the time the GM registers to the group (in either GSA_AUTH or GSA_REGISTRATION exchanges).¶
This document creates a new IKEv2 IANA registry for the types of the Member Key Bag attributes which is initially filled as described in Section 9. In particular, the following attributes are initially added.¶
Member Key Bag Attributes Value Format Multi-Valued ---------------------------------------------------- Reserved 0 WRAP_KEY 1 TLV YES AUTH_KEY 2 TLV NO GM_SENDER_ID 3 TLV YES¶
The attributes follow the format defined in the IKEv2 [RFC7296] section 3.3.5.¶
The WRAP_KEY attribute (1) contains a key that is used to encrypt other keys. One or more these attributes are sent to GMs if the GCKS key management method relies on some key hierarchy (e.g. LKH). This attribute MUST NOT be used if inband rekey (via the GSA_INBAND_REKEY exchange) is employed by the GCKS for the GM.¶
The content of the attribute has a format defined in Section 4.5.4 with a precondition that the Key ID field MUST NOT be zero. The algorithm associated with the key is from the Encryption Transform for the SA the WRAP_KEY attributes was sent in. The size of the key MUST be equal to the key size for this algorithm.¶
Multiple instances of the WRAP_KEY attributes MAY be present in the key bag.¶
The AUTH_KEY attribute (2) contains the key that is used to authenticate the GSA_REKEY messages. The content of the attribute depends on the authentication method the GCKS specified in the Authentication Method transform in the GSA payload.¶
If digital signatures are used for the GSA_REKEY messages authentication then the content of the AUTH_KEY attribute is a public key used for digital signature authentication. The public key MUST be represented as DER-encoded ASN.1 object SubjectPublicKeyInfo, defined in section 4.1.2.7 of Internet X.509 Public Key Infrastructure Certificate and CRL Profile [RFC5280]. The signature algorithm that will use this key was specified in the Signature Algorithm Identifier attribute of the Authentication Method transform. The key MUST be compatible with this algorithm. An RSA public key format is defined in PKCS#1 Version 2.2 [RFC8017], Section A.1. DSS public key format is defined in Algorithms and Identifiers for the Internet X.509 Public Key Infrastructure Certificate and CRL Profile [RFC3279] Section 2.3.2. For ECDSA Public keys, use format described in Elliptic Curve Cryptography Subject Public Key Information [RFC5480] Section 2. Other algorithms added to the IKEv2 Authentication Method registry are also expected to include a format of the SubjectPublicKeyInfo object included in the algorithm specification.¶
Multiple instances of the AUTH_KEY attributes MUST NOT be sent. This attribute MUST NOT appear in the rekey operations (in the GSA_REKEY or GSA_INBAND_REKEY exchanges).¶
The GM_SENDER_ID attribute (3) is used to download one or more Sender-ID values for the exclusive use of a group member. One or more of this attributes MUST be sent by the GCKS if the GM informed the GCKS that it would be a sender (by inclusion the SENDER notification to the request) and at least one of the Data-Security SAs included in the GSA payload uses counter-based mode of encryption.¶
If the GMs has requested multiple Sender-ID values in the SENDER notification, then the GCKS SHOULD provide it with the requested number of Sender-IDs by sending multiple instances of the GM_SENDER_ID attribute. The GCKS MAY send fewer values than requested by the GM (e.g. if it is running out of Sender-IDs), but it MUST NOT send more than requested.¶
This attribute MUST NOT appear in the rekey operations (in the GSA_REKEY or GSA_INBAND_REKEY exchanges).¶
Symmetric keys in G-IKEv2 are never sent in clear inside G-IKEv2 messages. They are always protected with other symmetric keys. This protection is called key wrapping. Algorithms used for key wrapping are usually based on generic encryption algorithms, but their mode of operation is optimized for protecting short high-entropy data with minimal additional overhead. While in general key wrap algorithms can be generic, in practice they are often tied to the underlying encryption algorithms. For example, AES Key Wrap with Padding Algorithm [RFC5649] defines key wrapping using AES and Key Wrapping Constructions using SipHash and ChaCha [ARX-KW] defines key wrapping using Chacha20.¶
