Many Android-powered devices that offer NFC functionality already support NFC
card emulation. In most cases, the card is emulated by a separate chip in the
device, called a
secure element
. Many SIM cards provided by wireless carriers
also contain a secure element.
Android 4.4 and higher provide an additional method of card emulation that
doesn't involve a secure element, called
host-based card emulation
. This
allows any Android application to emulate a card and talk directly to the NFC
reader. This topic describes how host-based card emulation (HCE) works on
Android and how you can develop an app that emulates an NFC card using this
technique.
Card emulation with a secure element
When NFC card emulation is provided using a secure element, the card to be
emulated is provisioned into the secure element on the device through an Android
application. Then, when the user holds the device over an NFC terminal, the NFC
controller in the device routes all data from the reader directly to the secure
element. Figure 1 illustrates this concept:
Figure 1.
NFC card emulation with a secure element.
The secure element itself performs the communication with the NFC terminal, and
no Android application is involved in the transaction. After the transaction is
complete, an Android application can query the secure element directly for the
transaction status and notify the user.
Host-based card emulation
When an NFC card is emulated using host-based card emulation, the data is routed
directly to the host CPU instead of being routed to a secure element. Figure 2
illustrates how host-based card emulation works:
Figure 2.
NFC card emulation without a secure element.
Supported NFC cards and protocols
Figure 3.
Android's HCE protocol stack.
The NFC standards offer support for many different protocols, and there are
different types of cards that you can emulate.
Android 4.4 and higher supports several protocols that are common in the market
today. Many existing contactless cards are already based on these protocols,
such as contactless payment cards. These protocols are also supported by many
NFC readers in the market today, including Android NFC devices functioning as
readers themselves (see the
IsoDep
class). This allows you to build and deploy an end-to-end NFC solution around
HCE using only Android-powered devices.
Specifically, Android 4.4 and higher supports emulating cards that are based on
the NFC-Forum ISO-DEP specification (based on ISO/IEC 14443-4) and process
Application Protocol Data Units (APDUs) as defined in the ISO/IEC 7816-4
specification. Android mandates emulating ISO-DEP only on top of the Nfc-A
(ISO/IEC 14443-3 Type A) technology. Support for Nfc-B (ISO/IEC 14443-4 Type B)
technology is optional. Figure 3 illustrates the layering of all of these
specifications.
HCE services
The HCE architecture in Android is based around Android
Service
components (known as
HCE
services
). One of the key advantages of a service is that it can run in the
background without any user interface. This is a natural fit for many HCE
applications, like loyalty or transit cards, which the user shouldn't need to
launch an app to use. Instead, tapping the device against the NFC reader starts
the correct service if it is not already running and executes the transaction
in the background. Of course, you are free to launch additional UI (such as
user notifications) from your service when appropriate.
Service selection
When the user taps a device to an NFC reader, the Android system needs to know
which HCE service the NFC reader wants to communicate with. The ISO/IEC 7816-4
specification defines a way to select applications, centered around an
Application ID (AID). An AID consists of up to 16 bytes. If you are emulating
cards for an existing NFC reader infrastructure, the AIDs that those readers
look for are typically well-known and publicly registered (for example, the
AIDs of payment networks such as Visa and MasterCard).
If you want to deploy new reader infrastructure for your own application, you
must register your own AIDs. The registration procedure for AIDs is defined in
the ISO/IEC 7816-5 specification. We recommend registering an AID as per 7816-5
if you are deploying a HCE application for Android, because it avoids collisions
with other applications.
AID groups
In some cases, an HCE service may need to register multiple AIDs and be set as
the default handler for all of the AIDs in order to implement a certain
application. Some AIDs in the group going to another service isn't supported.
A list of AIDs that are kept together is called an AID group. For all AIDs in an
AID group, Android guarantees one of the following:
- All AIDs in the group are routed to this HCE service.
- No AIDs in the group are routed to this HCE service (for example, because the
user preferred another service which requested one or more AIDs in your group
as well).
In other words, there is no in-between state, where some AIDs in the group can
be routed to one HCE service, and some to another.
AID groups and categories
You can associate each AID group with a category. This allows Android to group
HCE services together by category, and that in turn allows the user to set
defaults at the category level instead of the AID level. Avoid mentioning AIDs
in any user-facing parts of your application, because they don't mean anything
to the average user.
