| ace | H. Tschofenig |
| Internet-Draft | ARM Ltd. |
| Intended status: Standards Track | W. Werner |
| Expires: May 3, 2018 | Werner Management Services e.U. |
| October 30, 2017 |
Security for Low-Latency Group Communication
draft-tschofenig-ace-group-communication-security-00.txt
Some Internet of Things application domains require secure group communication. This draft describes procedures for authorization, key management, and securing group messages. We specify the usage of object security at the application layer for group communication and assume that CoAP is used as the application layer protocol. The architecture allows the usage of symmetric and asymmetric keys to secure the group messages. The asymmetric key solution provides the ability to uniquely authenticate the source of all group messages and this is the recommended architecture for most applications. However, some applications have strict requirements on latency for group communication (e.g. in non-emergency lighting applications) and it may not always be feasible to use the secure source authenticated architecture. In such applications we recommend the use of dynamically generated symmetric group keys to secure group communications.
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There are low latency group communication use cases that require securing communication between a sender, or a group of senders, and a group of receivers. In the lighting use case, a set of lighting nodes (e.g., luminaires, wall-switches, sensors) are grouped together into a single "Application Group" and the following three requirements need to be addressed:
This document discusses a group communication security solution that satisfies these three requirements. As discussed in Section 4, we recommend the usage of an asymmetric key solution that allows unique source authentication of all group messages. However, in situations where the low latency requirements can not be met (e.g. in non-emergency lighting applications), the alternative architecture discussed in Section 3 based on symmetric keys is recommended.
This document uses the following terms from [I-D.ietf-ace-actors]: Authorization Server, Resource Owner, Client, Resource Server. The terms 'sender' and 'receiver' refer to the application layer messaging used for lighting control; other communication interactions with the supporting infrastructure uses unicast messaging.
When nodes are combined into groups there are different layers of those groups with unique characteristics. For clarity we introduce terminology for three different groups:
Typically, the four groups might not coincide due to the memory constraints on the devices and also security considerations. For instance, in a small room with windows, we may have three application groups: "room group", "luminaires close to the window group" and "luminaires far from the window group". However, we may choose to use only one multicast group for all devices in the room and one security group for all the devices in the room. Note that every application group belongs to a unique security group. However, the converse is not always true. This implies that the application group ID maybe used to determine the associated security group but not vice versa.
The fact that security groups may not coincide with application groups implies that
In this document we provide fields that may be used to specify the "scope of the key" and "application groups for which the key may be used". A commissioner has a lot of flexibility to assign nodes to multicast groups and to security groups while the application groups will be determined by the semantics of the application itself. The exact partitioning of the nodes into security and multicast groups is therefore deployment specific.
Each node in a lighting application group might be a sender, a receiver or both sender and receiver (even though in Figure 1, we show nodes that are only senders or only receivers for clarity). The low latency requirement implies that most of the communication between senders and receivers of application layer messages is done using multicast IP. On some occasions, a sender in a group will be required to send unicast messages to unique receivers within the same group and these unicast messages also need communication security.
Two logical entities are introduced and they have the following function:
Note that we assume that nodes are pre-configured with device credentials (e.g., a certificate and the corresponding private key) during manufacturing or during an initial provisioning phase. These device credentials are used in the interaction with the authorization server.
Figure 1 and Figure 2 provide an architectural overview. The dotted lines illustrate the use of unicast DTLS messages for securing the message exchange between all involved parties. The secured group messages between senders and receivers are indicated using lines with star/asterisk characters. The security of the group messages is accomplished at the application level using small modification to OSCOAP - Object Security of CoAP (see [I-D.selander-ace-object-security]) which are to be defined.
Figure 1 illustrates the information flow between an authorization server and the nodes participating in the lighting network, which includes all nodes that exchange lighting application messages. This step is typically executed during the commissioning phase for nodes that are fixed-mounted in buildings. The authorization server, as a logical function, may in smaller deployments be included in a device carried by the commissioner and only be present during the commissioning phase. Other use cases, such as employees using their smartphones to control lights, may require an authorization server that dynamically executes access control decisions.
