| Network Working Group | O. Friel |
| Internet-Draft | E. Lear |
| Intended status: Informational | M. Pritikin |
| Expires: January 3, 2019 | Cisco |
| M. Richardson | |
| Sandelman Software Works | |
| July 02, 2018 |
BRSKI over IEEE 802.11
draft-friel-brski-over-802dot11-01
This document outlines the challenges associated with implementing Bootstrapping Remote Secure Key Infrastructures over IEEE 802.11 and IEEE 802.1x networks. Multiple options are presented for discovering and authenticating to the correct IEEE 802.11 SSID. This initial draft is a discussion document and no final recommendations are made on the recommended approaches to take.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 3, 2019.
Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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Bootstrapping Remote Secure Key Infrastructures (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra] describes how a device can bootstrap against a local network using an Initial Device Identity X.509 [IEEE802.1AR] IDevID certificate that is pre-installed by the vendor on the device in order to obtain an [IEEE802.1AR] LDevID. The BRSKI flow assumes the device can obtain an IP address, and thus assumes the device has already connected to the local network. Further, the draft states that BRSKI use of IDevIDs:
The draft does not describe any mechanisms for how an [IEEE802.11] enabled device would discover and select a suitable [IEEE802.11] SSID when multiple SSIDs are available. A typical deployment scenario could involve a device begin deployed in a location were twenty or more SSIDs are being broadcast, for example, in a multi-tenanted building or campus where multiple independent organizations operate [IEEE802.11] networks.
In order to reduce the administrative overhead of installing new devices, it is desirable that the device will automatically discover and connect to the correct SSID without the installer having to manually provision any network information or credentials on the device. It is also desirable that the device does not discover, connect to, and automatically enroll with the wrong network as this could result in a device that is owned by one organization connecting to the network of a different organization in a multi-tenanted building or campus.
Additionally, as noted above, the BRSKI draft does not describe how BRSKI could potentially align with [IEEE802.1X] authentication mechanisms.
This document outlines multiple different potential mechanisms that would enable a bootstrapping device to choose between different available [IEEE802.11] SSIDs in order to execute the BRSKI flow. This document also outlines several options for how [IEEE802.11] networks enforcing [IEEE802.1X] authentication could enable the BRSKI flow, and describes the required device behaviour.
This document presents both [IEEE802.11] mechanisms and Wi-Fi Alliance (WFA) mechanisms. An important consideration when determining what the most appropriate solution to device onboarding should be is what bodies need to be involved in standardisation efforts: IETF, IEEE and/or WFA.
IEEE 802.11u: an amendment to the IEEE 802.11-2007 standard to add features that improve interworking with external networks.
ANI: Autonomic Networking Infrastructure
ANQP: Access Network Query Protocol
AP: IEEE 802.11 Access Point
CA: Certificate Authority
EAP: Extensible Authentication Protocol
EST: Enrollment over Secure Transport
HotSpot 2.0 / HS2.0: An element of the Wi-Fi Alliance Passpoint certificatoin program that enables cell phones to automatically discover capabilities and enroll into IEEE 802.11 guest networks (hotspots).
IE: Information Element
IDevID: Initial Device Identifier
LDevID: Locally Significant Device Identifier
OI: Organization Identifier
MASA: BRSKI Manufacturer Authorized Signing Authority service
SSID: IEEE 802.11 Service Set Identifier
STA: IEEE 802.11 station
WFA: Wi-Fi Alliance
WLC: Wireless LAN Controller
WPA/WPA2: Wi-Fi Protected Access / Wi-Fi Protected Access version 2
WPS: Wi-Fi Protected Setup
As will be seen in the following sections, there are several discovery scenarios where the device can choose an incorrect SSID and attempt to join the wrong network. For example, the device is being deployed by one organization in a multi-tenant building, and chooses to connect to the SSID of a neighbor organization. The device is dependent upon the incorrect network rejecting its BRSKI enrollment attempt. It is possible that the device could end up enrolled with the wrong network.
BRSKI allows optional sales channel integration which could be used to ensure only the “correct” network can claim the device. In theory, this could be achieved if the BRSKI MASA service has explicit knowledge of the network where every single device will be deployed. After connecting to the incorrect SSID and possibly authenticating to the network, the device would present network TLS information in its voucher-request, and the MASA server would have to reject the request based on this network TLS information and not issue a voucher. The device could then reject that SSID and attempt to bootstrap against the next available SSID.
This could possibly be acheieved via sales channel integration, where devices are tracked through the supply chain all the way from manufacturer factory to target deployment network operator. In practice, this approach may be challenging to deploy as it may be extremely difficult to implement this tightly coupled sales channel integration and ensure that the MASA actually has accurate deployment network information.