In G-IKEv2 the key wrap algorithm MUST be negotiated in the IKE_SA_INIT exchange, so that the GCKS be able to send encrypted keys to the GM in the GSA_AUTH exchange. In addition, if the GCKS the multicast Rekey SA for group rekey, then it MUST specify the key wrap algorithm in the GSA payload. If SAg payload is included in the GSA_AUTH request, then it MUST also indicate which key wrap algorithms are supported by the GM.¶
The key wrap algorithm is specified by augmenting the Encryption Algorithm transform with a new "Key Wrap Algorithm" attribute (<TBA by IANA>). This way the key wrap algorithm is tied to the encryption algorithm.¶
This document creates a new IKEv2 IANA registry for the key wrap algorithms which is initially filled as described in Section 9. In particular, the following entries are initially added.¶
Key Wrap Algorithm Value ------------------------------------- Reserved 0 KW_5649 1 ARX_KW 2¶
These algorithms are defined as follows.¶
KW_5649 -- Key wrap algorithm defined in [RFC5649]. This algorithm is designed for use with AES block cipher, but can also be used with other block ciphers.¶
ARX_KW -- The ARX-KW-8-2-4-GX key wrap algorithm defined in [ARX-KW]. This algorithm is designed for use with Chacha20 stream cipher.¶
More key wrap algorithms may be defined in future. The requirement is that these algorithms MUST be able to wrap key material of size up to 256 bytes.¶
The key wrap algorithm is used with the encryption algorithm that protects the message the wrapped keys are sent in: in case of unicast IKE SA (used for GMs registration and rekeying with GSA_INBAND_REKEY) the encryption algorithm will be the one negotiated during the IKE SA establishment, while for a GSA_REKEY message the algorithm will be provided by the GCKS in the Encryption Algorithm transform in the GSA payload when this multicast SA was being established. Note that key wrap algorithms for these SAs may be different - for the unicast SA the key wrap algorithms is negotiated between the GM and the GCKS, while for the multicast Rekey SA the key wrap algorithm is provided by the GCKS to the group members as part of the group policy.¶
The format of the wrapped key is shown in Figure 21.¶
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| KWK ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Encrypted Key ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Wrapped Key fields are defined as follows:¶
Key ID (4 octets) -- ID of the encrypted key. The value zero means that the encrypted key contains SA keys (in the form of keying material, see Section 3.4)), otherwise it contains some intermediate key.¶
KWK ID (4 octets) -- ID of the key that was used to encrypt key with specified Key ID. The value zero means that the default KWK was used to encrypt the key, otherwise some intermediate key was used.¶
Encrypted Key (variable) -- The encrypted key bits. These bits comprise either a single encrypted key or a result of encryption of a concatenation of keys (key material) for several algorithms. The format of this fields is determined by the key wrap algorithm for the SA the wrapped key is sent over.¶
Delete payload is used in G-IKEv2 when the GCKS wants to delete Data-Security and Rekey SAs. The interpretation of the Protocol field in the Delete payload is extended, so that zero protocol indicates deletion of whole Group SA (i.e. all Data-Security SAs and Rekey SA). See Section 2.4.3 for detail.¶
G-IKEv2 uses the same Notify payload as specified in [RFC7296], section 3.10.¶
There are additional Notify Message types introduced by G-IKEv2 to communicate error conditions and status (see Section 9).¶
INVALID_GROUP_ID (45) is a new error type notification that indicates that the group ID sent during the registration process is invalid. The Protocol ID and SPI Size fields in the Notify payload MUST be zero. There is no data associated with this notification and the content of the Notification Data field MUST be ignored on receipt.¶
AUTHORIZATION_FAILED (46) is a new error type notification that is sent in the response to a GSA_AUTH or GSA_REGISTRATION message when authorization failed. The Protocol ID and SPI Size fields in the Notify payload MUST be zero. There is no data associated with this notification and the content of the Notification Data field MUST be ignored on receipt.¶
