Wi-Fi Protected Setup: 简化安全设备网络接入方案

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"Wi-Fi Protected Setup (WPS) 是一种由Wi-Fi Alliance提出的简化Wi-Fi设备安全设置和网络接入的规范。该规范旨在满足Wi-Fi Protected Setup工作组的需求，目的是让不熟悉网络配置的用户也能轻松地将设备连接到支持WPA（Wi-Fi Protected Access）的安全802.11网络。" Wi-Fi Protected Setup 规范版本1.0h详细介绍了实现这一目标的方法和架构。文档共110页，涵盖了多个方面： 1. **介绍**： - **目的**：创建一个简单且安全的设备配置流程，适用于802.11网络。 - **范围**：涉及设备在WPA网络中的设置和引入。 - **相关文档**：可能包括其他Wi-Fi Alliance的技术规范或指南。 - **支持的使用模型**：主要模型和次要模型，如家庭、小型办公室和企业环境。 - **心理模型**：理解用户对设置过程的期望和理解。 - **设计方法**：设计考虑的因素和策略。 - **解决方案灵活性**：适应不同设备和环境的能力。 - **用户体验**：包括在带宽内和带宽外的设置方式。 2. **核心架构**： - **定义**：规范中的关键术语解释。 - **组件与接口**： - **架构概述**：描述了系统的主要组成部分及其相互作用。 - **接口E**：设备与接入点之间的通信接口。 - **接口M**：管理设备注册和认证的接口。 - **接口A**：用户设备与外部设置工具的交互接口。 - **注册协议**：设备如何加入网络的详细步骤。 - **安全概览**： - **在带宽内的配置**：描述了通过无线网络进行安全设置的过程。 - **指南和要求**：确保数据传输的安全性，防止未授权访问。文档还深入探讨了安全措施、认证流程、设备识别以及可能的交互模式。WPS的目标是通过简化网络配置，使非技术用户也能安全地连接到Wi-Fi网络，同时保持网络的安全性。通过这种方式，Wi-Fi Alliance致力于提升用户体验，促进Wi-Fi设备的广泛采用和使用。

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Wi-Fi Protected Setup Specification Page 16 of 110 Version: 1.0h

2.4. Security Overview

Wi-Fi Protected Setup offers a variety of choices for device manufacturers, and each choice has different

security implications. The security of a system is only as strong as its weakest component. Therefore, the

effective security strength when using Wi-Fi Protected Setup to set up a given WLAN corresponds to the

strength of the least secure method used for setting up any of the devices on that WLAN. Users who want

strong security should be encouraged to purchase products that support the higher-security Wi-Fi

Protected Setup options. There are two major modes of operation of Wi-Fi Protected Setup: in-band

configuration and out-of-band configuration.

With in-band configuration, a Diffie-Hellman key exchange is performed and authenticated using a

shared secret called a “device password.” The device password is obtained from the Enrollee and entered

into the Registrar either manually using a keypad entry or using a USB flash drive or NFC. If the USB or

NFC are used to enter the device password, the Registrar is also provided the hash of the Enrollee’s

Diffie-Hellman public key. This significantly strengthens the authentication of the Enrollee to the

Registrar and reduces risks associated with attackers stealing USB flash drives or NFC tokens used for

wireless setup.

With out-of-band configuration, WLAN Credentials are sent across an out-of-band channel to the

Enrollee. The Credentials and configuration are optionally encrypted on the out-of-band channel. The

two out-of-band channels currently supported by this specification are USB flash drive and NFC.

It is worth noting that the Wi-Fi Alliance may choose to eliminate one or more of the options supported

by this specification from the compliance tests that will eventually be used for Wi-Fi Protected Setup

certification. If this occurs, then the minimum security strength of the standardized solution may be

increased beyond what is claimed in this section.

