Verifying hardware-backed key pairs with Key Attestation | Android Developers
Android Keystore system | Android Developers
// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.kt
@Throws(GeneralSecurityException::class)
private fun generateKey(alias: String, useStrongBox: Boolean, includeProps: Boolean) {
val keyPairGenerator = KeyPairGenerator.getInstance(
KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore")
val now = Date()
val originationEnd = Date(now.time + 1000000)
val consumptionEnd = Date(now.time + 2000000)
val builder = KeyGenParameterSpec.Builder(alias, KeyProperties.PURPOSE_SIGN)
.setAlgorithmParameterSpec(ECGenParameterSpec("secp256r1"))
.setKeyValidityStart(now)
.setKeyValidityForOriginationEnd(originationEnd)
.setKeyValidityForConsumptionEnd(consumptionEnd)
.setAttestationChallenge(now.toString().toByteArray())
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S && includeProps) {
builder.setDevicePropertiesAttestationIncluded(true)
}
if (Build.VERSION.SDK_INT >= 28 && useStrongBox) {
builder.setIsStrongBoxBacked(true)
}
builder.setDigests(KeyProperties.DIGEST_NONE, KeyProperties.DIGEST_SHA256)
keyPairGenerator.initialize(builder.build())
keyPairGenerator.generateKeyPair()
}
@Throws(AttestationException::class)
private fun doAttestation(alias: String, useStrongBox: Boolean, includeProps: Boolean
): AttestationResult {
val certs: Array<X509Certificate?>?
val attestation: Attestation
val isGoogleRootCertificate: Boolean
try {
val keyStore = KeyStore.getInstance("AndroidKeyStore")
keyStore.load(null)
generateKey(alias, useStrongBox, includeProps)
val certificates = keyStore.getCertificateChain(alias)
certs = arrayOfNulls(certificates.size)
for (i in certs.indices) certs[i] = certificates[i] as X509Certificate
} catch (e: ProviderException) {
if (Build.VERSION.SDK_INT >= 28 && e is StrongBoxUnavailableException) {
throw AttestationException(AttestationException.CODE_STRONGBOX_UNAVAILABLE, e)
} else if (e.cause?.message?.contains("device ids") == true) {
// The device does not support device ids attestation
throw AttestationException(AttestationException.CODE_DEVICEIDS_UNAVAILABLE, e)
} else {
// The device does not support key attestation
throw AttestationException(AttestationException.CODE_NOT_SUPPORT, e)
}
} catch (e: Exception) {
// Unable to get certificate
throw AttestationException(AttestationException.CODE_NOT_SUPPORT, e)
}
try {
isGoogleRootCertificate = VerifyCertificateChain.verifyCertificateChain(certs)
} catch (e: GeneralSecurityException) {
// Certificate is not trusted
throw AttestationException(AttestationException.CODE_CERT_NOT_TRUSTED, e)
}
try {
attestation = Attestation(certs[0])
} catch (e: CertificateParsingException) {
// Unable to parse attestation record
throw AttestationException(AttestationException.CODE_CANT_PARSE_ATTESTATION_RECORD, e)
}
return AttestationResult(useStrongBox, attestation, isGoogleRootCertificate)
}关键在于 KeyStore.getCertificateChain ，返回的证书链的第一项包含一些信息与设备解锁状态、boot hash 有关。（API 24 开始）
https://developer.android.com/training/articles/security-key-attestation#root_certificate
google 给出了根证书的公钥，如果用公钥验证匹配，说明根证书是 google 信任的。
-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----
java.security.KeyStore:
libcore/ojluni/src/main/java/java/security/KeyStore.java
libcore/ojluni/src/main/java/java/security/Provider.java
libcore/ojluni/src/main/java/java/security/KeyStoreSpi.java
Android 的实现：
KeyStore2 (Android 12)
frameworks/base/keystore/java/android/security/KeyStore2.java
frameworks/base/keystore/java/android/security/keystore2/AndroidKeyStoreSpi.java
frameworks/base/keystore/java/android/security/keystore2/AndroidKeyStoreProvider.java
system/hardware/interfaces/keystore2/aidl/android/system/keystore2/IKeystoreService.aidl
KeyStore(Android 11):
frameworks/base/keystore/java/android/security/keystore/AndroidKeyStore.java
frameworks/base/keystore/java/android/security/keystore/AndroidKeyStoreSpi.java
system/security/keystore/binder/android/security/keystore/IKeystoreService.aidl
frameworks/base/keystore/java/android/security/KeyStore.java
binder 服务 IKeystoreService 由 /system/bin/keystore 实现。
# service list | grep keystore
18 android.security.keystore: [android.security.keystore.IKeystoreService]
# ps -ef|grep keyst
keystore 1537 1 0 05:01 ? 00:00:01 /system/bin/keystore /data/misc/keystore
# service call android.security.keystore 1599097156
Result: Parcel(00000601 '....') # 1537服务接口和实现（目前是 keystore2）：
system/hardware/interfaces/keystore2/aidl/android/system/keystore2/IKeystoreService.aidl
system/security/keystore2/src/service.rs
system/security/keystore/
android-key-attestation/Constants.java at master · google/android-key-attestation
// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.kt
private fun doAttestation(alias: String, useStrongBox: Boolean, includeProps: Boolean
): AttestationResult {
val certs: Array<X509Certificate?>?
