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[原创]Key Attestation原理理解
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发表于: 2024-8-31 02:00
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结合github项目KeyAttestation来学习KeyAttestation原理
页面展示来自于attestationResult这个回调结果
从代码流程中可以分为三步
doAttestation的入参有三个
获取方式是
是从PackageManager中获取系统属性，这三个属性指的是什么意思呢？这里需要引入Android KeyStore的演变历史
KeyStore是借助系统芯片 (SoC) 中提供的可信执行环境，由硬件支持的密钥库
在 Android 6.0 之前的版本中，Android 已有一个非常简单的由硬件支持的加密服务 API（由 0.2 和 0.3 版的 Keymaster 硬件抽象层 (HAL) 提供）。该密钥库能够提供数字签名和验证操作，以及不对称签名密钥对的生成和导入操作。该 API 在许多设备上都已实现，但有许多安全目标无法只通过一个签名 API 来轻松达成。Android 6.0 中的密钥库在该密钥库 API 的基础上进行了扩展，能够提供更广泛的功能
在 Android 6.0 中，密钥库不仅增加了对称加密基元（AES 和 HMAC），还增加了针对由硬件支持的密钥的访问权限控制系统。访问权限控制在密钥生成期间指定，并会在密钥的整个生命周期内被强制执行。可以将密钥限定为仅在用户通过身份验证后才可使用，并且只能用于指定的目的或只有在具有指定的加密参数时才可使用。如需了解详情，请参阅授权标记和函数页面。
在 Android 7.0 中，Keymaster 2 增加了对密钥认证和版本绑定的支持。密钥认证提供公钥证书，这些证书中包含密钥及其访问权限控制的详细描述，以使密钥存在于安全硬件中并使其配置可以远程验证。
在 Android 8.0 中，Keymaster 3 从旧式 C 结构硬件抽象层 (HAL) 转换到根据新硬件接口定义语言 (HIDL) 中的定义生成的 C++ HAL 接口。在此变更过程中，很多参数类型发生了变化，但这些类型和方法与旧的类型和 HAL 结构体方法一一对应。如需了解详情，请参阅函数页面
除了此接口修订之外，Android 8.0 还扩展了 Keymaster 2 的认证功能，以支持 ID 认证。 ID 认证提供了一种受限且可选的机制来严格认证硬件标识符，例如设备序列号、产品名称和手机 ID (IMEI/MEID)。为了实现此新增功能，Android 8.0 更改了 ASN.1 认证架构，添加了 ID 认证。Keymaster 实现需要通过某种安全方式来检索相关的数据项，还需要定义一种安全永久地停用该功能的机制。
Android 9 纳入了以下更新：
更新到 Keymaster 4
对嵌入式安全元件的支持
对安全密钥导入的支持
对 3DES 加密的支持
更改了版本绑定，以便 boot.img 和 system.img 分别设置版本以允许独立更新
从KeyStore的版本演变上看，在迭代过程中逐步加入了新的认证方式，而FEATURE_STRONGBOX_KEYSTORE、FEATURE_KEYSTORE_APP_ATTEST_KEY就是判断设备是否支持某种认证方式（原因是因为OEM厂商不一定会紧跟着Google的架构演变方案）
这里区分了生成key的类型，如果设备开启了App Attest Key特性的话生成的密钥可以用来做密钥认证（Key Attestation），否则就是正常的数字签名密钥
这里优先根据是否开启了App Attest Key特性及KeyStore中是否包含attestKeyAlias的密钥来进行密钥生成
此时，KeyStore中包含了名称为alias的密钥对
在上一步生成key之后根据alias获取到对应的证书链，证书链是一个认证过程，最终指向可信的根证书，因此获取到的证书链实际的形式是
终端证书->中间证书->根证书
遍历证书列表，进行三次校验
在checkStatus函数中存在状态流转的过程，涉及到两个函数verify、checkValidity，主要目的是为了确保一个证书的签名是有效的、且被信任的上级证书所签发
这一步是为了确定根证书的颁发机构，其中获取OEM Key的方式如下
根据代码逻辑的理解，是根据证书颁发机构来选择各自的认证方式
EatAttestation使用实体认证令牌 (Entity Attestation Token, EAT) 进行设备认证。EAT是一种基于IETF标准的轻量级认证格式，场景更多的是存在嵌入到IoT设备中
三星电子提供的一种设备认证和安全机制，核心逻辑还是走的Asn1
重点关注下Asn1的认证方式，也是官方选择的认证方式，参考文档security-key-attestation
根据传入的x509Cert解析成各个字段，这里可以参考ASN.1架构
checkAttestation的核心是
核心是要证明teeEnforced的purpose字段，根据上面那段注释我理解是将密钥用途分为了签名和认证
这里引入了认证的源头：根证书的可信度问题
Andorid通过Trusty TEE完成证书的存储，TEE在硬件层面解决了安全性问题，其中类似RootOfTrust这类数据都是由厂商在设备出产时烧录到硬件存储当中的，从根本上解决了根密钥不可信的问题，并以此根密钥为信任链根，派生密钥
