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
Abstract
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.
Status of This Memo
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
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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 May 3, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Architecture - Group Authentication . . . . . . . . . . . . . 5
3.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. AT-KDC Access Tokens . . . . . . . . . . . . . . . . . . 9
3.3. AT-R Access Tokens . . . . . . . . . . . . . . . . . . . 9
3.4. Multicast Message Content . . . . . . . . . . . . . . . . 10
3.5. Receiver Algorithm . . . . . . . . . . . . . . . . . . . 11
3.6. Sender Algorithm . . . . . . . . . . . . . . . . . . . . 12
4. Architecture - source authentication . . . . . . . . . . . . 14
4.1. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 16
4.2. AT-R Access Tokens . . . . . . . . . . . . . . . . . . . 17
4.3. Multicast Message Content . . . . . . . . . . . . . . . . 17
4.4. Receiver Algorithm . . . . . . . . . . . . . . . . . . . 18
4.5. Sender Algorithm . . . . . . . . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 20
5.1. Applicability statement . . . . . . . . . . . . . . . . . 20
5.2. Token Verification . . . . . . . . . . . . . . . . . . . 21
5.3. Token Revocation . . . . . . . . . . . . . . . . . . . . 21
5.4. Time . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6. Operational Considerations . . . . . . . . . . . . . . . . . 22
6.1. Persistence of State Information . . . . . . . . . . . . 22
6.2. Provisioning in Small Networks . . . . . . . . . . . . . 23
6.3. Client IDs . . . . . . . . . . . . . . . . . . . . . . . 23
6.4. Application Groups vs. Security Groups . . . . . . . . . 23
6.5. Lost/Stolen Device . . . . . . . . . . . . . . . . . . . 23
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . 24
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10.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Access Levels . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
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:
1. Only authorized members of the application group must be able to
read and process messages.
2. Receivers of group messages must be able to verify the integrity
of received messages as being generated within the group.
3. Message communication and processing must happen with a low
latency and in synchronous manner.
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.
2. Terminology
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:
Application Group:
An application group consists of the set of all nodes that have
been configured to respond to a single application layer request.
For example, a wall mounted switch and a set of luminaires in a
single room might belong to a single group and the switch may be
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used to turn on/off all the luminaires in the group simultaneously
with a single button press. In the remainder of this document we
will use GId to identify an application group.
Multicast Group:
A multicast group consists of the set of all nodes that subscribe
to the same multicast IP address.
Security Group:
A security group consists of the set of all nodes that have been
provisioned with the same keying material. All the nodes within a
security group share a security association or a sequence of
security associations wherein a single association specifies the
keying material, algorithm-specific information, lifetime and a
key ID.
Source-authenticated Security Group:
A source-authenticated security group consists of the set of
receiver nodes that have been provisioned with the public
verification keying material of all the sender nodes and the set
of sender nodes that are provisioned with their unique private
signing keying material. All the nodes within a source-
authenticated security group share a security association or a
sequence of security associations wherein a single association
specifies the the public or private keying material, algorithm-
specific information, lifetime and a key ID.
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
(1) an application must be able to specify which resources on a
resource server are accessible by a client that has access to the
group key, and
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(2) a method is required to associate the group key to the
application group(s) for which the group key may be used.
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.
3. Architecture - Group Authentication
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:
Key Distribution Center (KDC): This logical entity is responsible
for generating symmetric keys and distributing them to the nodes
authorized to receive them. The KDC ensures that nodes belonging
to the same security group receive the same key and that the keys
are renewed based on certain events, such as key expiry or change
in group membership.
Authorization Server (AS): This logical entity stores authorization
information about devices, meta-data about them, and their roles
in the network. For example, a luminaire is associated with
different groups, and may have meta-data about its location in a
building.
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
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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.
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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
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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.
3.1. Assumptions
1. The AT-KDC is a manifestation of the authorization granted to a
specific client (or user running a client). The AT-KDC is
longer-lived and can be used to request multiple AT-Rs.
2. Each AT-R is valid for use with one or multiple application
groups.
3. The AS and the KDC logical roles may reside in different physical
entities.
4. The AT-KDC as well as the AT-R may be self-contained tokens or
references. References are more efficient from a bandwidth point
of view but require an additional lookup.
5. The AT-KDC token is opaque to the client. Data that is meant for
processing by the client has to be conveyed to the client
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separately. The AT-R token on the other hand is meant for
consumption by the client.
6. The client requests AT-Rs for different application groups by
including additional information in the request to the KDC for
what application groups the AT-R(s) have to be requested. The
KDC may return multiple AT-Rs in a single response (for
performance reasons).
7. The AT-KDC and the AT-R are encoded as CBOR Web Tokens
[I-D.ietf-ace-cbor-web-token] and protected using COSE
[I-D.ietf-cose-msg].