val attestation: Attestation
val isGoogleRootCertificate: Boolean
try {
val keyStore = KeyStore.getInstance("AndroidKeyStore")
keyStore.load(null)
generateKey(alias, useStrongBox, includeProps)
val certificates = keyStore.getCertificateChain(alias)
certs = arrayOfNulls(certificates.size)
for (i in certs.indices) certs[i] = certificates[i] as X509Certificate
} catch (e: ProviderException) { private fun generateKey(alias: String, useStrongBox: Boolean, includeProps: Boolean) {
val keyPairGenerator = KeyPairGenerator.getInstance(
KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore")
val now = Date()
val originationEnd = Date(now.time + 1000000)
val consumptionEnd = Date(now.time + 2000000)
val builder = KeyGenParameterSpec.Builder(alias, KeyProperties.PURPOSE_SIGN)
.setAlgorithmParameterSpec(ECGenParameterSpec("secp256r1"))
.setKeyValidityStart(now)
.setKeyValidityForOriginationEnd(originationEnd)
.setKeyValidityForConsumptionEnd(consumptionEnd)
.setAttestationChallenge(now.toString().toByteArray())
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S && includeProps) {
builder.setDevicePropertiesAttestationIncluded(true)
}
if (Build.VERSION.SDK_INT >= 28 && useStrongBox) {
builder.setIsStrongBoxBacked(true)
}
builder.setDigests(KeyProperties.DIGEST_NONE, KeyProperties.DIGEST_SHA256)
keyPairGenerator.initialize(builder.build())
keyPairGenerator.generateKeyPair()
}首先需要用 AndroidKeyStore 生成一个密钥对，并给它指定一个别名 (alias) 。AndroidKeystore 会使用 keymaster 生成密钥对，如果使用硬件，具体的实现取决于厂商。
密钥对是一个 java.security.KeyPair 对象，包含一个 PrivateKey 和一个 PublicKey ，通过 AndroidKeyStore 生成的 PrivateKey 无法直接获取其密钥的字节（getEncoded 返回 null），即私钥存储在 Keystore 中，不能直接取出。
frameworks/base/keystore/java/android/security/keystore/AndroidKeyStorePrivateKey.java
frameworks/base/keystore/java/android/security/keystore/AndroidKeyStorePublicKey.java
frameworks/base/keystore/java/android/security/keystore/AndroidKeyStoreKey.java
通过 setAttestationChallenge 指定了一个 KeyAttestation 的 challenge ，这样得到的公钥的证书信息就会包含一个扩展字段，包含了我们需要的 Root Of Trust 等信息。
此处用的是日期的字符串字节作为 challenge 。
If attestationChallenge is not null, the public key certificate for this key pair will contain an extension that describes the details of the key's configuration and authorizations, including the attestationChallenge value. If the key is in secure hardware, and if the secure hardware supports attestation, the certificate will be signed by a chain of certificates rooted at a trustworthy CA key. Otherwise the chain will be rooted at an untrusted certificate.
