Near Field Communication (NFC) enables wireless communication between two
devices at close proximity, usually less than a few centimeters.
NFC is an international standard (ISO/IEC 18092) defining an interface and
protocol for simple wireless interconnection of closely coupled devices
operating at 13.56 MHz.
The hardware standard is defined in [[[NFC-STANDARDS]]].
This document defines an API to enable selected use cases based on
NFC technology. The current scope of this specification is NDEF.
Low-level I/O operations (e.g. ISO-DEP, NFC-A/B, NFC-F) and Host-based Card
Emulation (HCE) are not supported within the current scope.
Implementers need to be aware that this specification is considered
unstable.
Implementers who are not taking part in the discussions will find the
specification changing out from under them in incompatible ways. Vendors
interested in implementing this specification before it eventually reaches
the Candidate Recommendation phase should subscribe to the repository on
GitHub and take part in the discussions.
This document defines conformance criteria that apply to a single
product: the UA (user agent) that implements the interfaces it
contains.
Introduction
Web NFC user scenario is as follows: Hold a device in close proximity to a
passively powered NFC tag, such as a plastic card or sticker, in order to
read and/or write data.
NFC works using magnetic induction, meaning that the reader (an active,
powered device) will emit a small electric charge which then creates a
magnetic field.
This field powers the passive device which turns it into electrical impulses
to communicate data. Thus, when the devices are within range, a read is
always performed (see NFC Analog Specification and NFC Digital Protocol, NFC
Forum, 2006).
The peer-to-peer connection works in a similar way, as the device
periodically switches into a so-called initiator mode in order to scan for
targets, then later to fall back into target mode. If a target is found, the
data is read the same way as for tags.
As NFC is based on existing RFID standards, many NFC chipsets support
reading RFID tags, but some of these are only supported by single
vendors and not part of the NFC standards. As such, this document
specifies ways to interact with the NFC Data Exchange Format (NDEF).
Terminology and conventions
The Augmented Backus-Naur Form (ABNF) notation used is specified in
[[RFC5234]].
NFC stands for Near Field Communications, a short-range wireless
technology operating at 13.56 MHz which enables communication between
devices at a distance less than 10 cm. The NFC communications protocols and
data exchange formats, and are based on existing radio-frequency
identification (RFID) standards, including ISO/IEC 14443 and FeliCa.
The NFC standards include ISO/IEC 18092[5] and those defined by the NFC
Forum. See
NFC Forum Technical Specifications for a complete
listing.
An NFC adapter is the software entity in the underlying
platform which provides access to NFC functionality implemented in a
given hardware element (NFC chip). A device may have multiple NFC
adapters, for instance a built-in one, and one or more attached via USB.
An NFC tag is a passive NFC device that is not blocklisted.
The NFC tag is powered by magnetic induction when an active NFC
device is in proximity range. An NFC tag that supports NDEF
contains a single NDEF message.
The way of reading the message may happen through proprietary
technologies, which require the reader and the tag to be of the same
manufacturer. They may also expose an NDEF message.
An NFC peer is an active, powered device which can interact
with other devices in order to exchange data using NFC.
As currently spec'ed, peer-to-peer is not supported.
NDEF is an abbreviation for NFC Forum Data Exchange Format, a
lightweight binary message format that is standardized in [[!NFC-NDEF]].
An NDEF message encapsulates one or more application-defined
NDEF records. NDEF messages can be stored on an NFC tag or
exchanged between NFC-enabled devices.
The term NFC content denotes all bytes sent to or received from
an NFC tag. In the current API it is synonym to NDEF message.
The NFC Standard
NFC is standardized in the NFC Forum and described in [[NFC-STANDARDS]].
NDEF compatible tag types
The NFC Forum has mandated the support of five different tag types to be
operable with NFC devices. The same is required on operating systems, such
as Android.
In addition to that, the MIFARE Standard specifies a way
for NDEF to work on top of the older MIFARE Standard, which may
be optionally supported by implementers.
NFC Forum Type 1: This tag is based on the ISO/IEC 14443-3A
(NFC-A). The tags are rewritable and can be
configured to become read-only. Memory size can be between `96` bytes and
`2` Kbytes. Communication speed is `106` kbit/s. In contrast to all other
types, these tags have no anti-collision protection for dealing with multiple
tags within the NFC field.
NFC Forum Type 2: This tag is based on the
ISO/IEC 14443-3A (NFC-A). The tags are rewritable and can be configured
to become read-only. Memory size can be between `48` bytes and `2` Kbytes.
Communication speed is `106` kbit/s.
NFC Forum Type 3: This tag is based on the Japanese Industrial
Standard (JIS) X 6319-4 (ISO/IEC 18092), commonly known as FeliCa. The tags are
preconfigured to be either rewritable or read-only. Memory is `2` kbytes.
Communication speed is `212` kbit/s or `424` kbit/s.
NFC Forum Type 4: This tag is based on the ISO/IEC 14443-4 A/B
(NFC A, NFC B) and thus supports either NFC-A or NFC-B
for communication. On top of that the tag may optionally support ISO-DEP
(Data Exchange Protocol defined in ISO/IEC 14443 (ISO/IEC 14443-4:2008
Part 4: Transmission protocol). The tags are preconfigured
to be either rewritable or read-only. Variable memory, up to `32` kbytes.
Supports three different communication speeds `106` or `212` or
`424` kbit/s.
NFC Forum Type 5: This tag is based on ISO/IEC 15693 (NFC-V) and
allows reading and writing an NDEF message on an ISO/IEC 15693 RF tag
that is accessible by long range RFID readers as well. The NFC communication
is limited to short distance and may use the Active Communication Mode of
ISO/IEC 18092 where the sending peer generates the field which balances
power consumption and improves link stability. Variable memory, up to `64` kbytes.
Communication speed is `26.48` kbit/s.
MIFARE Standard: This tag, often sold under the brand names MIFARE
Classic or MIFARE Mini, is based on the ISO/IEC 14443-3A (also known as NFC-A,
as defined in ISO/IEC 14443-3:2011, Part 3: Initialization and anticollision).
The tags are rewritable and can be configured to become read-only. Memory size
can be between `320` and `4` kbytes. Communication speed is `106` kbit/s.
MIFARE Standard is not an NFC Forum type and can only be read by devices
using NXP hardware. Support for reading and writing to tags based on the
MIFARE Standard is thus non-nominative, but the type is included
due to the popularity and use in legacy systems.
In addition to data types standardized for NDEF records by the NFC
Forum, many commercial products such as bus cards, door openers may be based
on the MIFARE Standard which requires specific NFC chips (same vendor of
card and reader) in order to function.
The NDEF record and fields
An NDEF record is a part of an NDEF message. Each record
is a binary structure that contains a data payload, as well as associated
type information. In addition to this, it includes information about how
the data is structured, like payload size, whether the data is chunked over
multiple records etc.
A generic record looks like the following:
Only the first three bytes (lines in figure) are mandatory. First the
header byte, followed by the TYPE LENGTH field and PAYLOAD
LENGTH field, both of which may be zero.
The TNF field (bit `0-2`, type name format) indicates the format
of the type name and is often exposed by native NFC software stacks. The
field can take binary values denoting the following NDEF record payload types:
The IL field (bit `3`, id length) indicates whether an
ID LENGTH field is present. If the IL field is `0`, then the
ID field is not present either.
The SR field (bit `4`, short record) indicates a short record,
one with a payload length <= `255` bytes. Normal records can have payload
lengths exceeding `255` bytes up to a maximum of `4` GB. Short records only
use one byte to indicate length, whether as normal records use `4` bytes
(`2``32``-1` bytes).
The CF field (bit `5`, chunk flag) indicates whether the payload
is chunked across multiple records.
Web NFC turns all received chunked records into logical records and
transparently chunks sent payload when that is needed.
The ME field (bit `6`, message end) indicates whether this record
is the last in the NDEF message.
The MB field (bit `7`, message begin) indicates whether this
record is the first of the NDEF message.
The TYPE LENGTH field is an unsigned 8-bit integer that denotes
the byte size of the TYPE field.
The TYPE field is a globally unique and maintained identifier
that describes the type of the PAYLOAD field in a structure,
encoding and format dictated by value of the TNF field.
The [[[!NFC-RTD]]] requires that the TYPE field names MUST be
compared in case-insensitive manner.
The ID LENGTH field is an unsigned 8-bit integer that denotes
the byte size of the ID field.