REGISTRATION_FAILED (<TBA>) is a new error type notification that is sent by the GCKS when the GM registration request cannot be satisfied for the reasons not related to this particular GM, for example if the capacity of the group is exceeded. The Protocol ID and SPI Size fields in the Notify payload MUST be zero. There is no data associated with this notification and the content of the Notification Data field MUST be ignored on receipt.¶
SENDER (16429) is a new status type notification that is sent in the GSA_AUTH or the GSA_REGISTRATION exchanges to indicate that the GM intends to be sender of data traffic. The data includes a count of how many Sender-ID values the GM desires. The count MUST be 4 octets long and contain the big endian representation of the number of requested Sender-IDs. The Protocol ID and SPI Size fields in the Notify payload MUST be zero.¶
G-IKEv2 uses the same Authentication payload as specified in [RFC7296], section 3.8, to authenticate the rekey message. However, if it is used in the GSA_REKEY messages the content of the payload is computed differently, as described in Section 2.4.1.1.¶
G-IKEv2 defines a number of attributes, that are used to convey information from GCKS to GMs. There are some restrictions on where and when these attributes can appear in G-IKEv2 messages, which are defined when the attributes are introduced. For convenience these restrictions are summarized in Table 2 (for multicast rekey operations) and Table 3 (for inband rekey operations) below.¶
The following notation is used:¶
Note, that the restrictions are defined per a substructure corresponding attributes are defined for and not per whole G-IKEv2 message.¶
| Attributes | GSA_AUTH GSA_REGISTRATION | GSA_REKEY | Notes |
|---|---|---|---|
| GSA Attributes (Section 4.4.2.2) | |||
| GSA_KEY_LIFETIME | S | S | |
| GSA_INITIAL_MESSAGE_ID | [S] | [S] | |
| GSA_NEXT_SPI | [M] | [M] | |
| GW Policy Attributes (Section 4.4.3.1) | |||
| GWP_ATD | [S] | [S] | |
| GWP_DTD | [S] | [S] | |
| GWP_SENDER_ID_BITS | S | - | 1 |
| Key Bag Attributes (Section 4.5.1) | |||
| SA_KEY | S | S[M] | 2 |
| WRAP_KEY | [M] | [M] | 3 |
| AUTH_KEY | S | [S] | 4 |
| GM_SENDER_ID | S[M] | - | 1 |
|
Notes:¶
|
|||
| Attributes | GSA_AUTH GSA_REGISTRATION | GSA_INBAND_REKEY | Notes |
|---|---|---|---|
| GSA Attributes (Section 4.4.2.2) | |||
| GSA_KEY_LIFETIME | [S] | [S] | |
| GSA_INITIAL_MESSAGE_ID | - | - | |
| GSA_NEXT_SPI | - | - | |
| GW Policy Attributes (Section 4.4.3.1) | |||
| GWP_ATD | [S] | [S] | |
| GWP_DTD | [S] | [S] | |
| GWP_SENDER_ID_BITS | S | - | 1 |
| Key Bag Attributes (Section 4.5.1) | |||
| SA_KEY | S | S | |
| WRAP_KEY | - | - | |
| AUTH_KEY | - | - | |
| GM_SENDER_ID | S[M] | - | 1 |
|
Notes:¶
|
|||
A number of IKEv2 extensions is defined that can be used to extend protocol functionality. G-IKEv2 is compatible with most of them. In particular, EAP authentication defined in [RFC7296] can be used to establish registration IKE SA, as well as EAP-only authentication [RFC5998] and Secure Password authentication [RFC6467]. G-IKEv2 is compatible with and can use IKEv2 Redirect Mechanism [RFC5685] and IKEv2 Session Resumption [RFC5723]. G-IKEv2 is also compatible with Multiple Key Exchanges in IKEv2 framework, defined in [RFC9370].¶
The above list of compatible IKEv2 extensions is not exhaustive, however some IKEv2 extensions require special handling if used in G-IKEv2.¶
G-IKEv2 can take advantage of the protection provided by Postquantum Preshared Keys (PPK) for IKEv2 [RFC8784]. However, the use of PPK leaves the initial IKE SA susceptible to quantum computer (QC) attacks. Group SA keys are protected with the default KWK (GSK_w), which is derived from SK_d and thus cannot be broken even by attacker equipped with a QC. However, other data sent over the initial IKE SA may be susceptible to an attacker equipped with a QC of a sufficient size. Such an attacker can store all the traffic until it obtains such a QC and then decrypt it (Store Now Decrypt Later attack). See Section 6 of [RFC8784] for details.¶
While the group keys are protected with PPK and thus are immune to QC, GCKS implementations that care about other data sent over initial IKE SA MUST rely on IKEv2 extensions that protect even initial IKE SA against QC (like [I-D.ietf-ipsecme-ikev2-qr-alt]).¶
Few extensions were defined for GDOI protocol [RFC6407], like GDOI Support for IEC 62351 Security Services [RFC8052] or GDOI GROUPKEY-PUSH Acknowledgement Message [RFC8263]. It is expected that these extensions will be redefined for G-IKEv2 in separate documents, if needed.¶
