FIDO Bluetooth Specification v1.0

The FIDO U2F framework was designed to be able to support multiple Authenticator form factors. This document describes the communication protocol with Authenticators over Bluetooth including both Basic Rate Enhanced Data Rate (BR/EDR) and Bluetooth Smart (refered to in this document as Bluetooth Low Enegry or BLE).

There are multiple form factors possible for Authenticators. Some might be low cost, low power devices, and others might be implemented as an additional feature of a more powerful device, such as a smartphone. The design proposed here is meant to support multiple form factors, including but not necessarily limited to these two examples.

Framing

Conceptually, framing defines an encapsulation of U2F raw messages responsible for correct transmission of a single request and its response by the transport layer (BR/EDR or Bluetooth Low Energy).

All requests and their responses are conceptually written as a single frame. The format of the requests and responses is given first as complete frames. Fragmentation is discussed next for each type of transport layer.

Request from Client to Authenticator

Request frames must have the following format

Offset	Length	Mnemonic	Description
0	1	`CMD`	Command identifier
1	1	`HLEN`	High part of data length
2	1	`LLEN`	Low part of data length
3	s	`DATA`	Data (s is equal to the length)

Supported commands are PING and MSG. The constant values for them are described below.

Data format is defined in [U2FRAWMESSAGES].

Response from Authenticator to Client

Response frames must have the following format, which share a similar format to the request frames:

Offset	Length	Mnemonic	Description
0	1	`STAT`	Response status
1	1	`HLEN`	High part of data length
2	1	`LLEN`	Low part of data length
3	s	`DATA`	Data (s is equal to the length)

When the status byte in the response is the same as the command byte in the request, the response is a successful response. The value ERROR indicates an error, and the response data contains an error code as a variable-length, big-endian integer. The constant value for ERROR is described below.

Note that the errors sent in this response are errors at the encapsulation layer, e.g., indicating an incorrectly formatted request, or possibly an error communicating with the Authenticator’s U2F message processing layer. Errors reported by the U2F message processing layer itself are considered a success from the encapsulation layer’s point of view, and are reported as a complete MSG response.

Data format is defined in [U2FRAWMESSAGES]. Note that as per [U2FRAWMESSAGES] (and unlike the NFC transport specification), all communication SHALL be done using extended length APDU format.

Command and status constants

Constant	Value
`PING`	0x81
`KEEPALIVE`	0x82
`MSG`	0x83
`ERROR`	0xbf

The KEEPALIVE command contains a single byte with the following possible values:

Status Constant	Value
`Processing`	0x01
`TUPNeeded`	0x02
`RFU`	0x00, 0x03-0xFF

Bluetooth Low Energy

Authenticator and Client devices using BLE SHALL conform to Bluetooth Core Specification 4.0 or later [BTCORE]

Bluetooth(tm) SIG specified UUID values SHALL be found on the Assigned Numbers website [BTASSNUM]

GATT Service Description

This profile defines two roles: FIDO Authenticator and FIDO Client.

The FIDO Client shall be a GATT Client
The FIDO Authenticator shall be a GATT Server

The following graphic illustrates the mandatory services and characteristics that SHALL be offered by a FIDO Authenticator as part of its GATT server:

The table below summarizes additional GATT sub-procedure requirements for a FIDO Authenticator (GATT Server) beyond those required by all GATT Servers.

GATT Sub-Procedure	Requirements
Write Characteristic Value	`Mandatory`
Notifications	`Mandatory`
Read Characteristic Descriptors	`Mandatory`
Write Characteristic Descriptors	`Mandatory`

The table below summarizes additional GATT sub-procedure requirements for a FIDO Client (GATT Client) beyond those required by all GATT Clients.

GATT Sub-Procedure	Requirements
Discover All Primary Services	`(*)`
Discover Primary Services by Service UUID	`(*)`
Discover All Characteristics of a Service	`(**)`
Discover Characteristics by UUID	`(**)`
Discover All Characteristic Descriptors	`Mandatory`
Read Characteristic Value	`Mandatory`
Write Characteristic Value	`Mandatory`
Notification	`Mandatory`
Read Characteristic Descriptors	`Mandatory`
Write Characteristic Descriptors	`Mandatory`

(*): Mandatory to support at least one of these sub-procedures.

(**): Mandatory to support at least one of these sub-procedures.

Other GATT sub-procedures may be used if supported by both client and server.

Specifics of each service are explained below. In the following descriptions: all values are big-endian coded, all strings are in UTF-8 encoding, and any characteristics not mentioned explicitly are optional.