我们要用别名从 AndroidKeyStore 取出刚才生成的密钥对的证书链。
验证证书链是否合法，并确定是否是 google 签名的根证书
try {
isGoogleRootCertificate = VerifyCertificateChain.verifyCertificateChain(certs)
} catch (e: GeneralSecurityException) {具体方法就是从根证书开始，用每个证书的公钥验证对下一级证书的签名（根证书也要验证自己的签名）
还要检查根证书是否来自 google 。
public static boolean verifyCertificateChain(X509Certificate[] certs)
throws GeneralSecurityException {
X509Certificate parent = certs[certs.length - 1];
var context = AppApplication.getApp().getApplicationContext();
var stream = context.getResources().openRawResource(R.raw.status);
JsonObject entries = CertificateRevocationStatus.parseStatus(stream);
for (int i = certs.length - 1; i >= 0; i--) {
X509Certificate cert = certs[i];
cert.checkValidity();
cert.verify(parent.getPublicKey());
parent = cert;
var certStatus = CertificateRevocationStatus.decodeStatus(cert.getSerialNumber(), entries);
if (certStatus != null) {
throw new CertificateException("Certificate revocation status is " + certStatus.status);
}
}
var X509Factory = CertificateFactory.getInstance("X.509");
var root = certs[certs.length - 1].getTBSCertificate();
for (String certificate : GOOGLE_ROOT_CERTIFICATES) {
var inStream = new ByteArrayInputStream(certificate.getBytes(UTF_8));
var secureRoot = (X509Certificate) X509Factory.generateCertificate(inStream);
if (Arrays.equals(secureRoot.getTBSCertificate(), root)) {
return true;
}
}
return false;
}Attestation 的内容在证书链的第一个 X509 证书的扩展字段，使用 ASN.1 格式存储
// app/src/main/java/io/github/vvb2060/keyattestation/attestation/Attestation.java
public Attestation(X509Certificate x509Cert) throws CertificateParsingException {
ASN1Sequence seq = getAttestationSequence(x509Cert);
unexpectedExtensionOids = retrieveUnexpectedExtensionOids(x509Cert);
attestationVersion = Asn1Utils.getIntegerFromAsn1(seq.getObjectAt(ATTESTATION_VERSION_INDEX));
attestationSecurityLevel = Asn1Utils.getIntegerFromAsn1(seq.getObjectAt(ATTESTATION_SECURITY_LEVEL_INDEX));
keymasterVersion = Asn1Utils.getIntegerFromAsn1(seq.getObjectAt(KEYMASTER_VERSION_INDEX));
keymasterSecurityLevel = Asn1Utils.getIntegerFromAsn1(seq.getObjectAt(KEYMASTER_SECURITY_LEVEL_INDEX));
attestationChallenge =
Asn1Utils.getByteArrayFromAsn1(seq.getObjectAt(Attestation.ATTESTATION_CHALLENGE_INDEX));
uniqueId = Asn1Utils.getByteArrayFromAsn1(seq.getObjectAt(Attestation.UNIQUE_ID_INDEX));
softwareEnforced = new AuthorizationList(seq.getObjectAt(SW_ENFORCED_INDEX));
teeEnforced = new AuthorizationList(seq.getObjectAt(TEE_ENFORCED_INDEX));
}
static final String KEY_DESCRIPTION_OID = "1.3.6.1.4.1.11129.2.1.17";
private ASN1Sequence getAttestationSequence(X509Certificate x509Cert)
throws CertificateParsingException {
byte[] attestationExtensionBytes = x509Cert.getExtensionValue(KEY_DESCRIPTION_OID);
if (attestationExtensionBytes == null || attestationExtensionBytes.length == 0) {
throw new CertificateParsingException(
"Did not find extension with OID " + KEY_DESCRIPTION_OID);
}
return Asn1Utils.getAsn1SequenceFromBytes(attestationExtensionBytes);
}如果应用访问了 keystore 服务，则应用可能进行了 key attestation 。
我们可以用 binder debugfs 来观察应用是否访问 keystore 服务：
cat /d/binder/proc/$(pidof keystore)可以看到第一个 binder node 引用的进程较多
binder proc state:
proc 1537
context binder
thread 1537: l 12 need_return 0 tr 0
thread 22091: l 00 need_return 0 tr 0
node 1803: ub4000079f9615560 cb4000079f960d5c8 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 7 iw 7 tr 1 proc 28455 29271 28303 28869 5251 1838 569
node 191752788: ub4000079f96158c0 cb4000079f96066c0 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 29271
node 191495115: ub4000079f9615ae0 cb4000079f9606720 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 28869
node 191790298: ub4000079f9615da0 cb4000079f7c4e120 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 29271
ref 1801: desc 0 node 4 s 1 w 1 d 0000000000000000
ref 191790310: desc 1 node 191790309 s 1 w 1 d 0000000000000000