可以利用KeyAttestation来做什么呢？
// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.ktval useStrongBox = hasStrongBox && preferStrongBoxval includeProps = hasDeviceIds && preferIncludePropsval useAttestKey = hasAttestKey && preferAttestKeyval result = try {
val attestationResult = doAttestation(useStrongBox, includeProps, useAttestKey)
Resource.success(attestationResult)
} catch (e: Throwable) {
val cause = if (e is AttestationException) e.cause else e
Log.w(AppApplication.TAG, "Do attestation error.", cause)
when (e) {
is AttestationException -> Resource.error(e, null)
else -> Resource.error(AttestationException(CODE_UNKNOWN, e), null)
}
}// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.ktval useStrongBox = hasStrongBox && preferStrongBoxval includeProps = hasDeviceIds && preferIncludePropsval useAttestKey = hasAttestKey && preferAttestKeyval result = try {
val attestationResult = doAttestation(useStrongBox, includeProps, useAttestKey)
Resource.success(attestationResult)
} catch (e: Throwable) {
val cause = if (e is AttestationException) e.cause else e
Log.w(AppApplication.TAG, "Do attestation error.", cause)
when (e) {
is AttestationException -> Resource.error(e, null)
else -> Resource.error(AttestationException(CODE_UNKNOWN, e), null)
}
}// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.kt@Throws(AttestationException::class)
private fun doAttestation(useStrongBox: Boolean,
includeProps: Boolean,
useAttestKey: Boolean): AttestationResult {
val certs = ArrayList<Certificate>()
val alias = if (useStrongBox) "${AppApplication.TAG}_strongbox" else AppApplication.TAG
val attestKeyAlias = if (useAttestKey) "${alias}_persistent" else null
try {
// 1. generateKey
if (useAttestKey && !keyStore.containsAlias(attestKeyAlias)) {
generateKey(attestKeyAlias!!, useStrongBox, includeProps, attestKeyAlias)
}
generateKey(alias, useStrongBox, includeProps, attestKeyAlias)
// 2. certs collect
val certChain = keyStore.getCertificateChain(alias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in certChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
if (useAttestKey) {
val persistChain = keyStore.getCertificateChain(attestKeyAlias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in persistChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
}
} catch (e: ProviderException) {
// 异常流程，可忽略
......