3.2. AT-KDC Access Tokens
The AT-KDC contains
1. Issuer: Entity creating the access token. This information needs
to be cryptographically bound to the digital signature/keyed
message digest protecting the content of the token, as provided
by the CBOR Web Token (CWT).
2. Expiry date: Information can be omitted if tokens do not expire
(for example, in a small enterprise environment).
3. Scope: Permissions of the entity holding the token. This
includes information about the resources that may be accessed
with the token (e.g., access level) and application layer group
IDs for the groups for which the tokens may be used.
4. Recipient/Audience: Indication to whom the AT-KDC was issued to.
In this case, it is the KDC.
5. Client ID: Information about the client that was authenticated by
the authorization server.
6. Issued at: Indicates date and time when the AT-KDC was created by
the authorization server.
3.3. AT-R Access Tokens
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.
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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
1. Issuer: Entity creating the access token. This information needs
to be cryptographically bound to the digital signature/keyed
message digest protecting the content of the token, as provided
by the CBOR Web Token (CWT).
2. Expiry date: Information can be omitted if tokens do not expire
(for example, in a small enterprise environment).
3. Scope: Permissions of the entity holding the token. This
includes information about the resources that may be accessed
with the token (e.g., access level) and application layer group
IDs for the groups for which the tokens may be used.
4. Security Group Key: Key to use for the group communication.
5. Algorithm: Used for secure group communication.
6. KID: Sequentially increasing ID of the key for the security group
(the devices may store an older key to help with key rolling.)
7. Issued at: Indicates date and time when the AT-R was created by
the KDC.
3.4. Multicast Message Content
The following information is needed for the cryptographic algorithm,
which is assumed to be in the COSE header:
1. Nonce value consisting of
* Client ID (unencrypted, integrity protected): Every sender
managed by a key distribution center MUST have a unique client
ID.
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* Sequence Number (unencrypted, integrity protected): Used for
replay protection.
* An implicit IV that is either derived from the keys at the
end-points or fixed to a certain value by standard (not sent
in the message)
2. MAC (not integrity protected): For integrity protection.
The following information is additionally required to process the
secure message:
1. Destination IP address and port (not encrypted, integrity
protected): Integrity protection of the IP address and port
ensures that the message content cannot be replayed with a
different destination address or on a different port.
2. CoAP Path (encrypted, integrity protected): Uniquely identifies
the target resource of a CoAP request.
3. Application Group id in CoAP header (unencrypted, integrity
protected): Is used to identify a sequence of security
associations to use to decrypt the message. The CoAP header
option is TBD.
4. Key ID (unencrypted, integrity protected): Is used to select the
current security association from the sequence of security
associations identified by the application group id.
5. CoAP Header Options other than application group id (encrypted -
if desired, integrity protected)
6. CoAP Payload (encrypted, integrity protected).
3.5. Receiver Algorithm
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).
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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:
o Application Group ID lookup does not return any security
association.
o Key ID lookup among the previously retrieved sequence of security
associations does not identify a unique security association.
o Integrity check fails.
o Decryption fails.
o Replay protection check failed. The (client ID || sequence
number), which are both part of the nonce, have already been
received in an earlier message.
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.
3.6. Sender Algorithm
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
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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
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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.
4. Architecture - source authentication
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
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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.
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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.
4.1. Assumptions
1. The AT-R is a manifestation of the authorization granted to a
specific client (or user running a client). The AT-R is longer-
lived and can be used directly for source-authenticated group
communication until it is revoked or expired.
2. Each AT-R is valid for use with one or multiple application
groups.
3. The AT-R may be self-contained tokens or references. References
are more efficient from a bandwidth point of view but require an
additional lookup.
4. The AT-R token is not opaque to the client and is meant for
consumption by the client.
5. The client requests AT-Rs for different application groups by
including additional information in the request to the AS for
what application groups the AT-R(s) have to be requested. The AS
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may return multiple AT-Rs in a single response (for performance
reasons).
6. The AT-R is encoded as CBOR Web Tokens
[I-D.ietf-ace-cbor-web-token] and protected using COSE
[I-D.ietf-cose-msg].
4.2. AT-R Access Tokens
The AT-R contains
1. Issuer: Entity creating the access token. This information needs
to be cryptographically bound to the digital signature/keyed
message digest protecting the content of the token, as provided
by the CBOR Web Token (CWT).
2. Expiry date: Information can be omitted if tokens do not expire
(for example, in a small enterprise environment).
3. Scope: Permissions of the entity holding the token. This
includes information about the resources that may be accessed
with the token (e.g., access level) and application layer group
IDs for the groups for which the tokens may be used.
4. Recipient/Audience: Indication to whom the AT-R was issued to.
In this case, it is the receivers.
5. Client ID: Information about the client that was authenticated by
the authorization server.