Figure 1 shows the commissioning phase where the nodes obtain configuration information, which includes the AT-KDC. The AT-KDC is an access token and includes authorization claims for consumption by the key distribution center. We use the access token terminology from [RFC6749]. The AT-KDC in this architecture may be a bearer token or a proof-of-possession (PoP) token. The bearer token concept is described in [RFC6750] and the PoP token concept is explained in [I-D.ietf-oauth-pop-architecture]. The AT-KDC is created by the authorization server after authenticating the requesting node and contains authorization-relevant information. The AT-KDC is protected against modifications using a digital signature or a message authentication code. It is verified in Figure 2 by the KDC.
Config +-------------+ Config
+-----------+Authorization+------------+
| .........>| Server |<.......... |
| . DTLS +-------------+ DTLS . |
| . ^^ . |
| . |. . |
| . |. . |
v v |. v v
+-----+ Config|.DTLS +-----+
+-----+| |. +-----+|
+-----+|+ |. +-----+|+
| A |+ vv | C |+
+-----+ +-----+ +-----+
. E.g. +-----+| E.g.
Light +-----+|+ Luminaires
Switches | B |+
+-----+
E.g.
Presence
Sensors
Legend:
Config (Configuration Data): Includes configuration
parameters, authorization information encapsulated
inside the access token (AT-KDC) and other meta-
data.
Figure 1: Architecture: Commissioning Phase.
In the simplified message exchange shown in Figure 2 a sender requests a security group key and the access token for use with the receivers (called AT-R). The request contains information about the resource it wants to access, such as the application group and other resource-specific information, if applicable, and the previously obtained AT-KDC access token. Once the sender has successfully obtained the requested information it starts communicating with receivers in that group using group messages. The symmetric key obtained from the KDC is used to secure the groups messages. The AT-R may be attached to the initial request.
Receivers need to perform two steps, namely to obtain the necessary group key to verify the incoming messages and to determine what resource the requestor is authorized to access. Both pieces of information can be found in the AT-R access token.
Group messages need to be protected such that replay and modification can be detected. The integrity of the message is accomplished using a keyed message digest in combination with the group key. The use of symmetric keys is envisioned in this specification due to latency requirements. For unicast messaging between the group members and the AS or KDC, we assume the use of DTLS for transport security. However, the use of TLS, and application layer security is possible but is outside the scope of this document.
Request Request
+AT-KDC +------------+ +AT-KDC
+------------>| Key |<----------+
|+------------|Distribution|----------+|
||Reply | Center | Reply ||
||+AT-R +------------+ +AT-R ||
||+Group ..^ ^.. +Group ||
|| Key .. .. Key ||
|| ...DTLS DTLS .. ||
|v .. .. v|
+-----+<. .>+-----+
+-----+| +-----+|
+-----+|+ Secure Multicast Msg +-----+|+
| A |+*****************************> | B |+
+-----+ +-----+
Sender(s) Receiver(s)
e.g. Light Switch e.g. Luminaires
Figure 2: Architecture: Group Key Distribution Phase.
The AT-KDC contains
Clients send the AT-KDC to the KDC in order to receive an AT-R.
The KDC MUST maintain a table consisting of scope values, which includes the application group id. These entries point to a sequence of security associations. A security association specifies the key material, algorithm-specific information, lifetime and a key ID and the key ID may be used to identify this security association.
The AS/KDC must guarantee the uniqueness of the client ids for its nodes. This may be accomplished by the AS/KDC assigning values to the nodes or by using information that is already unique per device (such as an EUI-64).
The KDC furthermore needs to be configured with information about the authorization servers it trusts. This may include a provisioned trust anchor store, or shared credentials (similar to a white list).
The KDC MUST generate new group keys after the validity period of the current group key expires.
The AT-R contains
The following information is needed for the cryptographic algorithm, which is assumed to be in the COSE header:
The following information is additionally required to process the secure message:
All receiving devices MUST maintain a table consisting of mappings of application group id, to a sequence of security associations.
