Host-based Card Emulation（基于主机的卡片仿真）

Many SIM cards provided by wireless carriers also contain a secure element.Android 4.4 introduces an additional method of card emulation that does not involve a secure element, called host-based card emulation. 

Android 4.4 supports several protocols that are common in the market today.

Specifically, Android 4.4 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.

》The HCE architecture in Android is based around Android Service components (known as "HCE services").

This is where the ISO/IEC 7816-4 specification comes in: it defines a way to select applications, centered around an Application ID (AID). An AID consists of up to 16 bytes. 

An AID group is a list of AIDs that should be considered as belonging together by the OS. 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.

》 The first step is therefore to extend  HostApduService .

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 need to be overridden and implemented.

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 as well. APDUs are the application-level packets being exchanged between the NFC reader and your HCE service. 

Note: The ISO/IEC 7816-4 specification also defines the concept of multiple logical channels, where you can have multiple parallel APDU exchanges on separate logical channels. Android’s HCE implementation however only supports a single logical channel, so there’s only a single-threaded exchange of APDUs.

》 Finally, you must 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.

Here's 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. An example of such a file with a single AID group declaration containing two proprietary AIDs is shown below:

<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>

》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. The Android platform resolves AID conflicts depending on which category an AID belongs to. Each category may have a different conflict resolution policy.

This asset should be sized 260x96 dp, and can be specified in your meta-data XML file by adding theandroid:apduServiceBanner attribute to the <host-apdu-service> tag, which points to the drawable resource. An example is shown below:

<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>

》 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_ervice"
                 android:resource="@xml/apduservice"/>
  </service>

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>

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 on Android security, read Security Tips .

》As noted earlier, HCE implementations only support a single logical channel. Attempting to select applications on different logical channels will not work on a HCE device.