G-IKEv2 registration exchange uses IKEv2 IKE_SA_INIT protocols, inheriting all the security considerations documented in the Section 5 of [RFC7296], including authentication, confidentiality, protection against man-in-the-middle, protection against replay/reflection attacks, and denial of service protection. The GSA_AUTH and GSA_REGISTRATION exchanges also take advantage of those protections. In addition, G-IKEv2 brings in the capability to authorize a particular group member regardless of whether they have the IKEv2 credentials.¶
The GSA maintenance channel is cryptographically and integrity protected using the cryptographic algorithm and key negotiated in the GSA member registration exchange.¶
The authentication key is distributed during the GM registration, and the receiver of the rekey message uses that key to verify the message came from the authorized GCKS. An implicit authentication can also be used, in which case the ability of the GM to decrypt and to verify ICV of the received message proved that a sender of the message is a member of the group. However, implicit authentication doesn't provide source origin authentication, so the GM cannot be sure that the message came from the GCKS. For this reason using implicit authentication is NOT RECOMMENDED unless in a small group of trusted parties.¶
Confidentiality is provided by distributing a confidentiality key as part of the GSA member registration exchange.¶
GSA maintenance channel is integrity protected by using a digital signature.¶
The GSA_REKEY message includes a monotonically increasing sequence number to protect against replay and reflection attacks. A group member will recognize a replayed message by comparing the Message ID number to that of the last received rekey message, any rekey message containing a Message ID number less than or equal to the last received value MUST be discarded. Implementations should keep a record of recently received GSA rekey messages for this comparison.¶
A new set of registries is created for G-IKEv2 on IKEv2 parameters page [IKEV2-IANA]. The terms Reserved, Expert Review and Private Use are to be applied as defined in [RFC8126].¶
This document creates a new IANA registry "Transform Type <TBA> - Group Key Management Methods". The initial values of the new registry are:¶
Value Group Key Management Method ------------------------------------------------------- Reserved 0 Wrapped Key Download 1 Unassigned 2-1023 Private Use 1024-65535¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document creates a new IANA registry "GSA Attributes". The initial values of the new registry are:¶
GSA Attributes Value Format Multi-Valued Protocol --------------------------------------------------------------------- Reserved 0 GSA_KEY_LIFETIME 1 TLV NO GIKE_REKEY, AH, ESP GSA_INITIAL_MESSAGE_ID 2 TLV NO GIKE_REKEY GSA_NEXT_SPI 3 TLV YES GIKE_REKEY, AH, ESP Unassigned 5-16383 Private Use 16384-32767¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document creates a new IANA registry "Group-Wise Policy Attributes". The initial values of the new registry are:¶
GW Policy Attributes Value Format Multi-Valued -------------------------------------------------------- Reserved 0 GWP_ATD 1 TV NO GWP_DTD 2 TV NO GWP_SENDER_ID_BITS 3 TV NO Unassigned 4-16383 Private Use 16384-32767¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document creates a new IANA registry "Group Key Bag Attributes". The initial values of the new registry are:¶
Group Key Bag
Attributes Value Format Multi-Valued Protocol
------------------------------------------------------------
Reserved 0
SA_KEY 1 TLV YES GIKE_REKEY,
NO AH, ESP
Unassigned 2-16383
Private Use 16384-32767
¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document creates a new IANA registry "Member Key Bag Attributes". The initial values of the new registry are:¶
Member Key Bag Attributes Value Format Multi-Valued ---------------------------------------------------- Reserved 0 WRAP_KEY 1 TLV YES AUTH_KEY 2 TLV NO GM_SENDER_ID 3 TLV YES Unassigned 4-16383 Private Use 16384-32767¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document creates a new IANA registry "Key Wrap Algorithms". The initial values of the new registry are:¶
Key Wrap Algorithm Value ----------------------------- Reserved 0 KW_5649 1 ARX_KW 2 Unassigned 3-1023 Private Use 1024-65535¶
Changes and additions to the unassigned range of this registry are by the Expert Review Policy [RFC8126].¶