The ID field is an identifier in the form of a URI reference
([[RFC3986]]) that is unique, and can be absolute of relative (in the
latter case the application must provide a base URI). Middle and terminating
chunk records MUST NOT have an ID field, other records MAY have it.
The PAYLOAD LENGTH field denotes the byte size of the
PAYLOAD field. If the SR field is `1`, its size is one byte,
otherwise 4 bytes, representing an 8-bit or 32-bit unsigned integer,
respectively.
The PAYLOAD field carries the application bytes. Any internal
structure of the data is opaque to NDEF. Note that in certain cases
discussed later, this field MAY contain an NDEF message as data.
The NFC Forum has standardized a small set of useful sub record types in
[[NFC-RTD]] (Resource Type Definition specifications) called
well-known type records, for instance text, URL, media and others.
In addition, there are record types designed for more
complex interactions, such as smart posters (containing optional embedded
records for url, text, signature and actions), and handover records.
NFC Forum local type that are defined by the NFC Forum or
by an application, and always start with lowercase character or a
number. Those are usually short strings that are unique only within
the local context of the containing record. They are used when the
meaning of the types doesn't matter outside of the local context of the
containing record and when storage usage is a hard constraint. See
Smart poster for an example on how local types are used.
A [=local type=] is thus defined in terms of a containing record type,
and thus doesn't need any namespacing. For this reason the same local
type name can be used within another record type with different meaning
and different payload type.
Well-known global types
NFC Forum global types are defined and managed by the
NFC Forum and usually start with an uppercase character.
Examples: "`T`" for text, "`U`" for URL, "`Sp`" for smart poster,
"`Sig`" for signature, "`Hc`" for handover carrier, "`Hr`" for
handover request, "`Hs`" for handover select, etc.
Text record
The Text record is a [=well-known type record=] that is defined
in the [[NDEF-TEXT]] specification.
The TNF field is `1` and the TYPE field is "`T`" (`0x54`).
The first byte of the PAYLOAD field is a status byte, followed
by the [=language tag=] in US-ASCII encoding.
The rest of the payload is the actual text, encoded either in UTF-8 or
UTF-16, as indicated by the status byte as follows:
Bits 0 to 5 define the length of the [=language tag=].
Bit 6 is `0`.
If bit 7 if set, means the payload is encoded in UTF-8, otherwise in
UTF-16.
URI record
URI record is defined in [[NDEF-URI]].
The TNF field is `1` and the TYPE field is "`U`" (`0x55`).
The first byte of the PAYLOAD field is a URI identifier code,
in fact an index in an abbreviation table where the values are prepended
to the rest of the URI. For instance the value `0` denotes no prepending,
`1` denotes "`http://www.`", `0x04` denotes "`https://`"" and so on.
The rest of the payload contains the rest of the URI as a UTF-8 string
(and if the first byte is `0`, then it denotes the whole URI).
The URI is defined in [[RFC3987]] and in fact is a UTF-8 encoded IRI
that can be a URN or a URL.
Smart poster record
Smart poster is defined in [[NDEF-SMARTPOSTER]] to describe a
given web content as an NDEF record that contains an NDEF message
as payload, including the following records:
The [[NDEF-SMARTPOSTER]] states that applications SHALL use only the
smart poster record if it is present in an NDEF message
that also contains other URI records.
Zero or more Text records that act as a title record
related to the content. When there are more than one title record
present, they MUST be with different language tags.
Applications SHOULD select one title record for presentation
to the end user.
Zero or more MIME type records that act as icon record
related to the content. The MIME type is usually "`image/jpg`",
"`image/png`", "`image/gif`", or even "`video/mpeg`".
Applications SHOULD select one icon record for presentation
to the end user.
One optional type record that has a [=local type name=]
"`t`" specific to smart poster and the PAYLOAD field
contains a UTF-8 encoded MIME type for the content referred to by the
URI record.
One optional size record that has [=local type name=] "`s`"
specific to smart poster and the PAYLOAD field contains
a 4-byte 32 bit unsigned integer that denotes the size of the object
referred to by the URL in the URI record of the
smart poster.
One optional action record that has a [=local type name=]
"`act`" specific to smart poster and the PAYLOAD field
contains a single byte, whose value has the following meaning:
At the time of NDEF standardization the value `0` ("do the action") was
intended for use cases like send an SMS, make a call or launch browser.
Similarly, the value `1`, ("save the content for later processing") was
intended for use cases like store the SMS in inbox, save the URL in
bookmarks, or save the phone number to contacts. Also, the value `2`
("open for editing") was meant to open the smart poster content with a
default application for editing.
Implementations don't need to implement any standardized behavior for the
actions defined here. In this API it's up to the applications what actions
they define (that may include the use cases above). However, Web NFC
just provides the values.
A smart poster MAY also contain other records, which can be
handled in an application specific manner.
The example below shows a smart poster record that embeds a text and a
URL record.
Signature records
NDEF Signature is defined [[NDEF-SIGNATURE]].
Its TYPE field contains "`Sig`" (`0x53`, `0x69`, `0x67`) and its
PAYLOAD field contains version, signature and a certificate chain.
As currently spec'ed, this is not supported.
Handover records
NFC handover is defined [[NFC-HANDOVER]] and the
corresponding message structure that allows negotiation and activation of
an alternative communication carrier, such as Bluetooth or WiFi.
The negotiated communication carrier would then be used (separately) to
perform certain activities between the two devices, such as sending photos
to the other device, printing to a Bluetooth printer or streaming video to
a television set.
As currently spec'ed, this is not supported.
MIME type records (TNF 2)
The MIME type records are records that store binary
data with associated MIME type.
NOTE: Some platforms, like Windows Phone have stored additional data
in the payload, but any payload data in these records are ignored by
other platforms such as Android. On Android, reading such a record,
will attempt to load the URL in Chrome and it is as such not intended
for client applications.
External type records (TNF 4)
The NFC Forum external type records are for
application specified data types and are defined in [[[NFC-RTD]]].
The external type is a URN with the prefix `"urn:nfc:ext:"` followed by
the name of the owner [=domain=], adding a `U+003A` (`:`), then a non-zero
type name, for instance `"urn:nfc:ext:w3.org:atype"`, stored as
`"w3.org:atype"` in the TYPE field.
Unknown type records (TNF 5)
The unknown records are records that store
opaque data without associated MIME type, meaning that the
`application/octet-stream` default MIME type MAY be assumed.
The [[NFC-NDEF]] specification recommends that NDEF parsers store
or forward the payload without processing it.
Unchanged type records (TNF 6)
The unchanged records are record chunks of
a chunked data set, and is used for any, but the first record.
A chunked payload is spread across multiple NDEF records
that undergo the following rules:
The initial chunk record has the CF field set, its
TYPE field set to the type of the whole chunked payload and
its ID field MAY be set to an identifier used for the whole
chunked payload. Its PAYLOAD LENGTH field denotes the size of
the payload chunk in this record only.
Reading an NFC tag containing an NDEF message, while the
{{Document}} of the top-level browsing context using Web NFC is
visible. For instance, a web page instructs the user to tap an NFC
tag, and then receives information from the tag.
Writing to other, writable NFC tags (i.e. overwriting a
generic tag).
Note that an NFC write operation to an NFC tag always involves
also a read operation.
Support for multiple NFC adapters
Users may attach one or more external NFC adapters to their
devices, in addition to a built-in adapter. Users may use either
NFC adapter.
Features
High level features for the Web NFC specification include the following:
Support devices with single or multiple NFC adapters.
If there are multiple adapters present when invoking an NFC function
then the UA operates all NFC adapters in parallel.
Support communication with passive (smart cards, tags, etc.) NFC devices.
Allow users to act on (e.g. read, write or transceive) discovered
passive NFC devices, as well as access the payload which were read in
the process as NDEF messages.
Allow users to write a payload via NDEF records to compatible
devices, such as writable tags, when they come in range, as
NDEF messages.
Examples
This section shows how developers can make use of the various features of
this specification.
Feature support
Detecting if Web NFC is supported can be done by checking the {{NDEFReader}}
object. Note that this does not guarantee that NFC hardware is available.
if ("NDEFReader" in window) { /* Scan and write NDEF Tags */ }
General information about writing data
Writing data is generally straightforward, but there are a few
gotcha's around how writing works with NFC.
