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The FIDO U2F framework was designed to be able to support multiple authenticator form factors. This document describes the communication protocol with authenticators over Near Field Communication (NFC).
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THIS FIDO ALLIANCE SPECIFICATION IS PROVIDED “AS IS” AND WITHOUT ANY WARRANTY OF ANY KIND, INCLUDING, WITHOUT LIMITATION, ANY EXPRESS OR IMPLIED WARRANTY OF NON-INFRINGEMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Type names, attribute names and element names are written as code.
String literals are enclosed in “”, e.g. “UAF-TLV”.
In formulas we use “|” to denote byte wise concatenation operations.
DOM APIs are described using the ECMAScript [ECMA-262] bindings for WebIDL [WebIDL].
UAF specific terminology used in this document is defined in [FIDOGlossary].
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].
The general protocol between a FIDO client and authenticator over NFC is as follows:
The Authenticator MUST reply to all commands within 800ms.
The NFC protocol SHALL NOT use any additional framing (unlike the USB HID protocol, for example). Instead, messages sent to an NFC authenticator SHALL follow the U2F raw message format as defined in [U2FRawMsgs] in the bibliography. In the NFC protocol, either short or extended length APDU encoding is allowed.
Some responses may not fit into a short APDU response. For this reason, U2F authenticators MUST respond in the following way:
A FIDO client SHALL always send an applet selection command to begin interaction with a FIDO authenticator via NFC. The structure of the applection command SHALL follow the same APDU structure as in the raw message format mentioned above.
The FIDO U2F AID consists of the following fields:
| Field | Value |
|---|---|
RID | 0xA000000647 |
AC | 0x2F |
AX | 0x0001 |
As a result, the command for selecting the applet using the FIDO U2F AID is:
| Field | Value |
|---|---|
CLA | 0x00 |
INS | 0xA4 |
P1 | 0x04 |
P2 | 0x00 |
LEN | 0x08 |
DATA | 0xA0000006472F0001 |
In response to the applet selection command, the FIDO authenticator SHALL reply with its version string in the successful response. In this writing, the version string is "U2F_V2", hence a successful response to the applet selection command would consist of the following bytes:
0x5532465F56329000
Correct and reliable functioning of the NFC U2F authenticator requires a reliable contactless communication between the NFC U2F authenticator and the contactless reader device. However, there are currently several relevant specifications describing the contactless proximity interface often summarized under the term “NFC”.
In order to guarantee interoperability, the contactless interface of the NFC U2F authenticators and the various implementations of contactless readers should follow one of the following standards:
The testing and certification for the above listed specifications will ensure interoperability of NFC U2F authenticators and NFC mobile devices. Generally, all reader devices that may be used with unspecific types of NFC U2F authenticators (see a.) should be conformant to NFC Forum analog specification.