Android 4.4 and higher supports two categories:
Implement an HCE service
To emulate an NFC card using host-based card emulation, you need to create a
Service
component that handles the NFC transactions.
Check for HCE support
Your application can check whether a device supports HCE by checking for the
FEATURE_NFC_HOST_CARD_EMULATION
feature. Use the
<uses-feature>
tag in the manifest of your application to declare that your app uses the HCE
feature, and whether it is required for the app to function or not.
Service implementation
Android 4.4 and higher provides a convenience
Service
class that you can use
as a basis for implementing an HCE service: the
HostApduService
class.
The first step is to extend
HostApduService
, as shown in the following code
sample:
Kotlin
class MyHostApduService : HostApduService() {
override fun processCommandApdu(commandApdu: ByteArray, extras: Bundle?): ByteArray {
...
}
override fun onDeactivated(reason: Int) {
...
}
}
Java
public class MyHostApduService extends HostApduService {
@Override
public byte[] processCommandApdu(byte[] apdu, Bundle extras) {
...
}
@Override
public void onDeactivated(int reason) {
...
}
}
HostApduService
declares two abstract methods that you must override and
implement. One of those,
processCommandApdu()
,
is called whenever a NFC reader sends an Application Protocol Data Unit (APDU)
to your service. APDUs are defined in the ISO/IEC 7816-4 specification. APDUs
are the application-level packets being exchanged between the NFC reader and
your HCE service. That application-level protocol is half-duplex: the NFC reader
sends you a command APDU, and it waits for you to send a response APDU in
return.
As mentioned previously, Android uses the AID to determine which HCE service the
reader wants to talk to. Typically, the first APDU an NFC reader sends to your
device is a
SELECT AID
APDU; this APDU contains the AID that the reader wants
to talk to. Android extracts that AID from the APDU, resolves it to an HCE
service, and then forwards that APDU to the resolved service.
You can send a response APDU by returning the bytes of the response APDU from
processCommandApdu()
. Note that this method is called on the main thread
of your application, which you shouldn't block. If you can't compute and return
a response APDU immediately, return null. You can then do the necessary work on
another thread and use the
sendResponseApdu()
method defined in the
HostApduService
class to send the response when you are
done.
Android keeps forwarding new APDUs from the reader to your service, until either
of the following happens:
- The NFC reader sends another
SELECT AID
APDU, which the OS resolves to a
different service.
- The NFC link between the NFC reader and your device is broken.
In both of these cases, your class's
onDeactivated()
implementation is called with an argument indicating which of the two happened.
If you are working with existing reader infrastructure, you must implement the
existing application-level protocol that the readers expect in your HCE service.
If you are deploying new reader infrastructure which you control as well, you
can define your own protocol and APDU sequence. Try to limit the amount of APDUs
and the size of the data to exchange: this makes sure that your users only have
to hold their device over the NFC reader for a short amount of time. A
reasonable upper bound is about 1 KB of data, which can usually be
exchanged within 300 ms.
Service manifest declaration and AID registration
You must declare your service in the manifest as usual, but you must add some
additional pieces to the service declaration as well:
To tell the platform that it is a HCE service implementing a
HostApduService
interface, add an intent filter for the
SERVICE_INTERFACE
action to your service declaration.
To tell the platform which AIDs groups are requested by this service, include
a
SERVICE_META_DATA
<meta-data>
tag in the declaration of the service, pointing to an XML
resource with additional information about the HCE service.
Set the
android:exported
attribute to
true
, and require the
android.permission.BIND_NFC_SERVICE
permission in your service declaration.
The former ensures that the service can be bound to by external applications.
The latter then enforces that only external applications that hold the
android.permission.BIND_NFC_SERVICE
permission can bind to your service.
Since
android.permission.BIND_NFC_SERVICE
is a system permission, this
effectively enforces that only the Android OS can bind to your service.