2.4.1. In-band Configuration

The Wi-Fi Protected Setup in-band Registration protocol is designed to provide strong protection against

passive eavesdropping attacks and also to detect and to protect the system from an attempt to perform an

active brute force attack. This means that if a Registrar engages an attacker that it believes is the

legitimate Enrollee, it first detects that the attacker does not know the password. This detection occurs

before it has given enough information to expose the password to brute force attack. However, if the

Registrar runs the Registration Protocol multiple times with an attacker using the same PIN, the attacker

will be able to discover the PIN through brute force offline attack and run the protocol again to obtain the

network settings. To address this vulnerability, if a PIN authentication or communication error occurs

after sending message M6, the Registrar MUST warn the user and MUST NOT automatically reuse the

PIN. Furthermore, if the Registrar detects this situation and prompts the user for a new PIN from the

Enrollee device, it MUST NOT accept the same PIN again without warning the user of a potential

attack. If a strong device password (such as an OOB Device Password or a Machine-specified password)

with at least 32 bytes of randomness is used instead of a PIN, the Registrar is permitted to use this

password multiple times without warning the user when failures occur. The requirements regarding PIN

reuse do not apply to the PBC (Pushbutton) method.

Device Password

All devices supporting Wi-Fi Protected Setup must provide at least one numeric Device Password (PIN)

for initial setup that is unique and randomly generated per device. Although it is possible and permitted

for two devices to have the same Device Password, a group of devices should not intentionally be

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assigned the same Device Password, and the Device Password MUST not be based on other

characteristics of the device, such as MAC address or serial number.

Headless Devices

Headless devices (i.e., those without a display) are required by Wi-Fi Protected Setup to include an 8-

digit device password called a PIN (A PIN on a headless device is typically printed on a sticker or

otherwise physically inscribed on the device). The PIN value of a headless device must also be

configured into the device itself. This would typically be done during the manufacturing process.

PIN-based device passwords are the basic security level for Wi-Fi Protected Setup. Since one of the

digits in the PIN is used as a checksum, the PIN contains approximately 23 bits of entropy. This in itself

is not the biggest limitation, however. The biggest limitation is that this PIN may be a fixed value (when

it is on a label). Because a fixed PIN value is very likely to be reused, it is susceptible to active attack.

The protocol permits a user to override the default device password with a new value, which can help

security-conscious users reduce this vulnerability.

Probably the most significant class of headless devices in a WLAN is the AP itself. If possible, an AP

should generate and display a fresh PIN for establishing external Registrars each time the Registration

Protocol is run in the initial AP setup mode. However, if a sticker-based PIN is used, the AP should track

multiple failed attempts to authenticate as an external Registrar and then enter a lock-down state (This

state is signified by setting the attribute AP Setup Locked to TRUE).

In this state, the AP MUST refuse to run the Registration Protocol in initial AP setup mode (with the AP

acting as an Enrollee) with any external Registrars. This technique protects the AP’s PIN against brute

force attack by an attacker posing as a new external Registrar. During the AP Setup Locked state, it is

still possible to add new Enrollee devices to the WLAN, but it is not possible to add new external

Registrars using the AP’s PIN. The AP Setup Locked state can be reset to FALSE through the

SetAPSettings action (The SetAPSettings action can only be invoked by a previously authorized external

Registrar) or through some other AP-specific method. The AP may include, for example, a way to reset

this state using the AP’s administrative Web page. It is up to the AP implementation to decide when to

enter the AP Setup Locked state.

In addition to the PIN method, headless devices may implement the push button configuration (PBC)

method (Devices with richer UIs may also optionally implement the PBC method). The PBC method has

zero bits of entropy and only protects only against passive eavesdropping attacks. The PBC method

should only be used if no PIN-capable Registrar is available and the WLAN user is willing to accept the

security risks associated with PBC.

Although the security properties of these methods are weaker than the other options, they are included in

this specification to accommodate devices without displays or other out-of-band channels.

Devices with Displays

If an Enrollee device is capable of displaying either four or eight numeric digits, it is required to generate

a fresh device password each time it runs the Registration Protocol. This has two significant advantages.

First, because the password is single-use, it is not susceptible to the brute force attack described above.

Second, it is simpler to manufacture devices that dynamically generate keys than to have them pre-

configured and printed on stickers at the factory. There is also no risk that a display will fall off or get

lost, which is possible with a sticker. Devices with displays may display either 4 or 8 digit PINs. The last

digit of an 8-digit PIN is a checksum of the first 7 digits. Section 6.4.1 specifies how the checksum is

generated. Four-digit PINs do not include a checksum digit.

Devices with NFC or USB

If the Registrar supports the same out-of-band channel as the Enrollee, that channel can be used to deliver

strong device passwords (such as 256 bit random values) to the Registrar. The hash of the Enrollee’s

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public key is also included. As far as the WLAN is concerned, this approach resists even attackers that

succeed in reading the data sent across the out-of-band channel. However, if the attacker is able to read

the USB flash drive or the device’s NFC token before completion of the enrollment process, then it may

be possible for them to perform a rogue network attack against the Enrollee.