This document defines new Exchange Types in the "IKEv2 Exchange Types" registry:¶
Value Exchange Type ---------------------------- 39 GSA_AUTH 40 GSA_REGISTRATION 41 GSA_REKEY <TBA> GSA_INBAND_REKEY¶
This document defines new Payload Types in the "IKEv2 Payload Types" registry:¶
Value Next Payload Type Notation ---------------------------------------------------- 50 Group Identification IDg 51 Group Security Association GSA 52 Key Download KD¶
This document makes the following changes to the "Transform Type Values" registry:¶
Defines two new transform types -- "Authentication Method (AUTHMETH)" and "Group Key Management Method (GKM)";¶
Renames existing transform type "Extended Sequence Numbers (ESN)" to "Anti-Replay Protection (ARP)";¶
Changes the "Used In" column for the existing allocations as follows;¶
Type Description Used In
---------------------------------------------------------------------
1 Encryption Algorithm (ENCR) IKE, GIKE_REKEY and ESP
2 Pseudo-random Function (PRF) IKE, GIKE_REKEY
3 Integrity Algorithm (INTEG) IKE, GIKE_REKEY, AH,
optional in ESP
4 Key Exchange Method (KE) IKE, optional in AH, ESP
5 Anti-Replay Protection (ARP) AH and ESP
<TBA> Authentication Method (AUTHMETH) GIKE_REKEY
<TBA> Group Key Management Method (GKM) GIKE_REKEY
¶
This document defines two new Attribute Types in the "IKEv2 Transform Attribute Types" registry:¶
Value Attribute Type Format ------------------------------------------------------ <TBA> Signature Algorithm Identifier TLV <TBA> Key Wrap Algorithm TV¶
This document renames the "Transform Type 5 - Extended Sequence Numbers Transform IDs" registry to "Transform Type 5 - Anti-Replay Protection Transform IDs" and also adds a new value into this registry:¶
Number Name --------------------- <TBA> Not Used¶
This document defines new Notify Message types in the "IKEv2 Notify Message Error Types" registry:¶
Value Notify Message Error Type ----------------------------------------- 45 INVALID_GROUP_ID 46 AUTHORIZATION_FAILED <TBA> REGISTRATION_FAILED¶
The Notify type with the value 16429 was allocated earlier in the development of G-IKEv2 document in the "IKEv2 Notify Message Status Types" registry with the name SENDER_REQUEST_ID. This document renames it as follows:¶
Value Notify Message Status Type ------------------------------------------ 16429 SENDER¶
This document defines a new Security Protocol Identifier in the "IKEv2 Security Protocol Identifiers" registry:¶
Protocol ID Protocol -------------------------- <TBA> GIKE_REKEY¶
This document renames the "Reserved" value in the "IKEv2 Authentication Method" registry to "NONE".¶
The authors thank Lakshminath Dondeti and Jing Xiang for first exploring the use of IKEv2 for group key management and providing the basis behind the protocol. Mike Sullenberger and Amjad Inamdar were instrumental in helping resolve many issues in several versions of the document.¶
The authors are grateful to Tero Kivinen, Daniel Migault, Gorry Fairhurst and Russ Housley for their careful reviews and valuable proposals for improving the document quality.¶
The following individuals made substantial contributions to early versions of this memo.¶
Sheela Rowles Cisco Systems 170 W. Tasman Drive San Jose, California 95134-1706 USA Phone: +1-408-527-7677 Email: [email protected]¶
Aldous Yeung Cisco Systems 170 W. Tasman Drive San Jose, California 95134-1706 USA Phone: +1-408-853-2032 Email: [email protected]¶
Paulina Tran Cisco Systems 170 W. Tasman Drive San Jose, California 95134-1706 USA Phone: +1-408-526-8902 Email: [email protected]¶
Yoav Nir Dell EMC 9 Andrei Sakharov St Haifa 3190500 Israel Email: [email protected]¶
Section 5.4 of [RFC2627] describes the LKH architecture, and how a GCKS uses LKH to exclude group members. This section clarifies how the LKH architecture is used with G-IKEv2.¶
In this section we will use the notation X{Y} where a key with ID Y is encrypted with the key with ID X. The notation GSK_w{Y} means that the default wrap key GSK_w (with zero KWK ID)is used to encrypt key Y, and the notation X{K_sa} means key X is used to encrypt the SA key K_sa (wich always has zero Key ID). Note, that GSK_w{K_sa} means that the SA key is encrypted with the default wrap key, in which case both KWK ID and Key ID are zero.¶