The following is an example of a
HostApduService
manifest declaration:
<service android:name=".MyHostApduService" android:exported="true"
android:permission="android.permission.BIND_NFC_SERVICE">
<intent-filter>
<action android:name="android.nfc.cardemulation.action.HOST_APDU_SERVICE"/>
</intent-filter>
<meta-data android:name="android.nfc.cardemulation.host_apdu_service"
android:resource="@xml/apduservice"/>
</service>
This meta-data tag points to an
apduservice.xml
file. The following is an
example of such a file with a single AID group declaration containing two
proprietary AIDs:
<host-apdu-service xmlns:android="http://schemas.android.com/apk/res/android"
android:description="@string/servicedesc"
android:requireDeviceUnlock="false">
<aid-group android:description="@string/aiddescription"
android:category="other">
<aid-filter android:name="F0010203040506"/>
<aid-filter android:name="F0394148148100"/>
</aid-group>
</host-apdu-service>
The
<host-apdu-service>
tag must contain an
<android:description>
attribute
that contains a user-friendly description of the service that you can show in
the app UI. You can use the
requireDeviceUnlock
attribute to specify that the
device is unlocked before you invoke this service to handle APDUs.
The
<host-apdu-service>
must contain one or more
<aid-group>
tags. Each
<aid-group>
tag is required to do the following:
- Contain an
android:description
attribute that contains a user-friendly
description of the AID group, suitable for display in UI.
- Have its
android:category
attribute set to indicate the category the AID
group belongs to, such as the string constants defined by
CATEGORY_PAYMENT
or
CATEGORY_OTHER
.
- Contain one or more
<aid-filter>
tags, each of which contains a single AID.
Specify the AID in hexadecimal format, and make sure it contains an even
number of characters.
Your application also needs to hold the
NFC
permission to register as an
HCE service.
AID conflict resolution
Multiple
HostApduService
components may be installed on a single device, and
the same AID can be registered by more than one service. Android resolves AID
conflicts differently depending on which category an AID belongs to. Each
category may have a different conflict resolution policy.
For some categories, such as payment, the user might be able to select a default
service in the Android settings UI. For other categories, the policy might be to
always ask the user which service to invoke in case of conflict. For information
about how to query the conflict resolution policy for a certain category, see
getSelectionModeForCategory()
.
Check if your service is the default
Applications can check whether their HCE service is the default service for a
certain category by using the
isDefaultServiceForCategory()
API.
If your service isn't the default, you can request it to be made the default
using
ACTION_CHANGE_DEFAULT
.
Payment applications
Android considers HCE services that have declared an AID group with the
payment
category as payment applications. Android 4.4 and higher contains a
top-level
Settings
menu entry called
tap & pay
, which enumerates all
such payment applications. In this settings menu, the user can select the
default payment application to invoke when a payment terminal is tapped.
Required assets for payment applications
To provide a more visually attractive user experience, HCE payment applications
are required to provide a service banner.
Android 13 and higher
To better fit the default payment selection list in the Settings UI, adjust the
banner requirement to a square icon. Ideally, it should be identical to the
application launcher icon design. This adjustment creates more consistency and a
cleaner look.
Android 12 and lower
Set the service banner's size to 260x96 dp, then set the service banner's size
in your metadata XML file by adding the
android:apduServiceBanner
attribute to
the
<host-apdu-service>
tag, which points to the drawable resource. The
following is an example:
<host-apdu-service xmlns:android="http://schemas.android.com/apk/res/android"
android:description="@string/servicedesc"
android:requireDeviceUnlock="false"
android:apduServiceBanner="@drawable/my_banner">
<aid-group android:description="@string/aiddescription"
android:category="payment">
<aid-filter android:name="F0010203040506"/>
<aid-filter android:name="F0394148148100"/>
</aid-group>
</host-apdu-service>
Screen off and lock-screen behavior
The behavior of HCE services varies based on the version of Android running on
the device.
Android 12 and higher
In apps that target Android 12 (API level 31) and higher, you can enable NFC
payments without the device's screen on by setting
requireDeviceScreenOn
to
false
.
Android 10 and higher
Devices running Android 10 (API level 29) or higher support
Secure
NFC
. While Secure
NFC is on, all card emulators (host applications and off-host applications) are
unavailable when the device screen is off. While Secure NFC is off, off-host
applications are available when the device screen is off. You can check for
Secure NFC support using
isSecureNfcSupported()
.