An alternative to sales channel integration is to provide the device owners with a, possibly authenticated, interface or API to the MASA service whereby they would have to explicitly claim devices prior to the MASA issuing vouchers for that device. There are similar problems with this approach, as there could be a complex sales and channel partner chain between the MASA service operator and the device operator who owns and deploys the device. This could make exposure of APIs by the MASA operator to the device operator untenable.
If a device connects to the wrong network, the correct network operator could detect this after the fact by integration with MASA and checking audit logs for the device. The MASA audit logs should indicate all networks that have been issued vouchers for a specific device. This mechanism also relies on the correct network operater having a list, bill or materials, or similar of all device identities that should be connecting to their network in order to check MASA logs for devices that have not come online, but are known to be physically deployed.
An obvious mechanism is to rely on network administrators to be good citizens and explicitly reject devices that attempt to bootstrap against the wrong network. This is not guaranteed to work for two main reasons:
Protocols such as the WFA Device Provisioning Profile [DPP] require that a network provisoining entity demonstrate knowledge of device information such as the device’s bootstrapping public key prior to the device attempting to connect to the network. This gives a higher level of confidence to the device that it is connecting to the correct SSID. These mechanisms could leverage a key that is printed on the device label, or included in a sales channel bill of materials. The security of these types of key distribution mechanisms relies on keeping the device label or bill of materials content from being compromised prior to device installation.
[IEEE802.11i] allows an SSID to advertise different authentication mechanisms via the AKM Suite list in the RSNE. A very brief introduction to [IEEE802.11i] is given in the appendices. An SSID could advertise PSK or [IEEE802.1X] authentication mechanisms. When a network operator needs to enforce two different authentication mechanisms, one for pre-BRSKI devices and one for post-BRSKI devices, the operator has two options:
If devices have to be flexible enough to handle both options, then this adds complexity to the device firmware and internal state machines. Similarly, if network infrastructure (APs, WLCs, AAAs) potentially needs to support both options, then this adds complexity to network infrastructure configuration flexibility, software and state machines. Consideration must be given to the practicalities of implementation for both devices and network infrastructure when designing the final bootstrap mechanism and aligning [IEEE802.11], [IEEE802.1X] and BRSKI protocol interactions.
Devices should be flexible enough to handle potential options defined by any final draft. When discovering a pre-BRSKI SSID, the device should also discover the authentication mechanism enforced by the SSID that is advertising BRSKI support. If the device supports the authentication mechanism being advertised, then the device can connect to the SSID in order to initiate the BRSKI flow. For example, the device may support [IEEE802.1X] as a pre-BRSKI authentication mechanism, but may not support PSK as a pre-BRSKI authentication mechanism.
Once the device has completed the BRKSI flow and has obtained an LDevID, a mechanism is needed to tell the device which SSID to use for post-BRSKI network access. This may be a different SSID to the pre-BRSKI SSID. The mechanism by which the post-BRSKI SSID is advertised to the device is out-of-scope of this version of this document.
If a device has to perform two different authentications, one for pre-BRSKI and one for post-BRSKI, network policy will typically assign the device to different VLANs for these different stages, and may assign the device different IP addresses depending on which network segment the device is assigned to. This could be true even if a single SSID is used for both pre-BRSKI and post-BRSKI connections. Therefore, the bootstrapping device may need to completely reset its network connection and network software stack, and obtain a new IP address between pre-BRSKI and post-BRSKI connections.
When evaluating all possible SSID discovery mechanism and authentication mechanisms outlined in this document, consideration must be given to the complexity of the required client and server implementation and state machines. Consideration must also be given to the network operator configuration complexity if multiple permutations and combinations of SSID discovery and network authentication mechanisms are possible.
This section outlines multiple different mechanisms that could potentially be leveraged that would enable a bootstrapping device to choose between multiple different available [IEEE802.11] SSIDs. As noted previously, this draft does not make any final recommendations.
The discovery options outlined in this document include:
These mechanisms are described in more detail in the following sections.
A standardized naming convention for SSIDs offering BRSKI services is defined such as:
Where:
Device manufacturers would bake the well-known prefix string and character delimiter into device firmware. Network operators configuring SSIDs which offer BRSKI services would have to ensure that the SSID of those networks begins with this prefix. On bootstrap, the device would scan all available SSIDs and look for ones with this given prefix.
If multiple SSIDs are available with this prefix, then the device could simply round robin through these SSIDs and attempt to start the BRSKI flow on each one in turn until it succeeds.
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against an incorrect network.