When a node receives an incoming multicast message it looks up the application group id and the key id (which are both found in the CoAP header) to determine the correct security association.
The key id is used for situations where the group key is updated by the KDC (for example in situations where a device in a group is lost or stolen).
To check for replay attacks the receiver has to consult the state stored with the security association to obtain the current sequence number and to compare it against the sequence number found in the request payload for that sender based on the Sender ID. The receiver needs to store the latest correctly verified nonce values to detect replay attacks
The receiver MUST silently discard an incoming message in the following cases:
Once the cryptographic processing of the message is completed, the receiver must check whether the sender is authorized to access the protected resource, indicated by the CoAP request URI at the right level. For this purpose the receiver consults the locally stored authorization database that was populated with the information obtained via the AT-R token and the static authorization levels described in Appendix A.
Once all verification steps have been successful the receiver executes the CoAP request and returns an appropriate response. Since the response message will also be secured the message protection processing described in Section 3.6 must be executed. Additionally, the nonce value corresponding to the security association MUST be updated to the nonce value in the message.
Figure 3 describes the algorithm for obtaining the necessary credentials to transmit a secure group message. When the sender wants to send a message to the application group, it checks if it has the respective group key. If no group key is available then it determines whether it has an access token for use with the KDC (i.e., AT-KDC). If no AT-KDC is found in the cache then it contacts the authorization server to obtain that AT-KDC. Note that this assumes that the authorization server is online, which is only true in scenarios where granting authorization dynamically is supported. In the other case where the AT-KDC is already available the sender contacts the KDC to obtain a group key. If a group key is already available then the sender can transmit a secured message to the group immediately.
_______
/ \
| Start |
\_______/
|
v
/\
/ \
/ \
/ \
/ \
___No____/Group Key \____
| \Available?/ |
| \ / |
v \ / Yes
/\ \ / |
/ \ \ / v
/ \ \/ +-------------+
/ \ ^ |Transmit |
/ \ | |multicast |
____/ AT+KDC \__ | |mesg to group|
| \Available?/ | | +-------------+
| \ / | |
No \ / Yes |
| \ / | |
| \ / | |
v \/ v |
+-----+-----+ ^ +----------+ |
|Request | | |Request | |
|AT-KDC | | |Group Key | |
|from |---+ |from KDC |--+
|Auth Server| | |
+-----------+ +----------+
Figure 3: Steps to Transmit Multicast Message (w/o Failure Cases).
Note that the sender does not have to wait until it has to transmit a message in order to request a group key; the sender is likely to be pre-configured with information about which application group it belongs to and can therefore pre-fetch the required information.
Group keys have a lifetime, which is configuration-dependent, but mechanisms need to be provided to update the group keys either via the sender asking for a group key renewal or via the KDC pushing new keys to senders and receivers. The lifetime can be based on time or on the number of transmitted messages.
This section discusses the usage of asymmetric keys to achieve source authentication of group messages and is the recommend architecture for securing group messages. However, this solution may not meet the low latency requirement without adequate hardware support but still most of the group communication between senders and receivers of application layer messages is done using multicast IP.
Unlike the previous architecture, the current architecture requires only the Authorization Server (AS) logical entity as defined in the previous section.
As in the previous case we assume that nodes are pre-configured with device credentials (e.g., a certificate and the corresponding private key) during manufacturing or during an initial provisioning phase. These device credentials are used in the interaction with the authorization server.
Figure 4 and Figure 5 provide an architectural overview for the source authenticated case. The main differences from the previous case is that the AS provides directly the AT-R tokens. Further no KDC is required in this case since the senders and receivers can use their public-private key pair credentials to secure messages. The AS may provide authorization based on the pre-existing device credentials or issue new credentials to the devices. The security of the group messages is accomplished at the application level using small modification to OSCOAP - Object Security of CoAP (see [I-D.selander-ace-object-security]) but based on public key signatures which are to be defined.