On devices running Android 10 and higher, the same functionality for setting
android:requireDeviceUnlock
to
true
applies as with devices
running Android 9 and lower, but only when Secure NFC is turned off. That is, if
Secure NFC is turned on, HCE services can't function from the lock-screen
regardless of the setting of
android:requireDeviceUnlock
.
Android 9 and lower
On devices that run Android 9 (API level 28) and lower, the NFC controller and
the application processor are turned off completely when the screen of the
device is turned off. HCE services therefore don't work when the screen is off.
Also on Android 9 and lower, HCE services can function from the lock screen.
However, this is controlled by the
android:requireDeviceUnlock
attribute in
the
<host-apdu-service>
tag of your HCE service. By default, device unlock is
not required, and your service is invoked even if the device is locked.
If you set the
android:requireDeviceUnlock
attribute to
true
for your HCE
service, Android prompts the user to unlock the device when the following
happen:
- the user taps an NFC reader.
- the NFC reader selects an AID that is resolved to your service.
After unlocking, Android shows a dialog prompting the user to tap again to
complete the transaction. This is necessary because the user may have moved the
device away from the NFC reader in order to unlock it.
Coexistence with secure element cards
This section is of interest for developers who have deployed an application
that relies on a secure element for card emulation. Android's HCE implementation
is designed to work in parallel with other methods of implementing card
emulation, including the use of secure elements.
This coexistence is based on a principle called
AID routing
. The NFC
controller keeps a routing table that consists of a (finite) list of routing
rules. Each routing rule contains an AID and a destination. The destination can
either be the host CPU, where Android apps are running, or a connected secure
element.
When the NFC reader sends an APDU with a
SELECT AID
, the NFC controller parses
it and checks whether the AIDs match with any AID in its routing table. If it
matches, that APDU and all APDUs following it are sent to the destination
associated with the AID, until another
SELECT AID
APDU is received or the NFC
link is broken.
Figure 4 illustrates this architecture:
Figure 4.
Android operating with both secure element and host-card emulation.
The NFC controller typically also contains a default route for APDUs. When an
AID is not found in the routing table, the default route is used. While this
setting might be different from device to device, Android devices are required
to ensure that the AIDs being registered by your app are properly routed to the
host.
Android applications that implement an HCE service or that use a secure element
don't have to worry about configuring the routing table; that is handled by
Android automatically. Android merely needs to know which AIDs can be handled
by HCE services and which ones can be handled by the secure element. The routing
table is configured automatically based on which services are installed and
which the user has configured as preferred.
The following section explains how to declare AIDs for applications that use a
secure element for card emulation.
Secure element AID registration
Applications using a secure element for card emulation can declare an
off-host service
in their manifest. The declaration of such a service is
almost identical to the declaration of an HCE service. The exceptions are as
follows:
- The action used in the intent filter must be set to
SERVICE_INTERFACE
.
- The meta-data name attribute must be set to
SERVICE_META_DATA
.
The meta-data XML file must use the
<offhost-apdu-service>
root tag.
<service android:name=".MyOffHostApduService" android:exported="true"
android:permission="android.permission.BIND_NFC_SERVICE">
<intent-filter>
<action android:name="android.nfc.cardemulation.action.OFF_HOST_APDU_SERVICE"/>
</intent-filter>
<meta-data android:name="android.nfc.cardemulation.off_host_apdu_service"
android:resource="@xml/apduservice"/>
</service>
The following is an example of the corresponding
apduservice.xml
file
registering two AIDs:
<offhost-apdu-service xmlns:android="http://schemas.android.com/apk/res/android"
android:description="@string/servicedesc">
<aid-group android:description="@string/subscription" android:category="other">
<aid-filter android:name="F0010203040506"/>
<aid-filter android:name="F0394148148100"/>
</aid-group>
</offhost-apdu-service>
The
android:requireDeviceUnlock
attribute doesn't apply to off-host services,
because the host CPU is not involved in the transaction and therefore cannot
prevent the secure element from executing transactions when the device is
locked.
The
android:apduServiceBanner
attribute is required for off-host services
that are payment applications and to be selectable as a default payment
application.
Off-host service invocation
Android never starts or binds to a service that is declared as "off-host,"
because the actual transactions are executed by the secure element and not by
the Android service. The service declaration merely allows applications to
register AIDs present on the secure element.