An NFC reader works by polling, so in order to be able to
write to a tag, a tag first needs to be found and read, which
means that polling needs to be initialized first.
If polling is not initiated already by first calling `scan()`,
then the `write()` method will initiate it temporarily until a
tag was found and read, and writing to it was attempted.
This means that the flow is that first a read it performed
as the tag is first found, then followed by a write.
This means that if `scan()` is running and you have an event
listener for the `reading` event, it will be dispatched once
during a `write()` operation, which might not be the intended
behavior.
In the following sections we will discuss how you can easily
deal with this behavior, but first a few simple examples.
Write a text string
Writing a text string to an NFC tag is straightforward.
In order to write, a tag needs to be found and thus read. This gives
you the ability to check whether it is actually a tag that you want to
write to or not, by checking existing data or serial number.
For this reason, it is recommended calling `write()` from a `reading`
event.
The below example shows how to coordinate between a common `reading`
handler and one used specifically for a single write.
const ndef = new NDEFReader();
let ignoreRead = false;
ndef.onreading = (event) => {
if (ignoreRead) {
return; // write pending, ignore read.
}
console.log("We read a tag, but not during pending write!");
};
function write(data) {
ignoreRead = true;
return new Promise((resolve, reject) => {
ndef.addEventListener("reading", event => {
// Check if we want to write to this tag, or reject.
ndef.write(data).then(resolve, reject).finally(() => ignoreRead = false);
}, { once: true });
});
}
await ndef.scan();
try {
await write("Hello World");
console.log("We wrote to a tag!")
} catch(err) {
console.error("Something went wrong", err);
}
Scheduling a write with a timeout
It can sometimes be useful to set a time limit on a write operation.
Like you ask the user to touch a tag, and if no tag is found within
a certain amount of time, then you time out.
const ndef = new NDEFReader();
ndef.onreading = (event) => console.log("We read a tag!");
function write(data, { timeout } = {}) {
return new Promise((resolve, reject) => {
const ctlr = new AbortController();
ctlr.signal.onabort = () => reject("Time is up, bailing out!");
setTimeout(() => ctlr.abort(), timeout);
ndef.addEventListener("reading", event => {
ndef.write(data, { signal: ctlr.signal }).then(resolve, reject);
}, { once: true });
});
}
await ndef.scan();
try {
// Let's wait for 5 seconds only.
await write("Hello World", { timeout: 5_000 });
} catch(err) {
console.error("Something went wrong", err);
} finally {
console.log("We wrote to a tag!");
}
Handle scanning errors
This example shows what happens when {{NDEFReader/scan}} promise rejects and
`readingerror` is fired.
const ndef = new NDEFReader();
ndef.scan().then(() => {
console.log("Scan started successfully.");
ndef.onreadingerror = (event) => {
console.log("Error! Cannot read data from the NFC tag. Try a different one?");
};
ndef.onreading = (event) => {
console.log("NDEF message read.");
};
}).catch(error => {
console.log(`Error! Scan failed to start: ${error}.`);
});
Read a single tag, once
This example show you how to easily create a convenience function
that just reads a single tag and then stops polling, saving
battery life by cutting unneeded work.
The example could easily be extended to time out after a given
amount of milliseconds.
This example shows reading various different kinds of data which can be
stored on a tag. If the tag is unformatted or contains an empty record,
a text message is written with the value "Hello World".
const ndef = new NDEFReader();
await ndef.scan();
ndef.onreading = async ({ message }) => {
if (message.records.length == 0 || // unformatted tag
message.records[0].recordType == "empty") { // empty record
await ndef.write({
records: [{ recordType: "text", data: "Hello World" }]
});
return;
}
const decoder = new TextDecoder();
for (const record of message.records) {
switch (record.recordType) {
case "text":
const textDecoder = new TextDecoder(record.encoding);
console.log(`Text: ${textDecoder.decode(record.data)} (${record.lang})`);
break;
case "url":
console.log(`URL: ${decoder.decode(record.data)}`);
break;
case "mime":
if (record.mediaType === "application/json") {
console.log(`JSON: ${JSON.parse(decoder.decode(record.data))}`);
}
else if (record.mediaType.startsWith("image/")) {
const blob = new Blob([record.data], { type: record.mediaType });
const img = document.createElement("img");
img.src = URL.createObjectURL(blob);
img.onload = () => window.URL.revokeObjectURL(this.src);
document.body.appendChild(img);
}
else {
console.log(`Media not handled`);
}
break;
default:
console.log(`Record not handled`);
}
}
};
Save and restore game progress with an NFC tag
Filtering of relevant data sources can be done by the use of
a custom record identifier, in this case "`/my-game-progress`".
When we read the data, we immediately update the game progress by issuing
a write with a custom NDEF data layout.
const ndef = new NDEFReader();
await ndef.scan();
ndef.onreading = async (event) => {
const decoder = new TextDecoder();
for (const record of event.message.records) {
console.log("Record type: " + record.recordType);
console.log("MIME type: " + record.mediaType);
console.log("=== data ===\n" + decoder.decode(record.data));
}
try {
await ndef.write("Overriding data is fun!");
} catch(error) {
console.log(`Write failed :-( try again: ${error}.`);
}
};
Stop listening to NDEF messages
Read NDEF messages for 3 seconds by using {{NDEFScanOptions/signal}}.
const ndef = new NDEFReader();
const ctrl = new AbortController();
await ndef.scan({ signal: ctrl.signal });
ndef.onreading = () => {
console.log("NDEF message read.");
};
ctrl.signal.onabort = () => {
console.log("We're done waiting for NDEF messages.");
};
// Stop listening to NDEF messages after 3s.
setTimeout(() => ctrl.abort(), 3_000);
Write a smart poster message
const ndef = new NDEFReader();
const encoder = new TextEncoder();
await ndef.write({ records: [
{
recordType: "smart-poster", // Sp
data: { records: [
{
recordType: "url", // URL record for the Sp content
data: "https://my.org/content/19911"
},
{
recordType: "text", // title record for the Sp content
data: "Funny dance"
},
{
recordType: ":t", // type record, a local type to Sp
data: encoder.encode("image/gif") // MIME type of the Sp content
},
{
recordType: ":s", // size record, a local type to Sp
data: new Uint32Array([4096]) // byte size of Sp content
},
{
recordType: ":act", // action record, a local type to Sp
// do the action, in this case open in the browser
data: new Uint8Array([0])
},
{
recordType: "mime", // icon record, a MIME type record
mediaType: "image/png",
data: await (await fetch("icon1.png")).arrayBuffer()
},
{
recordType: "mime", // another icon record
mediaType: "image/jpg",
data: await (await fetch("icon2.jpg")).arrayBuffer()
}
]}
}
]});
Read an external record with an NDEF message as payload
External type records can be used to create application defined records.
These records may contain an NDEF message as payload,
with its own NDEF records, including local types that are
used in the context of the application.
As NDEF gives no guarantee on the ordering of records, using
an external type record with an NDEF message as payload,
can be useful for encapsulating related data.
This example shows how to read an external record for social posts,
which contains an NDEF message, containing a text record and
a record with the local type "act" (action), with definition
borrowed from smart poster, but used in local application context.
const ndef = new NDEFReader();
await ndef.scan();
ndef.onreading = (event) => {
const externalRecord = event.message.records.find(
record => record.type == "example.com:smart-poster"
);
let action, text;
for (const record of externalRecord.toRecords()) {
if (record.recordType == "text") {
const decoder = new TextDecoder(record.encoding);
text = decoder.decode(record.data);
} else if (record.recordType == ":act") {
action = record.data.getUint8(0);
}
}
switch (action) {
case 0: // do the action
console.log(`Post "${text}" to timeline`);
break;
case 1: // save for later
console.log(`Save "${text}" as a draft`);
break;
case 2: // open for editing
console.log(`Show editable post with "${text}"`);
break;
}
};
Write an external record with an NDEF message as payload
External type records can be used to create application defined records
that may even contain an NDEF message as payload.
Write and read unknown records inside an external record
Unknown type records may be useful inside external type records as
developers know what they represent and therefore can avoid specifying the
mime type.
const encoder = new TextEncoder();
const ndef = new NDEFReader();
await ndef.write({ records: [
{
recordType: "example.com:shoppingItem", // External record
data: {
records: [
{
recordType: "unknown", // Shopping item name
data: encoder.encode("Food")
},
{
recordType: "unknown", // Shopping item description
data: encoder.encode("Provide nutritional support for an organism.")