Figure 4 illustrates the information flow between an authorization server and the nodes participating in the source-authenticated group network. Like the previous case, this step is typically executed during the commissioning phase for nodes that are fixed-mounted in buildings. The authorization server, as a logical function, may in smaller deployments be included in a device carried by the commissioner and only be present during the commissioning phase. Other use cases, such as employees using their smartphones to control lights, may require an authorization server that dynamically executes access control decisions.
Figure 4 shows the commissioning phase where the nodes obtain configuration information, which includes directly the AT-R. The AT-R is an access token and includes authorization claims for consumption by the receivers. The AT-R may be a bearer token or a proof-of-possession (PoP) token. The AT-R is created by the authorization server after authenticating the requesting node and contains authorization-relevant information. The AT-R is protected against modifications using a digital signature. It is verified in Figure 5 by the receivers.
Config +-------------+ Config
+-----------+Authorization+------------+
| .........>| Server |<.......... |
| . DTLS +-------------+ DTLS . |
| . ^^ . |
| . |. . |
| . |. . |
v v |. v v
+-----+ Config|.DTLS +-----+
+-----+| |. +-----+|
+-----+|+ |. +-----+|+
| A |+ vv | C |+
+-----+ +-----+ +-----+
. E.g. +-----+| E.g.
Light +-----+|+ Luminaires
Switches | B |+
+-----+
E.g.
Presence
Sensors
Legend:
Config (Configuration Data): Includes configuration
parameters, authorization information encapsulated
inside the access token (AT-R) and other meta-
data.
Figure 4: Architecture - Source-authenticated: Commissioning Phase.
In the simplified message exchange shown in Figure 5 a sender starts communicating with receivers in that source-authenticated group using public-key signed group messages. The AT-R may be attached to the initial request.
Receivers need to perform two steps, namely to obtain the necessary public verification key of the senders (or a root verification key if they are certified by the same authority) to verify the incoming messages and the public verification key of the AS to determine what resource the requestor is authorized to access. Both pieces of information can either be found in the AT-R access token or separately configured during the commissioning phase.
Source-authenticated Group messages also need to be protected such that replay and modification can be detected. The integrity of the message is accomplished using a public-key signature. This may not achieve the latency requirements and used where source-authentication is more important. For unicast messaging between the group members and the AS , we assume the use of DTLS for transport security.
+-----+ +-----+
+-----+| +-----+|
+-----+|+ Secure Multicast Msg +-----+|+
| A |+*****************************> | B |+
+-----+ +-----+
Sender(s) Receiver(s)
e.g. Light Switch e.g. Luminaires
Figure 5: Architecture - Source-authenticated: Group communication.
The AT-R contains
The following information is needed for the cryptographic algorithm, which is assumed to be in the COSE header:
The following information is additionally required to process the secure message:
When a node receives an incoming multicast message it looks up the application group id and the key id (which are both found in the CoAP header) to determine the correct security association to use to verify the message.
The key id is used for situations where the client may have different keys for different applications.
To check for replay attacks the receiver has to consult the state stored with the security association to obtain the current sequence number and to compare it against the sequence number found in the request payload for that sender based on the Sender ID. The receiver needs to store the latest correctly verified nonce values to detect replay attacks
The receiver MUST silently discard an incoming message in the following cases:
Once the cryptographic processing of the message is completed, the receiver must check whether the sender is authorized to access the protected resource, indicated by the CoAP request URI at the right level. For this purpose the receiver consults the locally stored authorization database that was populated with the information obtained via the AT-R token and the static authorization levels described in Appendix A.
Once all verification steps have been successful the receiver executes the CoAP request and returns an appropriate response. Since the response message will also be secured the message protection processing described in Section 3.6 must be executed. Additionally, the nonce value corresponding to the security association MUST be updated to the nonce value in the message.
Figure 6 describes the algorithm for obtaining the necessary credentials to transmit a source-authenticated secure group message. When the sender wants to send a message to the application group, it checks if it has the respective signing key that matches the KID in the AT-R. If no signing key is available then it contacts the authorization server to obtain the AT-R and corresponding signing keys. Note that this assumes that the authorization server is online, which is only true in scenarios where granting authorization dynamically is supported.