2.4.2. Guidelines and Requirements for PIN values

The recommended length for a manually entered device password is an eight digit numeric PIN. This

length does not provide a large amount of entropy for strong mutual authentication, but the design of the

Registration Protocol protects against dictionary attacks on PINs if a fresh PIN or a rekeying key is used

each time the Registration Protocol is run.

PIN values should be randomly generated, and they MUST NOT be derivable from any information that

can be obtained by an eavesdropper or active attacker. The device’s serial number and MAC address, for

example, are easily eavesdropped by an attacker on the in-band channel. Furthermore, if a device

includes multiple PIN values, these values MUST be cryptographically separate from each other. If, for

example, a device includes both a label-based PIN and a Device Password on an integrated NFC

Contactless Token, the two Device Passwords MUST be different and uncorrelated.

A Registrar may be preconfigured with a set of Enrollee PIN and UUID-E pairs as part of a packaged

solution or a Registrar may choose to store PIN values. PINs stored on the Registrar may remain valid

for an indeterminate amount of time, but Registrars should invalidate a PIN if a registration attempt

results in a failed PIN authentication. PINs that are stored on the Registrar should be cryptographically

protected and should not be read accessible via an interface on the Registrar.

2.4.3. Out-of-band Configuration

There are three options for using out-of-band channels for configuration in this proposal.

Unencrypted Settings

The first option places the WLAN Credential unencrypted onto the out-of-band media. Using this option

is based on the assumption that the user will maintain physical control over the out-of-band media (such

as an NFC token or USB flash drive). This control must be maintained even after the enrollment process

is complete. The primary advantage of this option is convenience: the out-of-band media can be reused

with new Enrollees without requiring the Registrar to be running at the time of introduction. Another

important advantage of this option is that it works well with legacy APs that do not forward messages

containing Enrollee public keys to the Registrar. The disadvantage is that if an attacker gains access to

the out-of-band media, they will immediately obtain valid WLAN Credentials.

Encrypted Settings

This option uses a key derived from the Diffie-Hellman public key of the Enrollee obtained over the in-

band channel, along with that of the Registrar, to encrypt settings for that specific Enrollee. Although the

settings are encrypted, it is still advisable to physically guard the out-of-band media from being read by

an attacker.

NFC Interfaces Operating in Peer-to-peer Mode

This mode has the strongest security properties supported by this specification because man-in-the-middle

attacks against NFC are not feasible. In this mode, a 1536-bit Diffie-Hellman exchange is performed over

the NFC interface, and WLAN settings are encrypted using 128-bit AES and delivered over the same

interface. The Diffie-Hellman public keys and WLAN settings are implicitly authenticated by both the

Registrar and the Enrollee, because they are received over the NFC channel.

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3. Initial WLAN Setup

There are three primary scenarios for initial WLAN setup with Wi-Fi Protected Setup. The first case is a

standalone AP that supports Wi-Fi Protected Setup. A “standalone AP” is one that includes a built-in

Registrar and does not use an external Registrar. The second case is a WLAN with a legacy AP that does

not support Wi-Fi Protected Setup. In this case, an external Registrar issues Credentials directly to

Enrollees, but the AP does not participate in this process. The third case is where a Wi-Fi Protected Setup

AP operates with one or more external Registrars. External Registrars are granted authority by the AP to

issue Credentials to Enrollees and to manage the AP’s configuration, WPA keys, and Registrar list.

3.1. Standalone AP

The simplest configuration for initial WLAN setup with Wi-Fi Protected Setup is a standalone AP. In

this case, the user simply plugs in the AP and optionally attaches its Internet connection. When

initializing in a standalone mode, a Wi-Fi Protected Setup AP must automatically choose an SSID

(preferably a random SSID) and channel. It should also by default turn on WPA or WPA2 with a strong,

randomly generated PSK. A standalone AP should include a Wi-Fi Protected Setup Registrar, issuing

keys to Enrollees via the Registration Protocol. A standalone AP may also include an option to turn

security on or off. An AP should also include a factory reset option that erases any configuration and

keys that have been established by the user and returns the AP to the state it had when originally

purchased.