} catch (e: Exception) {
throw AttestationException(CODE_UNKNOWN, e)
}
@Suppress("UNCHECKED_CAST")
// 3. parseCertificateChain
currentCerts = certs as List<X509Certificate>
return parseCertificateChain(certs)
}// app/src/main/java/io/github/vvb2060/keyattestation/home/HomeViewModel.kt@Throws(AttestationException::class)
private fun doAttestation(useStrongBox: Boolean,
includeProps: Boolean,
useAttestKey: Boolean): AttestationResult {
val certs = ArrayList<Certificate>()
val alias = if (useStrongBox) "${AppApplication.TAG}_strongbox" else AppApplication.TAG
val attestKeyAlias = if (useAttestKey) "${alias}_persistent" else null
try {
// 1. generateKey
if (useAttestKey && !keyStore.containsAlias(attestKeyAlias)) {
generateKey(attestKeyAlias!!, useStrongBox, includeProps, attestKeyAlias)
}
generateKey(alias, useStrongBox, includeProps, attestKeyAlias)
// 2. certs collect
val certChain = keyStore.getCertificateChain(alias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in certChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
if (useAttestKey) {
val persistChain = keyStore.getCertificateChain(attestKeyAlias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in persistChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
}
} catch (e: ProviderException) {
// 异常流程，可忽略
......
} catch (e: Exception) {
throw AttestationException(CODE_UNKNOWN, e)
}
@Suppress("UNCHECKED_CAST")
// 3. parseCertificateChain
currentCerts = certs as List<X509Certificate>
return parseCertificateChain(certs)
}// android.hardware.strongbox_keystoreuseStrongBox = pm.hasSystemFeature(PackageManager.FEATURE_STRONGBOX_KEYSTORE)// android.hardware.keystore.app_attest_keyhasAttestKey = pm.hasSystemFeature(PackageManager.FEATURE_KEYSTORE_APP_ATTEST_KEY)hasDeviceIds = pm.hasSystemFeature("android.software.device_id_attestation")
// android.hardware.strongbox_keystoreuseStrongBox = pm.hasSystemFeature(PackageManager.FEATURE_STRONGBOX_KEYSTORE)// android.hardware.keystore.app_attest_keyhasAttestKey = pm.hasSystemFeature(PackageManager.FEATURE_KEYSTORE_APP_ATTEST_KEY)hasDeviceIds = pm.hasSystemFeature("android.software.device_id_attestation")
if (useAttestKey && !keyStore.containsAlias(attestKeyAlias)) {
generateKey(attestKeyAlias!!, useStrongBox, includeProps, attestKeyAlias)
}generateKey(alias, useStrongBox, includeProps, attestKeyAlias)if (useAttestKey && !keyStore.containsAlias(attestKeyAlias)) {
generateKey(attestKeyAlias!!, useStrongBox, includeProps, attestKeyAlias)
}generateKey(alias, useStrongBox, includeProps, attestKeyAlias)private fun generateKey(alias: String,
useStrongBox: Boolean,
includeProps: Boolean,
attestKeyAlias: String?) {
val now = Date()
val attestKey = alias == attestKeyAlias
// 密钥用途判定
val purposes = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S && attestKey) {
KeyProperties.PURPOSE_ATTEST_KEY
} else {
KeyProperties.PURPOSE_SIGN
}
// 设置 KeyGenParameterSpec
val builder = KeyGenParameterSpec.Builder(alias, purposes)
.setAlgorithmParameterSpec(ECGenParameterSpec("secp256r1"))
.setDigests(KeyProperties.DIGEST_SHA256)
.setCertificateNotBefore(now)
.setAttestationChallenge(now.toString().toByteArray())
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P && useStrongBox) {
builder.setIsStrongBoxBacked(true)
}
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
if (includeProps) {
builder.setDevicePropertiesAttestationIncluded(true)
}
if (attestKey) {
builder.setCertificateSubject(X500Principal("CN=App Attest Key"))
} else {
builder.setAttestKeyAlias(attestKeyAlias)
}
}
// 获取 KeyPairGenerator 实例
val keyPairGenerator = KeyPairGenerator.getInstance(
KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore")
// 用 KeyGenParameterSpec 初始化 KeyPairGenerator
keyPairGenerator.initialize(builder.build())
// 生成密钥对
keyPairGenerator.generateKeyPair()
}private fun generateKey(alias: String,
useStrongBox: Boolean,
includeProps: Boolean,
attestKeyAlias: String?) {
val now = Date()
val attestKey = alias == attestKeyAlias
// 密钥用途判定
val purposes = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S && attestKey) {
KeyProperties.PURPOSE_ATTEST_KEY
} else {
KeyProperties.PURPOSE_SIGN
}
// 设置 KeyGenParameterSpec
val builder = KeyGenParameterSpec.Builder(alias, purposes)
.setAlgorithmParameterSpec(ECGenParameterSpec("secp256r1"))
.setDigests(KeyProperties.DIGEST_SHA256)
.setCertificateNotBefore(now)
.setAttestationChallenge(now.toString().toByteArray())
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P && useStrongBox) {
builder.setIsStrongBoxBacked(true)
}
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
if (includeProps) {
builder.setDevicePropertiesAttestationIncluded(true)
}
if (attestKey) {
builder.setCertificateSubject(X500Principal("CN=App Attest Key"))
} else {
builder.setAttestKeyAlias(attestKeyAlias)
}
}
// 获取 KeyPairGenerator 实例
val keyPairGenerator = KeyPairGenerator.getInstance(
KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore")
// 用 KeyGenParameterSpec 初始化 KeyPairGenerator
keyPairGenerator.initialize(builder.build())
// 生成密钥对
keyPairGenerator.generateKeyPair()
}val certChain = keyStore.getCertificateChain(alias) ?: throw CertificateException("Unable to get certificate chain")
for (cert in certChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}if (useAttestKey) {
val persistChain = keyStore.getCertificateChain(attestKeyAlias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in persistChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
}val certChain = keyStore.getCertificateChain(alias) ?: throw CertificateException("Unable to get certificate chain")
for (cert in certChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}if (useAttestKey) {
val persistChain = keyStore.getCertificateChain(attestKeyAlias)
?: throw CertificateException("Unable to get certificate chain")
for (cert in persistChain) {
val buf = ByteArrayInputStream(cert.encoded)
certs.add(certificateFactory.generateCertificate(buf))
}
}// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javapublic static AttestationResult parseCertificateChain(List<X509Certificate> certs) {
var infoList = new ArrayList<CertificateInfo>();
// 在certs中最后一个指向的是终端证书，逐步向上遍历
var parent = certs.get(certs.size() - 1);
for (int i = certs.size() - 1; i >= 0; i--) {
var parentKey = parent.getPublicKey();
var info = new CertificateInfo(certs.get(i));
infoList.add(info);
info.checkStatus(parentKey);
if (parent == info.cert) {
info.checkIssuer();
} else {
parent = info.cert;
}
if (info.checkAttestation()) {
break;
}
}
return AttestationResult.form(infoList);
// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javapublic static AttestationResult parseCertificateChain(List<X509Certificate> certs) {
var infoList = new ArrayList<CertificateInfo>();
// 在certs中最后一个指向的是终端证书，逐步向上遍历
var parent = certs.get(certs.size() - 1);
for (int i = certs.size() - 1; i >= 0; i--) {
var parentKey = parent.getPublicKey();
var info = new CertificateInfo(certs.get(i));
infoList.add(info);
info.checkStatus(parentKey);
if (parent == info.cert) {
info.checkIssuer();
} else {
parent = info.cert;
}
if (info.checkAttestation()) {
break;
}
}
return AttestationResult.form(infoList);
// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javaprivate void checkStatus(PublicKey parentKey) {