_______
/ \
| Start |
\_______/
|
v
/ \
/ \
/ \
/ \
/ \
___No____/Signing Key\____
| \Available? / |
| \ / |
| \ / Yes
| \ / |
| \ / v
v \ / +-------------+
+-----------+ ^ |Transmit |
|Request | | |multicast |
|AT-R | | |mesg to group|
|from |------+ +-------------+
|Auth Server|
+-----------+
Figure 6: Steps to Transmit Source-authenticated Multicast Message (w/o Failure Cases).
Note that the sender does not have to wait until it has to transmit a message in order to request a AT-R; the sender is likely to be pre-configured with information about which application group it belongs to and can therefore pre-fetch the required information.
This document describes two architectures based on symmetric group keys in Section 3 and asymmetric keys in Section 4.
The symmetric key solution is based on a group key that is shared between all group members including senders and receivers. As all members of the group posses the same key, it is only possible to authenticate group membership for the source of a message. In particular, it is not possible to authenticate the unique source of a message and consequently it is not possible to authorize a single node to control a group. Moreover, because the group key is shared across multiple nodes, it may be easier for an attacker to determine the group key by attacking any member of the group (note that this group key is dynamically generated and is usually stored in volatile memory which offers some addition protection). Subsequent to such an attack, it is also difficult to determine which of the group members was compromised and this makes it difficult to return the system to normal operation after an attack.
The asymmetric key solution distinguishes between a sender in the group and the receivers. In particular, the sender is in possession of a private key and the receivers are in possession of the corresponding public key. This allows the unique source of any group message to be authenticated. Moreover, an attacker cannot compromise the system by breaking into any of the receiving nodes. However, for constrained devices, the asymmetric key solution comes at a processing cost with cryptographic computations taking too long.
Therefore, it is recommended that whenever possible, the architecture with source authentication SHOULD be used to secure all multicast communication. However, in less sensitive applications (e.g. controlling luminaires in non-emergency applications), the architecture without source authentication MAY be used. When using the symmetric key solution two mitigating factors could improve system security. It is possible to achieve source authentication of messages at lower layers by requiring unique MAC layer keys for all devices within the network. The symmetric group keys are dynamically generated and therefore SHOULD be stored in volatile memory.
Due to the low latency requirements, token verification needs to be done locally and cannot be outsourced to other parties. For this reason a self-contained token must be used and the receivers are required to follow the steps outlined in Section 7.2 of RFC 7519 [RFC7519]. This includes the verification of the message authentication code protecting the contents of the token and the encryption envelope protecting the contained symmetric group key.
Tokens have a specific lifetime. Setting the lifetime is a policy decision that involves making a trade-off decision. Allowing a longer lifetime increases the need to introduce a mechanism for token revocation (e.g., a real-time signal from the KDC/Authorization Server to the receivers to blacklist tokens) but lowers the communication overhead during normal operation since new tokens need to be obtained only from time to time. Real-time communication with the receivers to revoke tokens may not be possible in all cases either, particularly when off-line operation is demanded or in small networks where the AS or even the KDC is only present during commissioning time.
We therefore recommend to issue short-lived tokens for dynamic scenarios like users accessing the lighting infrastructure of buildings using smartphones, tablets and alike to avoid potential security problems when tokens are leaked or where authorization rights are revoked. For senders that are statically mounted (like traditional light switches) we recommend a longer lifetime since re-configurations and token leakage is less likely to happen frequently.
To limit the authorization rights, tokens should contain an audience restriction, scoping their use to the intended receivers and to their access level.
Senders and receivers are not assumed to be equipped with real-time clocks but these devices are still assumed to interact with a time server. The lack of accurate clocks is likely to lead to clock drifts and limited ability to check for replays. For those cases where no time server is available, such as in small network installations, token verification cannot check for expired tokens and hence it might be necessary to fall-back to tokens that do not expire.