}
]
}
}
]});
The recordType property represents the NDEF record types.
The id property represents the record identifier,
which is an absolute or relative URL. The required uniqueness of the
identifier is guaranteed only by the generator, not by this specification.
The NFC NDEF specifications uses the terms "message identifier" and "payload identifier"
instead of record identifier, but the identifier is tied to each record and not
the message (collection of records), and it may be present when no payload is.
The encoding attribute represents the
[=encoding/name|encoding name=] used for encoding the payload in the
case it is textual data.
The lang attribute represents the [=language tag=]
of the payload in the case that was encoded.
A language tag is a string that matches the
production of a Language-Tag defined in the [[BCP47]]
specifications (see the IANA
Language Subtag Registry for an authoritative list of possible
values). That is, a language range is composed of one or more
subtags that are delimited by a U+002D HYPHEN-MINUS ("-").
For example, the 'en-AU' language range represents
English as spoken in Australia, and 'fr-CA' represents
French as spoken in Canada. Language tags that meet the validity
criteria of [[RFC5646]] section 2.2.9 that can be verified without
reference to the IANA Language Subtag Registry are considered
structurally valid.
The data property represents the PAYLOAD field data.
The NDEFRecordInit dictionary is used to initialize an
NDEF record with its record type recordType, and
optional record identifier id and payload data
data.
Additionally, there are additional optional fields that are only applicable
for certain record types:
"text": Optional [=encoding/label|encoding label=]
encoding and [=language tag=] lang.
The mapping from data types of an
NDEFRecordInit to NDEF record types is presented
in the algorithmic steps which handle the data and described in the
[[[#steps-receiving]]] and [[[#writing-content]]] sections.
To convert NDEFRecord.data bytes
given a |record:NDEFRecord|, run these steps:
Let |bytes:byte sequence| be the value of record's data attribute.
Let |recordType:record type| be the value of |record|'s
recordType attribute.
If the |recordType| value is "`smart-poster`", then return the result of
running parse records from bytes given |bytes| and
`"smart-poster"`.
In addition to [=well known type names=] it is also possible for
organizations to create a custom external type name,
which is a string consisting of a [=domain=] name and a custom type name,
separated by a colon `U+003A` (`:`).
Applications MAY also use a local type name, which is a
string that MUST start with lowercase character or a number,
representing a type for an NFC Forum [=local type=]. It is
typically used in a record of an NDEFMessage that is the payload
of a parent NDEFRecord, for instance in a smart poster.
The context of the local type is the parent record whose payload
is the NDEFMessage to which this record belongs and the
local type name SHOULD NOT conflict with any other type names
used in that context.
Any implementation of Web NFC MUST transparently expose chunked records
as single logical records, therefore unchanged records are not
explicitly represented.
Two external types MUST be compared character by character, in
case-insensitive manner.
The binary representation of any well-known type record and
external type MUST be written as a relative URI (RFC 3986),
omitting the namespace identifier (NID) "`nfc`" and namespace specific
string (NSS) "`wkt`" and "`ext`", respectively, i.e. omitting the
"`urn:nfc:wkt:`" and "`urn:nfc:ext:`" prefixes.
For instance, "`urn:nfc:ext:company.com:a" is stored as "`company.com:a`"
and the well-known type records of a Text record is
"`urn:nfc:wkt:T`", but it is stored as "`T`".
Data mapping
The mapping from data types of an NDEFRecordInit to
NDEF record types, as used in the [[[#writing-content]]]
section is as follows:
The NDEFReader is an object that exposes NFC functionality to the
browsing context: reading NDEF messages when a device, such
as a tag, is within the magnetic induction field. Also, it is used for
writing NDEF messages to NFC tags within range.
The NDEFMessageSource is a union type representing argument types
accepted by the {{NDEFReader/write()}} method.
The NDEFReadingEvent is the event being dispatched on new NFC readings.
The serialNumber property represents the serial number of
the device used for anti-collision and identification, or empty string in case none
is available. The message is an NDEFMessage object.
NDEFReadingEventInit is used to initialize a new event with a serial number
and the NDEFMessageInit data via the message member.
If serialNumber is not present or is `null`,
empty string will be used to init the event.
Though most tags will have a stable unique identifier (UID), not all
have one and some tags even create a random number on each read.
The serial number usually consists of 4 or 7 numbers, separated by `:`.
The pending write tuple is the value of the
[[\PendingWrite]] internal slot.
NFC is suspended if the
[[\Suspended]] internal slot is `true`.
To suspend NFC, set the [[\Suspended]]
internal slot to `true`.
To resume NFC, set the [[\Suspended]]
internal slot to `false`.
Internal slots are used only as a notation in this specification, and
implementations do not necessarily have to map them to explicit internal
properties.
Handling NFC adapters
Implementations MAY use multiple NFC adapters
according to the algorithmic steps described in this specification.
If it resolved with {{PermissionState["granted"]}}
(i.e. permission has been granted to the origin and
global object using the [[[PERMISSIONS]]] API),
return `true`.
Otherwise, if it resolved with {{PermissionState["prompt"]}},
then optionally
request permission
from the user for the Web NFC permission name.
If that is granted, return `true`.
When the user agent determines that the
visibility
state of the [=environment settings object / responsible document=]
of the current settings object changes, it must run these steps:
Let |document:Document| be the [=environment settings object /
responsible document=] of the current settings object.
The term suspended refers to NFC
operations being suspended, which means that no NFC content is
written by NDEFReaders, and no received NFC content is
presented to any {{NDEFReader}} while being suspended.
Aborting pending write operation
To attempt to abort a pending write operation,
run the following steps:
When the value of the overwrite property is
`false`, the write algorithm
will read the NFC tag to determine if it has
NDEF records on it, and if yes, it will not
execute any pending write.
The signal property allows to abort
the {{NDEFReader/write()}} operation.
The NDEFScanOptions dictionary
To stop listening to NDEF messages, an [= AbortSignal/aborted flag =]
can be used in the NDEFScanOptions dictionary:
The signal property allows to abort the
{{NDEFReader/scan()}} operation.
Writing content
This section describes how to write an NDEF message
to an NFC tag when it is next time in proximity range before a
timer expires. At any time there is a maximum of one
NDEF message that can be set for writing for an origin until
the current message is sent or the write is aborted.
The write() method
The
NDEFReader.write method, when invoked, MUST run the
write a message algorithm:
Let |p:Promise| be a new {{Promise}} object.
If not currently executing in the currently active top-level
browsing context, then reject |p| with and
{{"InvalidStateError"}} {{DOMException}} and return |p|.
Let |message:NDEFMessageSource| be the first argument.
Let |options:NDEFWriteOptions| be the second argument.
Let |signal:AbortSignal| be the |options|’ dictionary member
of the same name if present, or `null` otherwise.
If |signal|’s [= AbortSignal/aborted flag =] is set, then reject |p|
with an {{"AbortError"}} {{DOMException}} and return |p|.
If |p| was settled (fulfilled or rejected), then clear the
pending write tuple if it exists.
Return |p| and run the following steps in parallel:
If the obtain permission steps return `false`, then
reject |p| with a {{"NotAllowedError"}} {{DOMException}} and
abort these steps.
If there is no underlying NFC Adapter, or if a connection
cannot be established, then reject |p| with a
{{"NotSupportedError"}} {{DOMException}}
and abort these steps.
If the UA is not allowed to access the underlying NFC Adapter
(e.g. a user preference), then reject |p| with a
{{"NotReadableError"}} {{DOMException}}
and abort these steps.
If pushing data is not supported by the underlying
NFC Adapter, then reject |p| with a {{"NotSupportedError"}}
{{DOMException}} and abort these steps.
An implementation MAY reject |p| with
a {{"NotSupportedError"}} {{DOMException}}
and abort these steps.
The UA might abort message write at this point. The reasons
for termination are implementation details. For example, the
implementation might be unable to support the requested
operation.
Let |output| be the notation for the NDEF message
to be created by UA, as the result of invoking
create NDEF message with |message|, `""`, and `0`.
If this throws an exception, reject |p| with that
exception and abort these steps.
If the NFC tag in proximity range does not expose
NDEF technology for formatting or writing, then
reject |p| with a {{"NotSupportedError"}} {{DOMException}} and
return |p|.
If |device| is an NFC tag and if |options|'s overwrite is
`false`, read the tag to check whether there are NDEF
records on the tag. If yes, then reject |p| with a
{{"NotAllowedError"}} {{DOMException}} and return |p|.