The content of the KD payload will be shown as a sequence of key bags. The Group Key Bag substructure will be denoted as GP(SAn)(), when n is an SPI for the SA, and the Member Key Bag substructure will be denoted as MP(). The content of the key bags is shown as SA_KEY and WRAP_KEY attributes with the notation described above. For simplicity the type of the attribute will not be shown, because it is implicitly defined by the type of key bag.¶
Here is the example of KD payload.¶
KD(GP(SA1)(X{K_sa}),MP(Y{X},Z{Y},GSK_w{Z})
¶
For simplicity any other attributes in the KD payload are omitted.¶
We will also use the notation X->Y->Z to describe the Key Path. In this case key Y is needed to decrypt key X and key Z is needed to decrypt key Y. In the example above the keys had the following relation: K_sa->X->Y->Z->GSK_w.¶
When a GCKS forms a group, it creates a key tree as shown in the figure below. The key tree contains logical keys (which are represented as the values of their Key IDs in the figure) and a private key shared with only a single GM (the GMs are represented as letters followed by the corresponding key ID in parentheses in the figure). The root of the tree contains the multicast Rekey SA key (which is represented as SAn(K_san). The figure below assumes that the Key IDs are assigned sequentially; this is not a requirement and only used for illustrative purposes. The GCKS may create a complete tree as shown, or a partial tree which is created on demand as members join the group.¶
SA1(K_sa1)
+------------------------------+
1 2
+---------------+ +---------------+
3 4 5 6
+-------+ +-------+ +--------+ +--------+
A(7) B(8) C(9) D(10) E(11) F(12) G(13) H(14)
When GM A joins the group, the GCKS provides it with the keys in the KD payload of the GSA_AUTH or GSA_REGISTRATION exchange. Given the tree shown in figure above, the KD payload will be:¶
KD(GP(SA1)(1{K_sa1}),MP(3{1},7{3},GSK_w{7})
From these attributes the GM A will construct the Key Path K_sa1->1->3->7->GSK_w and since it ends up with GSK_w, it will use all the WRAP_KEY attributes present in the path as its Working Key Path: 1->3->7.¶
Similarly, when other GMs will be joining the group they will be provided with the corresponding keys, so after all the GMs will have the following Working Key Paths:¶
A: 1->3->7 B: 1->3->8 C: 1->4->9, D: 1->4->10 E: 2->5->11 F: 2->5->12 G: 2->6->13 H: 2->6->14¶
If the GCKS performs a simple SA rekey without changing group membership, it will only send group key bag in the KD payload with a new SA key encrypted with the default KWK.¶
KD(GP(SA2)(GSK_w{K_sa2}))
All the GMs will be able to decrypt it and no changes in their Working Key Paths will happen.¶
If the GKCS has reason to believe that a GM should be excluded, then it can do so by sending a GSA_REKEY message that includes a set of GM_KEY attributes which would allow all GMs except for the excluded one to get a new SA key.¶
In the example below the GCKS excludes GM F. For this purpose it changes the key tree as follows, replacing the key 2 with the key 15 and the key 5 with the key 16. It also generates a new SA key for a new SA3.¶
SA3(K_sa3)
+------------------------------+
1 15
+---------------+ +---------------+
3 4 16 6
+-------+ +-------+ +---- +--------+
A(7) B(8) C(9) D(10) E(11) F(12) G(13) H(14)
Then it sends the following KD payload for the new Rekey SA3:¶
KD(GP(SA3)(1{K_sa3},15{K_sa3}),MP(6{15},16{15},11{16})
While processing this KD payload:¶
GMs A, B, C and D will be able to decrypt the SA_KEY attribute 1{K_sa3} by using the "1" key from their key path. Since no new GM_KEY attributes are in the new Key Path, they won't update their Working Key Paths.¶
GMs G and H will construct new Key Path 15->6 and will be able to decrypt the intermediate key 15 using the key 6 from their Working Key Paths. So, they will update their Working Key Paths replacing their beginnings up to the key 6 with the new Key Path (thus replacing the key 2 with the key 15).¶
GM E will construct new Key Path 16->15->11 and will be able to decrypt the intermediate key 16 using the key 11 from its Working Key Path. So, it will update its Working Key Path replacing its beginnings up to the key 11 with the new Key Path (thus replacing the key 2 with the key 15 and the key 5 with the key 16).¶
GM F won't be able to construct any Key Path leading to any key he possesses, so it will be unable to decrypt the new SA key for the SA3 and thus it will be excluded from the group once the SA3 is used.¶
Finally, the GMs will have the following Working Key Paths:¶
A: 1->3->7 B: 1->3->8 C: 1->4->9, D: 1->4->10 E: 15->16->11 F: excluded G: 15->6->13 H: 15->6->14¶