Another issue with defining a specific naming convention for the SSID is that this may require network operators to have to deploy a new SSID. In general, network operators attempt to keep the number of unique SSIDs deployed to a minimum as each deployed SSID eats up a percentage of available air time and network capacity. A good discussion of SSID overhead and an SSID overhead [calculator] is available.
[IEEE802.11aq] is currently being worked by the IEEE, but is not yet finalized, and is not yet supported by any vendors in shipping product. [IEEE802.11aq] defines new elements that can be included in [IEEE802.11] Beacon, Probe Request and Probe Response frames, and defines new elements for ANQP frames.
The extensions allow an AP to broadcast support for backend services, where allowed services are those registered in the [IANA] Service Name and Transport Protocol Port Number Registry. The services can be advertised in [IEEE802.11] elements that include either:
Bloom filters simply serve to reduce the size of Beacon and Probe Response frames when a large number of services are advertised. If a bloom filter is used by the AP, and a device discovers a potential service match in the bloom filter, then the device can query the AP for the full list of service name hashes using newly defined ANQP elements.
If BRSKI were to leverage [IEEE802.11aq], then the [IEEE802.11aq] specification would need to be pushed and supported, and a BRSKI service would need to be defined in [IANA].
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against an incorrect network.
[IEEE802.11] defines Information Element (IE) number 221 for carrying Vendor Specific information. The purpose of this document is to define an SSID discovery mechanism that can be used across all devices and vendors, so use of this IE is not an appropriate long term solution.
[IEEE802.11u] defines mechanisms for interworking. An introduction to [IEEE802.11u] is given in the appendices. Existing IEs in [IEEE802.11u] include:
These existing IEs could be used to advertise a well-known, logical service that devices implicitly know to look for.
In the case of NAI Realm, a well-known service name such as “_bootstrapks” could be defined and advertised in the NAI Realm IE. In the case of Roaming Consortium, a well-known Organization Identifier (OI) could be defined and advertised in the Roaming Consortium IE.
Device manufacturers would bake the well-known NAI Realm or Roaming Consortium OI into device firmware. Network operators configuring SSIDs which offer BRSKI services would have to ensure that the SSID offered this NAI Realm or OI. On bootstrap, the device would scan all available SSIDs and use ANQP to query for NAI Realms or Roaming Consortium OI looking for a match.
The key concept with this proposal is that BRSKI uses a well-known NAI Realm name or Roaming Consortium OI more as a logical service advertisement rather than as a backhaul internet provider advertisement. This is conceptually very similar to what [IEEE802.11aq] is attempting to achieve.
Leveraging NAI Realm or Roaming Consortium would not require any [IEEE802.11] specification changes, and could possibly be defined by this IETF draft. Note that the authors are not aware of any currently defined IETF or IANA namespaces that define NAI Realms or OIs.
Additionally (or alternatively…) as NAI Realm includes advertising the EAP mechanism required, if a new EAP-BRSKI were to be defined, then this could be advertised. Devices could then scan for an NAI Realm that enforced EAP-BRSKI, and ignore the realm name.
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against an incorrect network.
Additionally, as the IEEE is attempting to standardize logical service advertisement via [IEEE802.11aq], [IEEE802.11aq] would seem to be the more appropriate option than overloading an existing IE. However, it is worth noting that configuration of these IEs is supported today by WLCs, and this mechanism may be suitable for demonstrations or proof-of-concepts.
It is possible that an SSID may be configured to provide unrestricted and unauthenticated internet access. This could be advertised in the Interworking Information IE by including:
If such a network were discovered, a device could attempt to use the BRSKI well-known vendor cloud Registrar. Possibly this could be a default fall back mechanism that a device could use when determining which SSID to use.
Of the various elements currently defined by [IEEE802.11u] for potentially advertising BRSKI, NAI Realm and Roaming Consortium IE are the two existing options that are a closest fit, as outlined above. Another possibility that has been suggested in the IETF mailers is defining an extension to [IEEE802.11u] specifically for advertising BRSKI service capability. Any extensions should be included in Beacon and Probe Response frames so that devices can discover BRSKI capability without the additional overhead of having to explicitly query using ANQP.
[IEEE802.11aq] appears to be the proposed mechanism for generically advertising any service capability, provided that service is registered with [IANA]. It is probably a better approach to encourage adoption of [IEEE802.11aq] and register a service name for BRSKI with [IANA] rather than attempt to define a completely new BRSKI-specific [IEEE802.11u] extension.
Wi-Fi Protected Setup (WPS) only works with Wi-Fi Protected Access (WPA) and WPA2 when in Personal Mode. WPS does not work when the network is in Enterprise Mode enforcing [IEEE802.1X] authentication. WPS is intended for consumer networks and does not address the security requirements of enterprise or IoT deployments.