Initiate data transfer to |device| using
|output| as buffer, using the NFC adapter
in communication range with |device|.
If the NFC tag in proximity range is unformatted and
NDEF-formatable, format it and write |output| as
buffer.
Multiple adapters should be used sequentially by users.
There is very little likelihood that a simultaneous tap
will happen on two or multiple different and connected
NFC adapters.
If it happens, the user will likely need to repeat the
taps until success, preferably one device at a time.
The error here gives an indication that the operation
needs to be repeated. Otherwise the user may think the
operation succeeded on all connected NFC adapters.
If the transfer fails, reject |p| with
{{"NetworkError"}} {{DOMException}}
and abort these steps.
When the transfer has completed, resolve |p|.
Creating NDEF message
To create NDEF message given |source:NDEFMessageSource|,
|context:string|, and |recordsDepth:unsigned short|,
run these steps:
Switch on |source:NDEFMessageSource|'s type:
{{DOMString}}
Let |textRecord| be an NDEFRecord initialized with its
|recordType| set to "`text`" and |data| set to |source|.
Let |records| be the list « |textRecord| ».
Set |source|'s records to |records|.
{{BufferSource}}
Let |mimeRecord| be an NDEFRecord initialized with its
|recordType| set to "`mime`", |data| set to |source|, and
|mediaType| set to "`application/octet-stream`".
Let |records| be the list « |mimeRecord| ».
Set |source|'s records to |records|.
{{NDEFMessageInit}}
If |source|'s records [= list/is empty =], [= exception/throw =]
a {{TypeError}} and abort these steps.
Increment |recordsDepth| by one.
If |recordsDepth| > `32`, [= exception/throw =] a {{TypeError}}
and abort these steps.
For practical purposes there is only so much room for storing
an NDEF message and so, this specification sets a limit on
the depth of nested messages which developers are unlikely to
hit. See
issue #620.
unmatched type
[= exception/throw =] a {{TypeError}} and abort these steps.
Let |output| be the notation for the NDEF message
to be created by the UA as a result of these steps.
[= list/For each =] |record:NDEFRecordInit| in the list
|source|'s records, run the following steps:
Let |ndef| be the result of running create NDEF record
given |record:NDEFRecordInit|, |context|, and |recordsDepth| or
make sure the underlying platform provides equivalent values to
|ndef|. If the algorithm throws an exception |e|, reject
|promise| with |e| and abort these steps.
Add |ndef| to |output|.
If running check created records given |output| and |context|
throw an |error: Error|, reject |promise| with |error| and abort these
steps.
Return |output|.
Check created records
To check created records given |records: NDEFRecord sequence|
and |context: string|, run these steps:
If |context| is `"smart-poster"` and |records| does not contain
exactly one URI record, or if it contains more than one
type record, size record or action record,
throw {{TypeError}} and abort these steps.
If |context| is `"smart-poster"`, move the URI record to
the beginning of |records|.
Icon record media types could be limited to `"image/"` or `"video/"`,
but the [[NDEF-SMARTPOSTER]] specification does actually allow other
media type records in a smart poster, which can be treated in
an application-specific manner, for instance a vCard contact card
using one of its associated
MIME types.
Creating NDEF record
To create NDEF record given |record:NDEFRecordInit|,
|context:string|, and |recordsDepth:unsigned short|, run these steps:
Let |ndef| be the representation of an NDEF record to be
created by the UA.
Switching on |record|'s recordType, invoke the algorithm
specified below with |record|, |ndef|, |context| and |recordsDepth|
and return the result. If an exception |e| is thrown, reject |promise|
with |e| and abort these steps.
If |record|'s recordType starts with a colon `U+003A` (`:`):
If |context| is `""` (i.e. |record| is not a payload to another
NDEF record), reject |promise| with a {{TypeError}} and
abort these steps.
If running the validate local type steps on |record|'s
recordType returns `false`, reject |promise| with a
{{TypeError}} and abort these steps.
Return the result of running
map local type to NDEF given |record|, |ndef|,
|context|, and |recordsDepth|. If that throws an exception |e|,
reject |promise| with |e| and abort these steps.
If running validate external type on |record|'s
recordType returns `true`,
return map external data to NDEF given |record|, |ndef|,
|context|, and |recordsDepth|. If that throws an exception |e|,
reject |promise| with |e| and abort these steps.
Otherwise, [= exception/throw =] a {{TypeError}} and abort
these steps.
Validating external type
The [[NFC-RTD]] specifies that external types MUST contain
the [=domain=] name of the issuing organization, a colon `U+003A` (`:`)
and a type name that is at least one character long, for instance
"`w3.org:member`".
The [[NFC-RTD]] specifies the URN prefix “`urn:nfc:ext:`” as well, but
it is not stored in the NDEF record, therefore Web NFC
applications SHOULD NOT specify the URN prefix when creating
external type records.
The [[NFC-RTD]] requires that external type names are represented
with the URN prefix “`urn:nfc:ext:`”, e.g. when reading
NDEF messages. However, since external type records are
distinguished by having the TNF FIELD set to `0x04`, there is
no risk seen for type name clashing. Also, there are
W3C TAG recommendations to avoid using URNs in the Web.
Therefore, Web NFC does not use the URN prefix neither when reading
or writing NDEF messages.
To validate external type given |input:USVString|, run
these steps:
If |input:string| is not an [=ASCII string=], is empty, or its
length exceeds 255 bytes, return `false`.
Let |domain| be the |input| from the start of |input| up to
but excluding the first occurrence of `U+003A` (`:`), or `null`
if that is not found.
Let |type| be the |input| after the first occurrence, if any, of
`U+003A` (`:`) up to the end of |input|, or `null` if that is
not found.
If |domain| or |type| is `null`, return `false`.
Let |asciiDomain| be the result of running
domain to ASCII given |domain| and `true` (as |beStrict|).
If |asciiDomain| is failure, return `false`.
If |asciiDomain| contains a [=forbidden host code point=] or
`U+005F` `LOW LINE` (`_`), return `false`.
If |type| contains [=code points=] that are not
[=ASCII alphanumeric=], or `U+0024` (`$`), `U+0027` (`'`),
`U+0028` `LEFT PARENTHESIS` (`(`), `U+0029` `RIGHT PARENTHESIS` (`)`),
`U+002A` (`*`), `U+002B` (`+`), `U+002C` (`,`), `U+002D` (`-`),
`U+002E` (`.`), `U+003B` (`;`), `U+003D` (`=`), `U+0040` (`@`),
`U+005F` (`_`), return `false`.
Return `true`.
Validating local type
To validate local type given an |input:USVString| run
these steps:
Let |localTypeName| be the |input| after the first occurrence of
`U+003A` (`:`) up to the end of |input|.
If |localTypeName| is not a {{USVString}} or its length exceeds 255
bytes, return `false` and abort these steps.
If |localTypeName| does not start with a lowercase character or a
number, return `false`.
To map text to NDEF given |record:NDEFRecordInit| and |ndef|,
run these steps:
This is useful when clients specifically want to write text in a
[=well-known type record=].
Other options would be to use the value "`mime`"
with an explicit MIME type text type, which allows for
better differentiation, e.g. when using "`text/xml`", or
"`text/vcard`".
If |record|'s mediaType is not `undefined`,
[= exception/throw =] a {{TypeError}} and abort these steps.
If the type of |record|'s data is not {{DOMString}} or
{{BufferSource}}, [= exception/throw =] a {{TypeError}} and abort
these steps.
Let |documentLanguage:string| be the [=document element=]'s
lang attribute.
If |documentLanguage| is the empty string, set it to "`en`".
Let |language:string| be |record|'s lang if it [= map/exists =],
or else to |documentLanguage|.
If |context| is `"smart-poster"`, |localTypeName| is
"`s`" (`0x73`) and if the type of
|record|'s data is not {{BufferSource}} or its byte length
is bigger than 4, [= exception/throw =] a {{TypeError}} and abort these steps.
If |context| is `"smart-poster"`, |localTypeName| is
"`act`" (`0x61` `0x63` `0x74`) and if the type of
|record|'s data is not {{BufferSource}} or its byte length
is not exactly one, [= exception/throw =] a {{TypeError}} and abort
these steps.