HCE and security
The HCE architecture provides one core piece of security: because your
service is protected by the
BIND_NFC_SERVICE
system permission, only the OS can bind to and communicate with your service.
This ensures that any APDU you receive is actually an APDU that was received by
the OS from the NFC controller, and that any APDU you send back only goes to
the OS, which in turn directly forwards the APDUs to the NFC controller.
The last remaining concern is where you get your data that your app sends
to the NFC reader. This is intentionally decoupled in the HCE design; it does
not care where the data comes from, it just makes sure that it is safely
transported to the NFC controller and out to the NFC reader.
For securely storing and retrieving the data that you want to send from your HCE
service, you can, for example, rely on the Android Application Sandbox, which
isolates your app's data from other apps. For more details about Android
security, read
Security tips
.
Protocol parameters and details
This section is of interest for developers who want to understand what protocol
parameters HCE devices use during the anti-collision and activation phases of
the NFC protocols. This allows building a reader infrastructure that is
compatible with Android HCE devices.
Nfc-A (ISO/IEC 14443 type A) protocol anti-collision and activation
As part of the Nfc-A protocol activation, multiple frames are exchanged.
In the first part of the exchange, the HCE device presents its UID; HCE devices
should be assumed to have a random UID. This means that on every tap, the UID
that is presented to the reader is a randomly generated UID. Because of this,
NFC readers should not depend on the UID of HCE devices as a form of
authentication or identification.
The NFC reader can subsequently select the HCE device by sending a
SEL_REQ
command. The
SEL_RES
response of the HCE device has at least the 6th bit
(0x20) set, indicating that the device supports ISO-DEP. Note that other bits in
the
SEL_RES
may be set as well, indicating for example support for the NFC-DEP
(p2p) protocol. Since other bits may be set, readers wanting to interact with
HCE devices should explicitly check for the 6th bit only, and
not
compare
the complete
SEL_RES
with a value of 0x20.
ISO-DEP activation
After the Nfc-A protocol is activated, the NFC reader initiates the ISO-DEP
protocol activation. It sends a RATS (Request for Answer To Select)
command. The NFC controller generates the RATS response, the ATS; the ATS isn't
configurable by HCE services. However, HCE implementations must meet NFC Forum
requirements for the ATS response, so NFC readers can count on these parameters
being set in accordance with NFC Forum requirements for any HCE device.
The section below provides more details on the individual bytes of the ATS
response provided by the NFC controller on a HCE device:
- TL: length of the ATS response. Must not indicate a length greater than 20
bytes.
- T0: bits 5, 6 and 7 must be set on all HCE devices, indicating TA(1), TB(1)
and TC(1) are included in the ATS response. Bits 1 to 4 indicate the FSCI,
coding the maximum frame size. On HCE devices the value of FSCI must be
between 0h and 8h.
- T(A)1: defines bitrates between reader and emulator, and whether they can be
asymmetric. There are no bitrate requirements or guarantees for HCE devices.
- T(B)1: bits 1 to 4 indicate the Start-up Frame Guard time Integer (SFGI). On
HCE devices, SFGI must be <= 8h. Bits 5 to 8 indicate the Frame Waiting
time Integer (FWI) and codes the Frame Waiting Time (FWT). On HCE devices, FWI
must be <= 8h.
- T(C)1: bit 5 indicates support for "Advanced Protocol features". HCE devices
may or may not support "Advanced Protocol features". Bit 2 indicates support
for DID. HCE devices may or may not support DID. Bit 1 indicates support for
NAD. HCE devices must not support NAD and set bit 1 to zero.
- Historical bytes: HCE devices may return up to 15 historical bytes. NFC
readers willing to interact with HCE services should make no assumptions about
the contents of the historical bytes or their presence.
Note that many HCE devices are likely made compliant with protocol requirements
that the payment networks united in EMVCo have specified in their "Contactless
Communication Protocol" specification. In particular:
- FSCI in T0 must be between 2h and 8h.
- T(A)1 must be set to 0x80, indicating only the 106 kbit/s bitrate is
supported, and asymmetric bitrates between reader and emulator are not
supported.
- FWI in T(B)1 must be <= 7h.
APDU data exchange
As noted earlier, HCE implementations support only a single logical channel.
Attempting to select applications on different logical channels doesn't work on
a HCE device.