[IEEE802.11i] introduced the RSNE element which allows an SSID to advertise multiple authentication mechanisms. A new Authentication and Key Management (AKM) Suite could be defined that indicates the STA can use BRSKI mechanisms to authenticate against the SSID. The authentication handshake could be an [IEEE802.1X] handshake, possibly leveraging an EAP-BRSKI mechanism, the key thing here is that a new AKM is defined and advertised to indicate the specific BRSKI-capable EAP method that is supported by [IEEE802.1X], as opposed to the current [IEEE802.1X] AKMs which give no indication of the supported EAP mechanisms. It is clear that such method would limit the SSID to BRSKI-supporting clients. This would require an additional SSID specifically for BRSKI clients.
The [DPP] specification defines how an entity that is already trusted by a network can assist an untrusted entity in enrolling with the network. The description below assumes the [IEEE802.11] network is in infrastructure mode. DPP introduces multiple key roles including:
In order to support a plug and play model for installation of devices, where the device is simply powered up for the first time and automatically discovers the network without the need for a helper or supervising application, for example an application running on a smart cell phone or tablet that performs the role of Configurator, then this implies that the AP must perform the role of the Configurator and the device or STA performs the role of Enrollee. Note that the AP may simply proxy DPP messages through to a backend WLC, but from the perspective of the device, the AP is the Configurator.
The DPP specification also mandates that the Initiator must be bootstrapped the bootstrapping public key of the Responder. For BRSKI purposes, the DPP bootstrapping public key will be the [IEEE802.1AR] IDevID of the device. As the boostrapping device cannot know in advance the bootstrapping public key of a specific operators network, this implies that the Configurator must take on the role of the Initiator. Therefore, the AP must take on the roles of both the Configurator and the Initiator.
More details to be added…
When the bootstrapping device determines which SSID to connect to, there are multiple potential options available for how the device authenticates with the network while bootstrapping. Several options are outlined in this section. This list is not exhaustive.
At a high level, authentication can generally be split into two phases using two different credentials:
The authentication options outlined in this document include:
These mechanisms are described in more detail in the following sections. Note that any mechanisms leveraging [IEEE802.1X] are [IEEE802.11] MAC layer authentication mechanisms and therefore the SSID must advertise WPA2 capability.
When evaluating the multiple authentication options outlined below, care and consideration must be given to the complexity of the software state machine required in both devices and services for implementation.
The device connects to an unauthenticated network pre-BRSKI. The device connects to a network enforcing EAP-TLS post-BRSKI. The device uses its LDevID as the post-BRSKI EAP-TLS credential.
To be completed..
The device connects to a network enforcing PSK pre-BRSKI. The mechanism by which the PSK is provisioned on the device for pre-BRSKI authentication is out-of-scope of this version of this document. The device connects to a network enforcing EAP-TLS post-BRSKI. The device uses the LDevID obtained via BRSKI as the post-BRSKI EAP-TLS credential.
When the device connects to the post-BRSKI network that is enforcing EAP-TLS, the device uses its LDevID as its credential. The device should verify the certificate presented by the server during that EAP-TLS exchange against the trusted CA list it obtained during BRSKI.
If the [IEEE802.1X] network enforces a tunneled EAP method, for example [RFC7170], where the device must present an additional credential such as a password, the mechanism by which that additional credential is provisioned on the device for post-BRSKI authentication is out-of-scope of this version of this document. NAI Realm may be used to advertise the EAP methods being enforced by an SSID. It is to be determined if guidelines should be provided on use of NAI Realm for advertising EAP method in order to streamline BRSKI.
Many AAA server state machine logic allows for the network to fallback to MAC Address Bypass (MAB) when initial authentication against the network fails. If the device does not present a valid credential to the network, then the network will check if the device’s MAC address is whitelisted. If it is, then the network may grant the device access to a network segment that will allow it to complete the BRSKI flow and get provisioned with an LDevID. Once the device has an LDevID, it can then reauthenticate against the network using its EAP-TLS and its LDevID.
The device connects to a network enforcing EAP-TLS pre-BRSKI. The device uses its IDevID as the pre-BRSKI EAP-TLS credential. The device connects to a network enforcing EAP-TLS post-BRSKI. The device uses its LDevID as the post-BRSKI EAP-TLS credential.
When the device connects to a pre-BRSKI network that is enforcing EAP-TLS, the device uses its IDevID as its credential. The deivce should not attempt to verify the certificate presented by the server during that EAP-TLS exchange, as it has not yet discovered the local domain trusted CA list.