6. Client public key: The public key to use for signing the source-
authenticated group communication. These public key may be
optionally certified using the AS key or a domain root key. This
reduces the need for additional per-device public key storage on
the receivers.
7. Algorithm: Used for source-authenticated secure group
communication.
8. Issued at: Indicates date and time when the AT-R was created by
the authorization server.
4.3. Multicast Message Content
The following information is needed for the cryptographic algorithm,
which is assumed to be in the COSE header:
1. Nonce value consisting of
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* Client ID (unencrypted, integrity protected): Every sender
managed by the AS MUST have a unique client ID.
* Sequence Number (unencrypted, integrity protected): Used for
replay protection.
2. Signature (not integrity protected): For source-authenticated
integrity protection.
The following information is additionally required to process the
secure message:
1. Destination IP address and port (not encrypted, integrity
protected): Integrity protection of the IP address and port
ensures that the message content cannot be replayed with a
different destination address or on a different port.
2. CoAP Path (encrypted, integrity protected): Uniquely identifies
the target resource of a CoAP request.
3. Application Group id in CoAP header (unencrypted, integrity
protected): Is used to identify a sequence of security
associations to use to decrypt the message. The CoAP header
option is TBD.
4. Key ID (unencrypted, integrity protected): Is used to select the
correct security association containing the verification key from
the sequence of security associations identified by the
application group id.
5. CoAP Header Options other than application group id (encrypted -
if desired, integrity protected)
6. CoAP Payload (encrypted, integrity protected).
4.4. Receiver Algorithm
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
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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:
o Application Group ID lookup does not return any security
association.
o Key ID lookup among the previously retrieved sequence of security
associations does not identify a unique security association.
o Integrity check fails.
o Replay protection check failed. The (client ID || sequence
number), which are both part of the nonce, have already been
received in an earlier message.
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.
4.5. Sender Algorithm
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.
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_______
/ \
| 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.
5. Security Considerations
5.1. Applicability statement
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
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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.
5.2. Token Verification
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.
5.3. Token Revocation
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
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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.
5.4. Time
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.
6. Operational Considerations
6.1. Persistence of State Information
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.
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6.2. Provisioning in Small Networks
In small networks the authorization server and the KDC may be
available only temporarily during the commissioning process and are
not available afterwards.
6.3. Client IDs
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.
6.4. Application Groups vs. Security Groups
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.
6.5. Lost/Stolen Device
The following procedure MUST be implemented if a device is stolen or
keys are lost.
1. The AS tells the KDC to invalidate the AT-KDC.
2. The KDC no longer returns a new group key if the invalidated AT-
KDC is presented to it.
3. The KDC generates new keys for all security groups to which the
compromised device belongs.
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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.
7. Acknowledgements
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.
8. IANA Considerations
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.
9. Contributors
We would like to thank our former co-authors, Abhinav Somaraju and
Sandeep Kumar for their contributions to earlier versions of the
draft.
10. References
10.1. Normative References
[I-D.ietf-ace-actors]
Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
architecture for authorization in constrained
environments", draft-ietf-ace-actors-05 (work in
progress), March 2017.
[I-D.ietf-ace-cbor-web-token]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", draft-ietf-ace-cbor-web-token-09
(work in progress), October 2017.
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[I-D.ietf-cose-msg]
Schaad, J., "CBOR Object Signing and Encryption (COSE)",
draft-ietf-cose-msg-24 (work in progress), November 2016.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
10.2. Informative References
[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", draft-ietf-oauth-pop-architecture-08 (work
in progress), July 2016.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-selander-ace-
object-security-06 (work in progress), October 2016.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
Appendix A. Access Levels
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.
Level 0: Service detection only
This is a service that is used with broadcast service detection
methods. No operational data is accessible at this level.
Level 1: Reporting only
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This level allows access to sensor and other (relatively
uncritical) operational data and the device error status. The
operation of the system cannot be influenced using this level.
Level 2: Standard use
This level allows access to all operational features, including
access to operational parameters. This is the highest level of
access that can be obtained using (secure) multicast.
Level 3: Commissioning use / Parametrization Services
This level gives access to certain parameters that change the day-
to-day operation of the system, but does not allow structural
changes.
Level 4: Commissioning use / Localization and Addressing Services
(including Factory Reset) This level allows access to all services
and parameters including structural settings.
Level 5: Software Update and related Services
This level allows the change and upgrade of the software of the
devices.
Note: The use of group security is disallowed for level higher than
Level 2 and unicast communication is used instead.
Authors' Addresses
Hannes Tschofenig
ARM Ltd.
Hall in Tirol 6060
Austria
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Walter Werner
Werner Management Services e.U.
Josef-Anton-Herrburgerstr. 10
Dornbirn 6850
Austria
Email: werner@werner-ms.at
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