If the type of a |record|'s data is {{BufferSource}},
To listen for NFC content, the client MUST activate an
{{NDEFReader}} instance by calling NDEFReader.scan(). When attaching
an event listener for the "`reading`" event on it, NFC content is
accessible to the client.
Incoming NFC content is matched using {{NDEFReader}} instances.
When the NDEFReader.scan method is invoked, the UA
MUST run the following
NFC listen algorithm:
Let |p:Promise| be a new {{Promise}} object.
If not currently executing in the currently active top-level
browsing context, then reject |p| with an
{{"InvalidStateError"}} {{DOMException}} and return |p|.
Let |reader:NDEFReader| be the {{NDEFReader}} instance.
Let |options| be first argument.
Let |signal:AbortSignal| be the |options|’ dictionary member
of the same name if present, or `null` otherwise.
If |signal|’s [= AbortSignal/aborted flag =] is set, then reject |p|
with an {{"AbortError"}} {{DOMException}} and return |p|.
Return |p| and run the following steps in parallel:
If the obtain permission steps return `false`, then
reject |p| with a {{"NotAllowedError"}} {{DOMException}} and
abort these steps.
If there is no underlying NFC Adapter, or if a connection
cannot be established, then reject |p| with a
{{"NotSupportedError"}} {{DOMException}}
and abort these steps.
If the UA is not allowed to access the underlying NFC Adapter
(e.g. a user preference), then reject |p| with a
{{"NotReadableError"}} {{DOMException}}
and abort these steps.
If |reader| is already in the activated reader objects,
then reject |p| with an {{"InvalidStateError"}} {{DOMException}}
and abort these steps.
Let |serialNumber:serialNumber| be the device identifier as a series of
numbers, or `null` if unavailable.
If |serialNumber| is not `null`, set it to the
string of U+003A (`:`) concatenating each number represented as
ASCII hex digit, in the same order.
Let |message:NDEFMessage| be a new NDEFMessage object, with
|message|'s records set to the empty list.
If the NFC tag in proximity range is unformatted and is
NDEF-formattable, let |input| be `null`.
Otherwise, let |input| be the notation for the NDEF message
which has been received.
The UA SHOULD represent an unformatted NFC tag as an
NDEF message containing no NDEF records, i.e. an empty
array for its {{NDEFMessage/records}} property.
[= list/For each =] NDEF record which is part of |input|, run the
following sub-steps:
Let |ndef| be the notation for the current NDEF record with
|typeNameField:number| corresponding to the TNF field and
|payload:byte sequence| corresponding to the PAYLOAD field
data.
Let |record:NDEFRecord| be the result of parse an NDEF record
given |ndef| and `""`.
If |record| is not `null`, append |record| to |message|'s
records.
To parse records from bytes given |bytes:byte sequence| and
|context: string|, run these steps:
If the length of |bytes| is `0`, return `null` and abort these steps.
Let |records| be the empty list.
As long as there are unread bytes of |bytes|, run the following
sub-steps:
If the remaining length of |bytes| is less than `3`, return `null`
and abort these sub-steps.
If any of the following steps requires reading bytes beyond the
remaining length of |bytes|, return `null` and abort these
sub-steps.
Let |ndef| be the notation for the current NDEF record.
Let |header:byte| be the next byte of |bytes|.
Let |messageBegin:boolean| (MB field) be the left most
bit (bit 7) of |header|.
If this is the first iteration of these sub-steps and
|messageBegin| is `false`, return `null` and abort these
sub-steps.
Let |messageEnd:boolean| (ME field) be bit 6 of |header|.
As chunked records are not allowed as sub records, ignore bit 5
(CF field) is ignored.
Let |shortRecord:boolean| (SR field) be bit 4 of |header|.
Let |hasIdLength:boolean| (IL field) be bit 3 of |header|.
Let |ndef|'s |typeNameField:number| (TNF field) be the
integer value of bit 2-0 of |header|.
Let |typeLength:number| be the integer value of next byte
(TYPE LENGTH field) of |bytes|.
If |shortRecord| is `true`, let |payloadLength:number|
be the integer value of next byte (PAYLOAD LENGTH field) of
|bytes|.
Otherwise, let |payloadLength| be the integer value of the next 4
bytes of |bytes|.
If |hasIdLength| is `true`, let |idLength:number| be
the integer value of next byte (ID LENGTH field) of |bytes|,
otherwise let it be `0`.
If |typeLength| > 0, let |ndef|'s |type:string| be result of
running UTF-8 decode on the next |typeLength|
(TYPE field) bytes, or else let |type| be the empty string.
If |idLength| > 0, let |ndef|'s |id:string| be result of
running UTF-8 decode on the next |idLength| (ID field)
bytes, or else let |ndef|'s |id| be the empty string.
Let |ndef|'s |payload| be the byte sequence of the last
|payloadLength| (PAYLOAD field) bytes, which may be `0` bytes.
Let |record:NDEFRecord| be the result of parse an NDEF record
given |ndef| and |context|.
If |record| is not `null`, append |record| to |records|.
If |messageEnd| is `true`,
If check parsed records given |records| and |context|
throws an |error|, reject |promise| with |error| and abort these
steps.
Otherwise abort these sub-steps (terminate the loop).
Return |records|.
Check parsed records
To check parsed records given |records: NDEFRecord sequence|
and |context: string|, run these steps:
If |context| is `"smart-poster"` and |records| does not contain
exactly one URI record, or if it contains more than one
type record, size record or action record,
throw {{TypeError}} and abort these steps.
Otherwise return `true`.
Parsing NDEF records
To parse an NDEF record given |ndef| and |context:string| into a
|record:NDEFRecord|, run these steps:
If |ndef|'s |typeNameField| is `1` ([=well-known type record=]), then
If |ndef|'s |type:string| is "`T`" (`0x54`),
set |record| to the result of running
parse an NDEF text record on |ndef|.
If |ndef|'s |type:string| is "`U`" (`0x55`),
set |record| to the result of running
parse an NDEF URL record on |ndef|.
If |ndef|'s |type:string| is "`Sp`" (`0x53` `0x70`),
set |record| to the result of running
parse an NDEF smart-poster record on |ndef|.
If |ndef|'s |type:string| is "`s`" (`0x73`)
and if |context| is equal to `"smart-poster"`,
set |record| to the result of running
parse a smart-poster size record on |ndef|.
If |ndef|'s |type:string| is "`t`" (`0x74`)
and if |context| is equal to `"smart-poster"`,
set |record| to the result of running
parse a smart-poster type record on |ndef|.
If |ndef|'s |type:string| is "`act`" (`0x61` `0x63` `0x74`)
and if |context| is equal to `"smart-poster"`,
set |record| to the result of running
parse a smart-poster action record on |ndef|.
If running the validate local type steps on
|ndef|'s |type:string| returns `true`,
If |context| is not `"external"` or `"smart-poster"`,
[= exception/throw =] a {{TypeError}} and abort these steps.
Otherwise [= exception/throw =] a {{TypeError}} and abort these
steps.
If |ndef|'s |typeNameField| is `2` (MIME type record), then
set |record| to the result of running
parse an NDEF MIME type record on |ndef|, or make sure that
the underlying platform provides equivalent values to the |record|
object's properties.
If |ndef|'s |typeNameField| is `4` (external type record),
then set |record| to the result of running
parse an NDEF external type record on |ndef|, or make sure that
the underlying platform provides equivalent values to the |record|
object's properties.
If |ndef|'s |typeNameField| is `5` (unknown record)
then set |record| to the result of running
parse an NDEF unknown record on |ndef|, or make sure that the
underlying platform provides equivalent values to the |record| object's
properties.
Otherwise [= exception/throw =] a {{TypeError}} and abort these steps.
Parsing NDEF well-known `T` records
To parse an NDEF text record given an |ndefRecord| into a
|record:NDEFRecord|, run these steps:
If |ndefRecord|'s PAYLOAD field is not present, set
|record|'s data to `null` and return |record|.
Let |header:byte| be the first byte of |ndefRecord|'s
PAYLOAD field.
Let |languageLength:octet| be the value given by bit `5`
to bit `0` of the |header|.
Let |language:string| be the result of running ASCII decode
on second byte to the |languageLength| + `1` byte, inclusive.
Let |buffer:byte sequence| be the byte sequence of the rest of the
|ndefRecords|'s PAYLOAD field (excluding |header| and |languageLength| number of bytes).
Let |prefixByte:byte| be the value of the first byte of
|buffer|.