When the device connects to the post-BRSKI network that is enforcing EAP-TLS, the device uses its LDevID as its credential. The device should verify the certificate presented by the server during that EAP-TLS exchange against the trusted CA list it obtained during BRSKI.
Again, if the post-BRSKI network enforces a tunneled EAP method, the mechanism by which that second credential is provisioned on the device is out-of-scope of this version of this document.
New TEAP TLVs are defined to transport BRSKI messages inside an outer EAP TLS tunnel such as TEAP [RFC7170]. [I-D.lear-eap-teap-brski] outlines a proposal for how BRSKI messages could be transported inside TEAP TLVs. At a high level, this enables the device to obtain an LDevID during the Layer 2 authentication stage. This has multiple advantages including:
There are two suboptions to choose from when tunneling BRSKI messages inside TEAP:
This section assumes that new TLVs are defined for transporting BRSKI messages inside the TEAP tunnel and that a new EAP BRSKI method type is not defined.
The device discovers and connects to a network enforcing TEAP. A high level TEAP with BRSKI extensions flow would look something like:
The EAP server continues with TLS tunnel establishment:
At this stage, the EAP server has some policy decisions to make:
Once the EAP flow has successfully completed, then:
It is assumed that the device will automatically handle LDevID certificate reenrolment via standard EST [RFC7030] outside the context of the EAP tunnel.
An item to be considered here is what information is included in Beacon or Probe Response frames to explicitly indicate that [IEEE802.1X] authentication using TEAP supporting BRSKI extensions is allowed. Currently, the RSNE included in Beacon and Probe Response frames can only indicate [IEEE802.1X] support.
[IEEE802.11] supports multiple authentication algorithms in its Authentication frame including:
Shared Key authentication is used to indicate that the legacy WEP authentication mechanism is to be used. Simultaneous Authentication of Equals is used to indicate that the Dragonfly-based shared passphrase authentication mechanism introduced in [IEEE802.11s] is to be used. One thing that these two methods have in common is that a series of handshake data exchanges occur between the device and the AP as elements inside Authentication frames, and these Authentication exchanges happen prior to [IEEE802.11] Association.
It would be possible to define a new Authentication Algorithm and define new elements to encapsulate BRSKI messages inside Authentication frames. For example, new elements could be defined to encapsulate BRSKI requestvoucher, voucher and voucher telemetry JSON messages. The full BRSKI flow completes and the device gets issued an LDevID prior to associating with an SSID, and prior to doing full [IEEE802.1X] authentication using its LDevID.
The high level flow would be something like:
[IEEE802.1X] defines multiple EAPOL packet types, including EAPOL-Announcement and EAPOL-Announcement-Req messages. EAPOL-Annoncement and EAPOL-Announcement-Req messages can include multiple TLVs. EAPOL-Annoncement messages can be sent prior to starting any EAP authentication flow. New TLVs could be defined to encapsulate BRSKI messages inside EAPOL-Announcement and EAPOL-Announcement-Req TLVs. For example, new TLVs could be defined to encapsulate BRSKI requestvoucher, voucher and voucher telemetry JSON messages. The full BRSKI flow could complete inside EAPOL-Announcement exchanges prior to sending EAPOL-Start or EAPOL-EAP messages.
The high level flow would be something like:
[[ TODO ]]
[[ TODO ]]
802.11i-2004 is an IEEE standard from 2004 that improves connection security. 802.11i-2004 is incorporated into 802.11-2014. 802.11i defines the Robust Security Network IE which includes information on:
The RSN IEs are included in Beacon and Probe Response frames. STAs can use this frame to determine the authentication mechanisms offered by a particular AP e.g. PSK or 802.1X.
802.11u-2011 is an IEEE standard from 2011 that adds features that improve interworking with external networks. 802.11u-2011 is incorporated into 802.11-2016.
STAs and APs advertise support for 802.11u by setting the Interworking bit in the Extended Capabilities IE, and by including the Interworking IE in Beacon, Probe Request and Probe Response frames.
The Interworking IE includes information on:
802.11u introduced Access Network Query Protocol (ANQP) which enables STAs to query APs for information not present in Beacons/Probe Responses.
ANQP defines these key IEs for enabling the STA to determine which network to connect to:
STAs can use one or more of the above IEs to make a suitable decision on which SSID to pick.
HotSpot 2.0 is an example of a specification built on top of 802.11u and defines 10 additional ANQP elements using the standard vendor extensions mechanisms defined in 802.11. It also defines a HS2.0 Indication element that is included in Beacons and Probe Responses so that STAs can immediately tell if an SSID supports HS2.0.