try {
status = CERT_SIGN;
cert.verify(parentKey);
status = CERT_REVOKED;
var certStatus = RevocationList.get(cert.getSerialNumber());
if (certStatus != null) {
throw new CertificateException("Certificate revocation " + certStatus);
}
status = CERT_EXPIRED;
cert.checkValidity();
status = CERT_NORMAL;
} catch (GeneralSecurityException e) {
securityException = e;
}
}// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javaprivate void checkStatus(PublicKey parentKey) {
try {
status = CERT_SIGN;
cert.verify(parentKey);
status = CERT_REVOKED;
var certStatus = RevocationList.get(cert.getSerialNumber());
if (certStatus != null) {
throw new CertificateException("Certificate revocation " + certStatus);
}
status = CERT_EXPIRED;
cert.checkValidity();
status = CERT_NORMAL;
} catch (GeneralSecurityException e) {
securityException = e;
}
}// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javaprivate void checkIssuer() {
var publicKey = cert.getPublicKey().getEncoded();
if (Arrays.equals(publicKey, googleKey)) {
issuer = KEY_GOOGLE;
} else if (Arrays.equals(publicKey, aospEcKey)) {
issuer = KEY_AOSP;
} else if (Arrays.equals(publicKey, aospRsaKey)) {
issuer = KEY_AOSP;
} else if (Arrays.equals(publicKey, knoxSakv2Key)) {
issuer = KEY_KNOX;
} else if (oemKeys != null) {
for (var key : oemKeys) {
if (Arrays.equals(publicKey, key.getEncoded())) {
issuer = KEY_OEM;
break;
}
}
}
}// app/src/main/java/io/github/vvb2060/keyattestation/attestation/CertificateInfo.javaprivate void checkIssuer() {
var publicKey = cert.getPublicKey().getEncoded();
if (Arrays.equals(publicKey, googleKey)) {
issuer = KEY_GOOGLE;
} else if (Arrays.equals(publicKey, aospEcKey)) {
issuer = KEY_AOSP;
} else if (Arrays.equals(publicKey, aospRsaKey)) {
issuer = KEY_AOSP;
} else if (Arrays.equals(publicKey, knoxSakv2Key)) {
issuer = KEY_KNOX;
} else if (oemKeys != null) {
for (var key : oemKeys) {
if (Arrays.equals(publicKey, key.getEncoded())) {
issuer = KEY_OEM;
break;
}
}
}
}private static Set<PublicKey> getOemPublicKey() {
var resName = "android:array/vendor_required_attestation_certificates";
var res = AppApplication.app.getResources();
// noinspection DiscouragedApi
var id = res.getIdentifier(resName, null, null);
if (id == 0) {
return null;
}
var set = new HashSet<PublicKey>();
try {
var cf = CertificateFactory.getInstance("X.509");
for (var s : res.getStringArray(id)) {
var cert = s.replaceAll("\\s+", "\n")
.replaceAll("-BEGIN\\nCERTIFICATE-", "-BEGIN CERTIFICATE-")
.replaceAll("-END\\nCERTIFICATE-", "-END CERTIFICATE-");
var input = new ByteArrayInputStream(cert.getBytes());
var publicKey = cf.generateCertificate(input).getPublicKey();
set.add(publicKey);
}
} catch (CertificateException e) {
Log.e(AppApplication.TAG, "getOemKeys: ", e);
return null;
}
set.removeIf(key -> Arrays.equals(key.getEncoded(), googleKey));
if (set.isEmpty()) {
return null;
}
set.forEach(key -> Log.i(AppApplication.TAG, "getOemKeys: " + key));
return set;
}private static Set<PublicKey> getOemPublicKey() {
var resName = "android:array/vendor_required_attestation_certificates";
var res = AppApplication.app.getResources();
// noinspection DiscouragedApi
var id = res.getIdentifier(resName, null, null);
if (id == 0) {
return null;
}
var set = new HashSet<PublicKey>();
try {
var cf = CertificateFactory.getInstance("X.509");
for (var s : res.getStringArray(id)) {
var cert = s.replaceAll("\\s+", "\n")
.replaceAll("-BEGIN\\nCERTIFICATE-", "-BEGIN CERTIFICATE-")
[招生]系统0day安全班，企业级设备固件漏洞挖掘，Linux平台漏洞挖掘！
最后于 2024-8-31 09:33
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，原因：