If the value of |prefixByte| matches the URL expansion codes in the
[[[NFC-STANDARDS]]] URI Record Type Definition specification,
Section 3.2.2, Table 3, then
Let |prefixString:string| be the byte sequence value
corresponding to the value of |prefixByte|.
Set |record|'s data to |prefixString| appended to |buffer|.
Otherwise, if there is no match for |prefixByte|, set
|record|'s data to |buffer|.
Return |record|.
Parsing NDEF well-known `Sp` records
To parse an NDEF smart-poster record given an |ndefRecord| into a
|record:NDEFRecord|, run these steps:
Applications can parse this value as a string that contains an
[[RFC2048]] media type that denotes the media type of the object the
URI record in the smart-poster refers to.
An NFC device is blocklisted if the blocklist’s
value contains the device's historical bytes hexadecimal values. In
ISO 14443-4 terminology, the historical bytes are a subset of the
RATS (Request for Answer To Select) response.
Security and Privacy
Chain of trust
Implementations need to make sure that when the user authorizes a method which is part of the Web NFC API, only that action is run, without side effects.
By default, NDEF doesn't provide any way to make the content trusted
beyond allowing tags to be made permanently read-only after writing
data to them. This can even be done from a factory setting.
Data written by the use of this API is not signed or encrypted automatically,
which follows existing native NFC APIs. In order to protect the
integrity and authenticity of NDEF messages, the NFC Forum introduced
[[NDEF-SIGNATURE]]. Using NDEF signature and
key management is the responsibility of the application.
For trusting the confidentiality of the data exchanged
via NFC, applications may use encrypted NFC content.
For trusting the integrity of the data exchanged via NFC,
applications may use an NDEF signature, with key management based
on Public Key Infrastructure (PKI).
Security considerations for MIME types in general are discussed in
[[RFC2048]] and [[RFC2046]].
Privacy implications and implementation considerations
The comparison to barcodes or QR codes is appropriate because NFC tags
are another non-human-readable method of exchanging data and sharing
them can have unforeseen privacy and security implications. For a web
site to read a QR code, a piece of interruptive UI must be used (the
camera) which captures an image and makes it apparent that the contents
of this image (including the QR code) will be available to the web page,
making it clear to the user that scanning is being performed.
Scanning a tag with NFC requires the user to place the scanning device
(e.g. phone) in close proximity to the NFC tag - usually 5-10 cm, 2-4
inches.
Scanning a tag when a Web NFC scan is not active, triggers the host OS
handling. Thus launching URLs and apps from scanning a NFC tag is not
handled and supported by Web NFC itself.
Furthermore, Web NFC scanning needs to be activated from a user interaction,
and scanning is paused when the web site is not in focus or the device
screen turns off (i.e. is not unlocked). This is put in place so that
accidental scans are not likely to happen.
Web NFC further recommends that the implementation makes it very clear
UX-wise to the user that data will be scanned when placing the scanning
device in close proximity to a NFC tag - basically mimicking the UX flow
of scanning a QR code.
There are many options for doing so, like playing back a sound or showing
some persistent UI while the scanning can happen, for instance a modal
dialog with the ability to cancel at any point.
An implementation could also show the data that is about to be uploaded,
postpone sharing the read data until the user OK'd it and even show some
UI allowing the user to select which records to share.
Reading and writing during a scan
When the user scans a tag, at that point the web application has access
to read the data on the tag, and in case it is not read-only, also to
write data to the tag. Consumer stickers for private usage (e.g. in the
maker community) are often unlocked (read + write), whereas commercial
deployment of NFC are read-only.
An older protocol SNEP (Simple NDEF Exchange Protocol) allowed active
devices (e.g. a phone) to receive NDEF data from another active device,
but it is unsupported by Web NFC and it is currently being deprecated
on supported native platforms.
A newer protocol TNEP (Tag NDEF Exchange Protocol) allows bidirectional
communication between a scanner device (e.g. phone) and actively powered
device like an IOT device. It is currently unsupported by Web NFC, and
furthermore it has restrictions on what input to accept and the IOT device
must ensure that the accepted records are valid.
If the tag contains privacy sensitive data, such data will be shared with
the site. Potentially, not immediately if the UX requires the user to
confirm the data exchange before doing so.
In some cases it might be obvious that the tag/device contains privacy
sensitive data, say in case of NFC equipped conference badgets and business
cards. This would also be the case with an NFC equipped glucose meter,
which can indicate that you, or someone in the close family, are a diabetes
patient.
In other cases it might be less obvious that that can happen, but a user
might have used an app or website to write data to a tag that unknowingly
to the user encoded a user id or the like, which can later be read back
by any other site.
Private and unexpected data can also be stored in files (e.g. word
pressing documents, PDFs or camera images) which are uploaded using
the file upload API. The mitigations associated with the Web NFC API
are stronger than those associated with file upload and the data less
likely to be personally identifiable.
Reading and writing during a scan
A scan of a tag might also reveal user location if the website knows
how to identify the tag and know the tags location in the real world,
like if it is mounted inside a museum. It might also be able to deduct
it somewhat as for instance FeliCa NFC tags are mostly used in Japan,
but Web NFC doesn’t reveal what tag technology is used.
This does not bring the web advertising and tracking model into the
real world, because it requires an action by the user and it cannot
be triggered in the background; and with proper UX it should be clear
that scanning is active.
Overwriting existing data
There is also the fear that writing to a NFC tag can ruin it or
“brick it”. As NFC tags were designed to be read by multiple user
applications, NDEF tags have been designed with an easy way to make
devices permanently read-only and can even be configured that way
from a factory.
NDEF is a simple exchange format for reading and writing data and
not for bi-directional communication. NFC supports multiple
communications formats based on lower tech (thus not locked as
read-only as NDEF) and none of these are supported by Web NFC.
Things that users should be made aware of when using NFC
This section details some of the things that users ought to be aware
of when using NFC. It is recommendated that implementations help educate
the users of given facts before or when related NFC actions are performed.
Data that is read is shared with site
When a site has access to read NFC content, then the data of the scanned
tags is shared with the site, in a similar way to uploading files and images.
As with any site, it is up to the user whether to trust that the site handles this data properly and in the
intended manner.
A site may modify and overwrite data of tags that are not made read-only
Deployed NFC solutions, like tags in stores etc, should always be made
read-only in order to ensure they are not modified by mistake or as part
of a malicious act.
Private tags and stickers are often unlocked (writable) from the factory
and the user should be aware that such tags might be overwritten/modified
by scanning them.
Reading a fixed (e.g. mounted) tag may expose reading location
A fixed tag may encode its ID or location in the data, meaning that reading
it exposes that information to the site that knows the physical location of the tag, which then can deduct the location
the read took place. That combined with being logged into a service, can
share your location data with the site.
Data written is readable by other apps and sites with granted read access
Any NDEF data on a tag can be read by any app or web site with the proper
access, so if that is not intended then the data should be encrypted in a
secure manner that only who is supposed to read it can.
Multiple tags may be within the reading field at the same time
NFC can only read one tag at the time, but multiple tags can be detected
and one of the tags can be selected as the tag to communicate with.
Use cases for this could be having multiple smart cards (NFC based) in
your wallet and not wanting to take the card out.
This is mostly useful for payment cards and travel cards that are read
by external hardware and thus not a use-case for Web NFC. For Web NFC,
we do not allow reading when there are multiple tags available, preventing the following attack vector.
There is an attack vector, where someone places another malicious NFC
tag/sticker on top of a legitimate tag, in order to load the wrong
app/site, or inject wrong data into the right app/site. They can
do so by cloning the data of the original tag and modifying it -
either by changing the URL to load a malicious app/site, or by
changing the data to inject malicious data in the right app/site.
Example: the tag is supposed to take you to https://example.com but is modified
to take you to https://exаmple.com (that is with a Cyrillic а) - it looks legitimate
and you might now to giving sensitive data to a malicious site.
Loading web sites from a tag is outside the scope of Web NFC, but it
is recommended for user agents to not auto load URLs when multiple
tags are available due to the above attack vector.
By disallowing reading when there are multiple tags available, Web
NFC protects well against injecting wrong/malicious data into a site
as shielding the existing NFC tag is quite difficult as it requires
ferrite shielding which is quite visible. Metal interferes with the
magnetic field and makes tags not readable.