Devices in the lighting system can often be powered down intentionally or unintentionally. Therefore the devices may need to store the authorization tokens and cryptographic keys (along with replay context) in persistent storage like flash. This is especially required if the authorization server is no more online because it was removed after the commissioning phase. However the decision on the data to be persistently stored is a trade-off between how soon the devices can be back online to normal operational mode and the memory wear caused due to limited program-erase cycles of flash over the 15-20 years life-time of the device.
The different data that may need to be stored are access tokens AT-KDC, AT-R and last seen replay counter.
In small networks the authorization server and the KDC may be available only temporarily during the commissioning process and are not available afterwards.
A single device should not be managed by multiple KDCs. However, a group of devices in a domain (such as a lighting installation within an enterprise) should either be managed by a single KDC or, if there are multiple KDCs serving the devices in a given domain, these KDCs MUST exchange information so that the assigned client id and application group id values are unique within the devices in that domain. We assume that only devices within a given domain communicate with each other using group messages.
Multiple application groups may use the same key for performance reasons, reducing the number of keys needed to be stored - leading to less RAM needed by each node. This is only a reasonable option if the attack surface is not increased. For example, a room A is configured to use three application groups to address a subset of the device. In addition to configuring all nodes in room A with these three application groups the nodes are configured with a special group that allows them to access all devices in room A, referred as the all-nodes-in-room-A group. In this case, having the nodes to use the same key for the all-nodes-in-room group and the three groups does not increase the attack surface since any node can already use the all-nodes-in-room-A group to control other devices in that room. The three application groups in room A are a subset of the larger all-nodes-in-room-A group.
The following procedure MUST be implemented if a device is stolen or keys are lost.
The KDC SHOULD inform all devices in the security group to update their group key. This requires the KDC to maintain a list of all devices that belong to the security group and to be able to contact them reliably.
The author would like to thank Esko Dijk for his help with this document.
Parts of this document are a byproduct of the OpenAIS project, partially funded by the Horizon 2020 programme of the European Commission. It is provided "as is" and without any express or implied warranties, including, without limitation, the implied warranties of fitness for a particular purpose. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the OpenAIS project or the European Commission.
This document defines one CoAP Header Option Application Group ID that MUST be allocated in the Registry "CoAP Option Numbers" of [RFC6749]. IANA is requested to allocation TBD option number to application group ID in this specification.
We would like to thank our former co-authors, Abhinav Somaraju and Sandeep Kumar for their contributions to earlier versions of the draft.
| [I-D.ietf-ace-actors] | Gerdes, S., Seitz, L., Selander, G. and C. Bormann, "An architecture for authorization in constrained environments", Internet-Draft draft-ietf-ace-actors-05, March 2017. |
| [I-D.ietf-ace-cbor-web-token] | Jones, M., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "CBOR Web Token (CWT)", Internet-Draft draft-ietf-ace-cbor-web-token-09, October 2017. |
| [I-D.ietf-cose-msg] | Schaad, J., "CBOR Object Signing and Encryption (COSE)", Internet-Draft draft-ietf-cose-msg-24, November 2016. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014. |
| [I-D.ietf-oauth-pop-architecture] | Hunt, P., Richer, J., Mills, W., Mishra, P. and H. Tschofenig, "OAuth 2.0 Proof-of-Possession (PoP) Security Architecture", Internet-Draft draft-ietf-oauth-pop-architecture-08, July 2016. |
| [I-D.selander-ace-object-security] | Selander, G., Mattsson, J., Palombini, F. and L. Seitz, "Object Security of CoAP (OSCOAP)", Internet-Draft draft-selander-ace-object-security-06, October 2016. |
| [RFC6749] | Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012. |
| [RFC6750] | Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012. |
| [RFC7519] | Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015. |
A characteristic of the lighting domain is that access control decisions are also impacted by the type of operation being performed and those categories are listed below. The following access levels are pre-defined.
Note: The use of group security is disallowed for level higher than Level 2 and unicast communication is used instead.