ref 191790293: desc 3 node 191790292 s 1 w 0 d 0000000000000000
ref 416917: desc 4 node 15351 s 1 w 1 d 0000000000000000
binder proc state:
proc 1537
context hwbinder
thread 1537: l 00 need_return 0 tr 0
thread 22091: l 00 need_return 0 tr 0
ref 1733: desc 0 node 1 s 1 w 1 d 0000000000000000
ref 1781: desc 2 node 122 s 1 w 1 d 0000000000000000
buffer 1789: 0000000000000000 size 8:0:0 delivered
buffer 191790316: 0000000000000000 size 168:24:176 delivered
我们启动 key attestation ，再 print ：
# cat /d/binder/proc/$(pidof keystore)
binder proc state:
proc 1537
context binder
thread 1537: l 12 need_return 0 tr 0
thread 22491: l 00 need_return 0 tr 0
node 1803: ub4000079f9615560 cb4000079f960d5c8 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 8 iw 8 tr 1 proc 22457 28455 29271 28303 28869 5251 1838 569
node 191752788: ub4000079f96158c0 cb4000079f96066c0 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 29271
node 191495115: ub4000079f9615ae0 cb4000079f9606720 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 28869
node 191790298: ub4000079f9615da0 cb4000079f7c4e120 pri 0:139 hs 1 hw 1 ls 0 lw 0 is 1 iw 1 tr 1 proc 29271
ref 1801: desc 0 node 4 s 1 w 1 d 0000000000000000
ref 191801757: desc 2 node 191801756 s 1 w 1 d 0000000000000000
ref 416917: desc 4 node 15351 s 1 w 1 d 0000000000000000
引用的进程 pid 出现了 22457 ，正是 key attestation 的。启动 momo 也类似。这些应用都进行了 key attestation 来检测解锁状态。
# ps -ef|grep key
keystore 1537 1 0 Nov27 ? 00:00:33 /system/bin/keystore /data/misc/keystore
u0_a266 22457 762 12 12:42 ? 00:00:00 io.github.vvb2060.keyattestation
root 22518 22220 1 12:42 pts/0 00:00:00 grep --color key
我们也可以用代理 binder 的方法：
这是 Rikka 曾经提出的通过 addService 来代理 binder 的做法
class ProxyBinder(private val orig: IBinder) : Binder() {
override fun onTransact(code: Int, data: Parcel, reply: Parcel?, flags: Int): Boolean {
return try {
println("transact $code from u${getCallingUid()}/p${getCallingPid()} (oneway=${flags and FLAG_ONEWAY != 0})")
orig.transact(code, data, reply, flags)
} catch (t: Throwable) {
println("error occurred while proxy binder transaction")
println(t)
false;
}
}
}
fun runBinderProxy(arg: ArgParser) {
val name = arg.getOption("n")
val oldBinder = ServiceManager.getService(name)!!
println("get binder for service $name : $oldBinder")
try {
val newBinder = ProxyBinder(oldBinder)
ServiceManager.addService(name, newBinder)
println("replaced service $name\nPress any key to exit")
System.`in`.read()
} finally {
println("recovery service $name")
ServiceManager.addService(name, oldBinder)
}
}上面的代码可以代理在 servicemanager 注册的 binder 服务，并观察 transaction ，打印 calling pid 。
这在 AVD Android 12 可用，在朋友的 MIUI (Android 12) 手机上也可用，但唯独我自己的 MIUI (Android 11) 始终没法代理到服务，而 addService 的过程并没有报错。
于是搜了一下源码，发现 Android 11 addService 的时候用的是 map::emplace ，这样如果服务存在不会发生替换，后来又被改成直接替换了。在 Android 11 以前也是会替换的，似乎只有这一个版本不行。
// Android 11
// https://cs.android.com/android/platform/superproject/+/android-11.0.0_r1:frameworks/native/cmds/servicemanager/ServiceManager.cpp;l=216;drc=e192a6721d3812c73cdee58bd775f6a489c7d844
auto entry = mNameToService.emplace(name, Service {
.binder = binder,
.allowIsolated = allowIsolated,
.dumpPriority = dumpPriority,
.debugPid = ctx.debugPid,
});
// https://cs.android.com/android/platform/superproject/+/master:frameworks/native/cmds/servicemanager/ServiceManager.cpp;l=378;drc=ce94b75b68df3e07e9c73d42eaa59cfce0929e53
// Overwrite the old service if it exists
mNameToService[name] = Service{
.binder = binder,
.allowIsolated = allowIsolated,
.dumpPriority = dumpPriority,
.ctx = ctx,
};
// Android 10
// https://cs.android.com/android/platform/superproject/+/android-10.0.0_r27:frameworks/native/cmds/servicemanager/service_manager.c;l=229;drc=5516d77f61c0553f20b7332842863bc511a97074
si = find_svc(s, len);
if (si) {
if (si->handle) {
ALOGE("add_service('%s',%x) uid=%d - ALREADY REGISTERED, OVERRIDE\n",
str8(s, len), handle, uid);
svcinfo_death(bs, si);
}
si->handle = handle;
} else {