U2F Service

An Authenticator SHALL implement the U2F Service described below. The UUID for the FIDO U2F GATT service is 0xFFFD, it shall be declared as a Primary Service. The service contains the following characteristics:

Characteristic Name	Mnemonic	Property	Length	UUID
U2F Control Point	`u2fControlPoint`	Write	Defined by Vendor (20-512 bytes)	F1D0FFF1-DEAA-ECEE-B42F-C9BA7ED623BB
U2F Status	`u2fStatus`	Notify	N/A	F1D0FFF2-DEAA-ECEE-B42F-C9BA7ED623BB
U2F Control Point Length	`u2fControlPointLength`	Read	2 bytes	F1D0FFF3-DEAA-ECEE-B42F-C9BA7ED623BB
U2F Service Revision	`u2fServiceRevision`	Read	Defined by Vendor (20-512 bytes)	0x2A28

u2fControlPoint is a write-only command buffer.

u2fStatus is a notify-only response attribute. The Authenticator will send a series of notifications on this attribute with a maximum length of (ATT_MTU-3) using the response frames defined above. This mechanism is used because this results in a faster transfer speed compared to a notify-read combination.

u2fControlPointLength defines the maximum size in bytes of a single write request to u2fControlPoint. This value SHALL be between 20 and 512.

u2fServiceRevision defines the revision of the U2F Service. The value is a UTF-8 string. For this version of the specification, the value u2fServiceRevision SHALL be 1.0 or in raw bytes: 0x312e30.

The u2fServiceRevision Characteristic MAY include a Characteristic Presentation Format descriptor with format value 0x19, UTF-8 String.

Device Information Service

An Authenticator SHALL implement the Device Information Service [BTDIS] with the following characteristics:

Manufacturer Name String
Model Number String
Firmware Revision String

All values for the Device Information Service are left to the vendors. However, vendors should not create uniquely identifiable values so that Authenticators do not become a method of tracking users.

Generic Access Service

Every Authenticator SHALL implement the Generic Access Service [BTGAS] with the following characteristics:

Device Name
Appearance

Protocol Overview

The general overview of the communication protocol follows:

Authenticator advertises the FIDO U2F service.
Client scans for Authenticator advertising the FIDO U2F service.
Client performs characteristic discovery on the Authenticator.
If not already paired, the Client and Authenticator SHALL perform BLE pairing and create a LTK. Authenticator SHALL only allow connections from previously bonded Clients without user intervention.
Client reads the u2fControlPointLength characteristic.
Client registers for notifications on the u2fStatus characteristic.
Client writes a request (e.g., an enroll request) into the u2fControlPoint characteristic.
Authenticator evaluates the request and responds by sending notifications over u2fStatus characteristic.
The connection is closed by the Client or the connection times out and is closed by the Authenticator.

Authenticator Advertising Format

When advertising, the Authenticator SHALL advertise the FIDO U2F service UUID.

When advertising, the Authenticator MAY include the TxPower value in the advertisement (see [BTXPLAD]).

The advertisement MAY also carry a device name which is distinctive and user-identifiable. For example, "ACME Key" would be an appropriate name, while "XJS4" would not be.

The Authenticator SHALL also implement the Generic Access Profile [BTGAP] and Device Information Service [BTDIS], both of which also provide a user friendly name for the device which could be used by the Client.

It is not specified when or how often an Authenticator should advertise, instead that flexibility is left to manufacturers.

Requests

Clients SHOULD make requests by connecting to the Authenticator and performing a write into the u2fControlPoint characteristic.

Responses

Authenticators SHOULD respond to Clients by sending notifications on the u2fStatus characteristic.

Some Authenticators might alert users or prompt them to complete the test of user presence (e.g., via sound, light, vibration, etc.) Upon receiving any request, the Authenticators SHALL respond within kMaxInitialResponseMillis. The Authenticators SHALL send KEEPALIVE BLE commands every kKeepAliveMillis milliseconds. While the Authenticator is processing the request the KEEPALIVE BLE command will contain status Processing. As soon the Authenticator has completed the processing it SHALL either send the reply or wait for user presence. If a wait-for-user-presence state is entered the KEEPALIVE will contain the TUPNeeded status. The Authenticators MAY alert the user (e.g., by flashing) in order to prompt the user to complete the test of user presence. Upon receiving a KEEPALIVE message, the Client SHALL assume the Authenticator is still processing the command; the Client SHALL not resend the command. The Authenticator SHALL continue sending KEEPALIVE messages at least every kKeepAliveMillis to indicate that it is still handling the request. Until a client-defined timeout occurs, the Client SHALL NOT move on to other devices when it receives a KEEPALIVE with TUPNeeded status, as it knows this is a device that can satisfy its request.

Framing fragmentation

A single request/response sent over BLE MAY be split over multiple writes and notifications, due to the inherent limitations of BLE which is not currently meant for large messages. Frames are fragmented in the following way:

A frame is divided into an initialization fragment and one or more continuation fragments.