If an AP includes a built-in Registrar that uses a Web-based interface to input Enrollee passwords or

perform other Registrar functions, the following suggestions are recommended:

• The AP’s Registrar pages should be protected with TLS

• HTTP Basic Authentication must not be used, even over TLS. At minimum, Digest

Authentication over TLS with the "response-auth" option should be used.

• It should be possible to disable the AP’s Registrar Web interface for adding Enrollees.

If the AP ships with a built-in device password for web page access and for setting up an external

Registrar, this password must be unique to that individual device. Furthermore, the user must be

permitted to change this password to a stronger value. If the default password is changed, then the

original password must be deactivated unless the AP is reset to its original factory settings.

Security Considerations

There are several security and usability challenges when using a standalone AP as a Registrar. These

challenges stem primarily from the limitations of the user interface and storage capabilities of an AP.

Ideally, the Registrar should guide the user step-by-step through the setup process and explain any errors

or problems that have been encountered. However, a standalone AP without a display will have difficulty

providing this level of feedback to the user unless it is operated through a browser interface. It is

important to understand that these usability issues also have an impact on security A user might not not be

able to make correct security decisions unless the system can provide sufficient information to inform

those decisions.

3.2. Legacy AP

Even if the AP does not support Wi-Fi Protected Setup, other devices on the network can still benefit

from supporting Wi-Fi Protected Setup. In this case, an external Registrar must be manually configured

with the key(s) and SSID that have been established for the WLAN via some proprietary method. The

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method for entering this legacy AP configuration data will depend on the implementation of the AP and

of the Registrar. Given this information, the Registrar can use Wi-Fi Protected Setup to provide Enrollees

with WLAN Credentials.

In the legacy AP case, a wireless Registrar receives Discovery frames from an Enrollee through Probe

Requests and may respond back to the Enrollee through an ad-hoc Probe Response. If both the Enrollee

and the Registrar support ad hoc connections, they may proceed directly to the rest of the Wi-Fi Protected

Setup protocol exchange over 802.1X using a point-to-point ad-hoc connection. If either the Registrar or

the Enrollee do not support the in-band protocol in ad hoc mode, then the Registrar still may use the

Enrollee’s Probe Request to help guide the user through the setup process. If the Registrar and the

Enrollee share a common out-of-band configuration method, then that method can be used to set up the

Enrollee even with a legacy AP.

This mode of operation will naturally have some limitations. The user experience of manually

configuring the Registrar, for example, may vary according to the AP implementation. Furthermore, it

may be difficult to configure strong keys for the network with this method because the keys would need

to be entered by hand. The legacy AP may also support only single shared key (no per-device keys).

Another potential limitation is that with a legacy AP, a wired Registrar may not be able to receive

Discovery messages sent by Enrollees in 802.11 frames. This means the user may have to invoke the out-

of-band Registrar function manually.

Security Considerations

The primary security problem with a legacy AP is that without Wi-Fi Protected Setup to distribute strong

keys between the Registrar and the AP, the user is likely to configure a weak WPA Personal pass phrase

that is susceptible to offline dictionary or brute force attack.

3.3. AP With an External Registrar

It is ultimately the responsibility of the AP to perform link layer access control on a Wi-Fi network in

infrastructure mode. Wi-Fi Protected Setup was developed based on the presumption that a person in

physical possession of the AP during the setup process is the de facto owner who is authorized to extend

Domain membership to other devices. If wireless security is enabled, each member device must first be

given a Credential (in home networks, this Credential is typically a WPA-Personal PSK). This

enrollment function can be performed by a standalone AP and it can also be delegated to one or more

external Registrar devices. If an external Registrar is used, then the external Registrar must also establish

a secure Management Interface with the AP. The Management Interface is specified in the WFADevice

and WFAWLANConfig Service documents.

An external Registrar issues Credentials to Enrollees and configures APs in the Domain to accept those

Credentials. It also provides diagnostic feedback to help the user resolve problems with the network and

to lead users through the device enrollment process. Secondary usage models such as guest access and

Credential revocation can also be facilitated by an external Registrar.

A user may want to use an external Registrar for any of the following reasons:

• The external Registrar may have a greater ability to store and display a comprehensive log of

network setup events.

• An external Registrar may have a richer UI that can help explain and resolve problems

encountered during setup.

• The external Registrar may support multiple out-of-band channels, so it is capable of easily

introducing a greater variety of Enrollee devices.

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