Assets
Assets to be protected include the following:
NDEF message as a whole,
NDEF records (including payload and header) in
particular, either in-transfer or in-storage state,
when they are being overwritten by a Web NFC triggered operation,
against data disclosure and data modification.
This also includes Denial of Service attacks against a solution
deployed with NFC tags (e.g. a malicious actor destroying tags linked
to a solution).
User identity or other privacy sensitive attributes
that can be directly or indirectly determined by using Web NFC, by the
NFC content creator, or by a web site using Web NFC.
This data could be used directly or leaked forward to third parties.
Examples are user location, device identifiers and user identifiers.
User data exposed to a web page using Web NFC.
While a web page might collect user data using other means than
Web NFC, it might embed this data into NDEF records and share via
Web NFC.
Integrity of user device.
A read of an NFC tag might result in a user device compromise
that can further lead to loss of other web NFC or platform assets.
Attacker model
The following attacker patterns have been considered:
Malicious web page creator: phishing user data,
identity or other privacy sensitive attributes, destroying or
modifying NFC tags to cause further damage through fake identities
and attack vectors.
Malicious NFC tag creator: same as above, but with the
additional possibility to create, delete or modify the NFC
tags locally.
As a result can compromise integrity of user device, cause data
injection, redirect to malicious web page, phishing user location,
causing side actions such as installing applications, trigger
automated dispatching or other actions.
Compromised device or user agent: man-in-the-middle (MITM) attack:
any MITM style attack between Web NFC implementation and an NFC adapter
in a user device, including attempts to interact with a web site using
Web NFC by presenting modified or replayed NDEF records.
Threats
An introduction to NFC security is found here.
Potential threats for Web NFC are given below.
Fingerprinting and data collection
Threat description
Malicious web page collects user data, identity or other privacy
sensitive attributes (such as location) without user consent and
exposes it to third parties (writing it to NFC tags).
Affected assets
User data, user identity or other privacy sensitive attributes
Actors
Malicious web page owner using Web NFC, malicious tag owner.
Mitigation, comments
The user SHOULD be able to be aware of what data can be shared using
NFC from the given web page.
Use permissions and user prompts for accessing personal data,
minimize user data exposed to NFC.
An NFC tag SHOULD NOT ever trigger a user’s device to navigate
to a web site without asking permission, unless the site has been in
the foreground or has been brought to the foreground and has been
granted permission. User agents SHOULD take into account the
security and privacy measures listed in the
Geolocation API.
NFC tag modification
Threat description
An NFC tag is being modified without user consent.
This might enable further attacks using a malicious tag
or can be a Denial of Service attack to make one or more tags
unusable.
Affected assets
NDEF message records, including payload and header in-storage.
Actors
Malicious web page creator, malicious user.
Mitigation, comments
Require permission and user prompt needed for writing tags.
Or, control what tags can be written by a given web page, for instance
a web page can write only a tag that can be connected to its
origin.
Or, allow overwriting since tags not meant to be written can be
protected by making them read only.
Use NDEF signature to detect a modification of NFC tags.
NDEF record modification in-transit
Threat description
NDEF records transferred between Web NFC and the
NFC adapter and user device are modified to cause various
man-in-the-middle attacks or denial-of-service (DoS) attacks.
Also, NDEF signature records can be removed or
replaced along with changed content.
This threat is out of scope for Web NFC implementations.
Applications can use NDEF signatures and appropriate tools
(signature algorithm, certificates, security policies) to protect the
NFC content. Additionally, harden the platform stack.
NDEF record payload disclosure
Threat description
Confidential payload of NDEF record in-storage (stored
on an NFC tag) or in-transfer between Web NFC and the
NFC adapter are read by unauthorized parties.
Affected assets
Confidential NDEF message payload in-transfer and in-storage.
Actors
Malicious man-in-the-middle user, malicious web page creator.
Mitigation, comments
To ensure confidentiality, use payload encryption and secure
communication for data exchange, authentication and authorization
between Web NFC and NFC adapters.
Active attack via malicious NFC tag
Threat description
Malicious tag may be involuntarily or voluntarily read by devices
and the data read may constitute an attack vector on the user agent.
For example it can attempt to trigger an action on
the device, which may be a threat, for instance launching a malicious
web site, or opening an image prepared for attacking the device.
Affected assets
Integrity of user device, all other Web NFC assets.
Actors
Malicious tag creator.
Mitigation, comments
This is a generic problem with all existing NFC tags.
The data is considered application specific.
Implementations need security hardening.
Involuntary touch is low probability due to short range and critical
angle for reading, and due to the focus requirements.
Automatic actions for smart posters and other tags should not be
allowed. The user must be made aware and given the ability to control
what is happening during the NFC communication. For instance, opening
content from smart poster, automatic connection to (possibly
malicious) WiFi via NFC handover, etc.
Do no allow actions from untrusted NFC tags, trust can be established
via the NDEF signature check.
Security mechanisms for implementations
Obtaining permission
Implementations SHOULD use a mechanism to obtain permission,
for instance an explicit permission given by the user.
The [[[PERMISSIONS]]] API is suggested to be used by UAs
for implementing NFC related permissions.
Implementations MAY use per-session/ephemeral permissions.
Warning user during NFC operations
Implementations MAY show an overlay dialog whenever the NFC adapter is
being accessed by the web page (e.g. there is an ongoing scan) in order
to warn user.
For trusting the confidentiality of the data exchanged via NFC,
applications may use encrypted NFC content with key management
based on Public Key Infrastructure (PKI).
Key management is out of the scope of Web NFC.
Signing NDEF records
For trusting the integrity of the data exchanged via NFC, user agents
MAY use an NDEF signature with a Public Key Infrastructure
for key management.
For tags signed with NDEF signature version 1.0 ([[NFC-SECURITY]]),
the signature is applied only to the TYPE field, ID field
and PAYLOAD field, leaving out the first byte of the NDEF header,
allowing surface to attacks. Version 2.0 of [[NFC-SECURITY]] included
tag hardware attributes in the signature and allowed for shorter
certificates.
In order to mitigate
known vulnerabilities of NDEF signature, it is recommended
that applications always sign a full NDEF message with a single
NDEF signature, and use the right tool chain and security
policies for creating and verifying signatures.
Security policies
This section lists the normative security policies for implementations.
Secure Context
Only secure contexts are allowed to access NFC content.
Browsers MAY ignore this rule for development purposes only.
Visible document
Web NFC functionality is allowed only for the {{Document}} of the
top-level browsing context, which must be
visible.
This also means that UAs should block access to the NFC radio if
the display is off or the device is locked.
For backgrounded web pages, receiving and writing NFC content
must be suspended.
All permission that are preserved beyond the current
browsing session MUST be revocable.
Blocklist
Web NFC includes a blocklist of vulnerable NFC devices to prevent
websites from taking advantage of them.
Warn about risk of physical location leak
When listening for and writing NFC content,
the UA MAY warn the user that the given origin may be able to
infer physical location.
Restrict automatic handling
When the payload data on NFC content is untrusted, it MUST NOT
be used by the UA to do automatic handling of the content, such as
opening a web page with a URL found in an NFC tag, or installing
an application, or other actions, unless the user approves that.
The following policies are recommended to be implemented by applications.
A smart poster MAY be trusted only if signed by using
a single NDEF signature record by the same issuer and
it is either the first record in a message, or it is preceded by
another NDEF signature.
User agents expose NDEF signature records without verifying
them. It is applications' responsibility to verify signatures and
to sign NFC content.
Applications SHOULD use appropriate signature algorithms,
certificates and security policies for NDEF signature
creation and verification. Also, take into account the known
attacks agains NDEF signatures, for instance removing
NDEF signature, replacing NDEF signature along with
modifying the NFC content, reordering records by changing
the record PAYLOAD LENGTH field, etc.
Acknowledgments
The editors would like to thank Jeffrey Yasskin, Anne van Kesteren,
Anssi Kostiainen, Domenic Denicola, Daniel Ehrenberg, Jonas Sicking,
Don Coleman, Salvatore Iovene, Rijubrata Bhaumik, Wanming Lin, Han Leon,
Ryan Sleevi, Balázs Engedy, Theodore Olsauskas-Warren, Reilly Grant,
Diego González and Daniel Appelquist for their contributions to this document.
Special thanks to Luc Yriarte and Samuel Ortiz for their initial
work
on exposing NFC to the web platform, and for their support for the current
approach. Also, special thanks to Elena Reshetova for the contributions to
the Security and Privacy section.