An initialization fragment is defined as:

Offset	Length	Mnemonic	Description
0	1	`CMD`	Command identifier
1	1	`HLEN`	High part of data length
2	1	`LLEN`	Low part of data length
3	maxLen - 3	`DATA`	Data

where maxLen is the maximum packet size supported by the characteristic or notification.

In other words, the start of an initialization fragment is indicated by setting the high bit in the first byte. The subsequent two bytes indicate the total length of the frame, in big-endian order. The first maxLen - 3 bytes of data follow.

Continuation fragments are defined as:

Offset	Length	Mnemonic	Description
0	1	`SEQ`	Packet sequence 0x00..0x7f (high bit always cleared)
1	maxLen - 1	`DATA`	Data

where maxLen is the maximum packet size supported by the characteristic or notification.

In other words, continuation fragments begin with a sequence number, beginning at 0, implicitly with the high bit cleared.

Example for sending a PING command with 40 bytes of data with a maxLen of 20 bytes:

Frame	Bytes
0	`[810028] [17 bytes of data]`
1	`[00] [19 bytes of data]`
2	`[01] [4 bytes of data]`

Example for sending a ping command with 400 bytes of data with a maxLen of 512 bytes:

Frame	Bytes
0	`[810190] [400 bytes of data]`

Implementation Considerations

Bluetooth pairing: Client considerations

As noted in the Pairing section, a disadvantage of using standard Bluetooth pairing is that the pairing is "system-wide" on most operating systems. That is, if an Authenticator is paired to a FIDO Client which resides on an operating system where Bluetooth pairing is "system-wide", then any application on that device might be able to interact with an Authenticator. This poses both security and privacy risks to users.

While Client operating system security is partly out of FIDO's scope, further revisions of this specification MAY propose mitigations for this issue.

Bluetooth pairing: Authenticator considerations

The method to put the Authenticator into Pairing Mode should be such that it is not easy for the user to do accidentally especially if the pairing method is Just Works. For example, the action could be pressing a physically recessed button or pressing multiple buttons. A visible or audible cue that the Authenticator is in Pairing Mode should be considered. As a counter example, a silent, long press of a single non-recessed button is not advised as some users naturally hold buttons down during regular operation.

Handling command completion

One of the benefits of the Bluetooth, and especially, Bluetooth Low Energy protocols is that they allow Authenticators to be battery-operated devices with low power requirements. A key consideration for such devices to to be able to conserve power by shutting down or switching to a lower-power state when they have satisfied a Client's requests.

On the other hand, the design of the protocol typically requires more than one command before completion. This is especially true when one command is rejected due to a key handle error: if a user has more than one key handle associated with an account or identity, multiple key handles may need to be tried before getting a successful outcome. At the same time, a Client that fails to send followup commands in a timely fashion may cause the Authenticator to drain its battery by staying powered up anticipating more commands.

A further consideration for an Authenticator is to ensure that a user is not confused about which command she is confirming by completing the test of user presence. That is, if a user performs the test of user presence, that action should perform exactly one operation.

We combine these considerations into the following series of recommendations:

Upon initial connection to an Authenticator, and upon receipt of a response from an Authenticator, if a Client has more commands to issue, the Client MUST transmit the next command or fragment within kMaxCommandTransmitDelayMillis milliseconds.
Upon successful completion of a command which required a test of user presence, e.g. upon a successful authentication or registration command, the Authenticator can assume the Client is satisfied, and MAY reset its state or power down.
Upon sending a command response that did not consume a test of user presence, the Authenticator MUST assume that the Client may wish to initiate another command, and leave the connection open until the Client closes it or until a timeout of at least kErrorWaitMillis elapses. Examples of command responses that do not consume user presence include failed authenticate or register commands, as well as get version responses, whether successful or not. After kErrorWaitMillis milliseconds have elapsed without further commands from a Client, an Authenticator MAY reset its state or power down.

Constant	Value
`kMaxCommandTransmitDelayMillis`	1500 milliseconds
`kErrorWaitMillis`	2000 milliseconds
`kMaxInitialResponseMillis`	500 milliseconds
`kKeepAliveMillis`	500 milliseconds

Data throughput

BLE does not have particularly high throughput, this can cause noticeable latency to the user if request/responses are large. Some ways that implementers can reduce latency are:

Support the maximum MTU size allowable by hardware (up to the 512 bytes max from the BLE specifications).
Make the attestation certificate as small as possible, do not include unnecessary extensions.

Advertising

Though the standard doesn’t appear to mandate it (in any way that we’ve found thus far), advertising and device discovery seems to work better when the Authenticators advertise on all 3 advertising channels and not just one.

Authenticator Address Type

In order to enhance the user's privacy and specifically to guard against tracking, it is recommended that Authenticators use Resolvable Private Addresses (RPAs) instead of static addresses.

Notation

Key Words

Pairing

Link Security