Key attestation support for cloud secure area keys. (#777)

Signed-off-by: Peter Sorotokin <[email protected]>
openwallet-foundation-labs · Nov 14, 2024 · e3d2252 · e3d2252
1 parent aeea796
commit e3d2252
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Showing 5 changed files with 122 additions and 26 deletions.
diff --git a/identity-issuance/src/main/java/com/android/identity/issuance/authenticationUtilities.kt b/identity-issuance/src/main/java/com/android/identity/issuance/authenticationUtilities.kt
@@ -3,6 +3,7 @@ package com.android.identity.issuance
 import com.android.identity.crypto.X509CertChain
 import com.android.identity.crypto.javaX509Certificate
 import com.android.identity.crypto.javaX509Certificates
+import com.android.identity.securearea.AttestationExtension
 import com.android.identity.util.AndroidAttestationExtensionParser
 import com.android.identity.util.Logger
 import com.android.identity.util.fromHex
@@ -36,26 +37,30 @@ fun extractAttestationSequence(chain: X509CertChain): ASN1Sequence {
     return seqInputStream.readObject() as ASN1Sequence
 }
 
+private fun X509CertChain.validate() {
+    val certs = certificates
+    // Check that all the certificates sign each other...
+    for (n in 0 until certs.size - 1) {
+        val cert = certs[n]
+        val nextCert = certs[n + 1]
+        try {
+            cert.verify(nextCert)
+        } catch (e: Throwable) {
+            throw IllegalArgumentException("Attestation error: error validating certificate chain", e)
+        }
+    }
+}
+
 fun validateKeyAttestation(
     chain: X509CertChain,
     clientId: String?,
     requireGmsAttestation: Boolean,
     requireVerifiedBootGreen: Boolean,
     requireAppSignatureCertificateDigests: List<String>,
 ) {
+    chain.validate()
     val x509certs = chain.javaX509Certificates
-
-    // First check that all the certificates sign each other...
-    for (n in 0 until x509certs.size - 1) {
-        val cert = x509certs[n]
-        val nextCert = x509certs[n + 1]
-        try {
-            cert.verify(nextCert.publicKey)
-        } catch (e: Throwable) {
-            throw IllegalArgumentException("Key attestation error: error validating chain", e)
-        }
-    }
-    val rootCertificatePublicKey = x509certs[x509certs.size - 1].publicKey
+    val rootCertificatePublicKey = x509certs.last().publicKey
 
     if (requireGmsAttestation) {
         // Must match the well-known Google root
@@ -101,8 +106,42 @@ fun validateKeyAttestation(
     }
 }
 
-
 // This public key is from https://developer.android.com/training/articles/security-key-attestation
 private val GOOGLE_ROOT_ATTESTATION_KEY =
     "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"
         .fromHex()
+
+fun isCloudKeyAttestation(chain: X509CertChain): Boolean {
+    return chain.certificates[0].javaX509Certificate
+        .getExtensionValue(AttestationExtension.ATTESTATION_OID) != null
+}
+
+fun validateCloudKeyAttestation(
+    chain: X509CertChain,
+    nonce: String,
+    trustedRootKeys: Set<ByteString>
+) {
+    chain.validate()
+    val certificates = chain.certificates
+    val leafX509Cert = certificates.first().javaX509Certificate
+    val extensionDerEncodedString = leafX509Cert.getExtensionValue(AttestationExtension.ATTESTATION_OID)
+        ?: throw IllegalStateException(
+            "No attestation extension at OID ${AttestationExtension.ATTESTATION_OID}")
+
+    val attestationExtension = try {
+        val asn1InputStream = ASN1InputStream(extensionDerEncodedString);
+        (asn1InputStream.readObject() as ASN1OctetString).octets
+    } catch (e: Exception) {
+        throw IllegalStateException("Error decoding attestation extension", e)
+    }
+
+    val challengeInAttestation = AttestationExtension.decode(attestationExtension)
+    if (!challengeInAttestation.contentEquals(nonce.toByteArray())) {
+        throw IllegalStateException("Challenge in attestation does match expected attestation")
+    }
+
+    val rootPublicKey = ByteString(certificates.last().javaX509Certificate.publicKey.encoded)
+    check(trustedRootKeys.contains(rootPublicKey)) {
+        "Unexpected cloud attestation root"
+    }
+}
diff --git a/...ty-issuance/src/main/java/com/android/identity/issuance/wallet/ApplicationSupportState.kt b/...ty-issuance/src/main/java/com/android/identity/issuance/wallet/ApplicationSupportState.kt
@@ -5,6 +5,7 @@ import com.android.identity.crypto.Crypto
 import com.android.identity.crypto.EcPrivateKey
 import com.android.identity.crypto.EcPublicKey
 import com.android.identity.crypto.X509Cert
+import com.android.identity.crypto.javaX509Certificate
 import com.android.identity.flow.annotation.FlowMethod
 import com.android.identity.flow.annotation.FlowState
 import com.android.identity.flow.server.Configuration
@@ -15,6 +16,8 @@ import com.android.identity.issuance.LandingUrlUnknownException
 import com.android.identity.issuance.WalletServerSettings
 import com.android.identity.issuance.common.cache
 import com.android.identity.issuance.funke.toJson
+import com.android.identity.issuance.isCloudKeyAttestation
+import com.android.identity.issuance.validateCloudKeyAttestation
 import com.android.identity.issuance.validateKeyAttestation
 import com.android.identity.securearea.KeyAttestation
 import com.android.identity.util.Logger
@@ -108,13 +111,22 @@ class ApplicationSupportState(
         val keyList = keyAttestations.map { attestation ->
             // TODO: ensure that keys come from the same device and extract data for key_type
             // and user_authentication values
-            validateKeyAttestation(
-                attestation.certChain!!,
-                nonce,
-                settings.androidRequireGmsAttestation,
-                settings.androidRequireVerifiedBootGreen,
-                settings.androidRequireAppSignatureCertificateDigests
-            )
+            if (isCloudKeyAttestation(attestation.certChain!!)) {
+                val trustedRootKeys = getCloudSecureAreaTrustedRootKeys(env)
+                validateCloudKeyAttestation(
+                    attestation.certChain!!,
+                    nonce,
+                    trustedRootKeys.trustedKeys
+                )
+            } else {
+                validateKeyAttestation(
+                    attestation.certChain!!,
+                    nonce,
+                    settings.androidRequireGmsAttestation,
+                    settings.androidRequireVerifiedBootGreen,
+                    settings.androidRequireAppSignatureCertificateDigests
+                )
+            }
             attestation.publicKey.toJson(null)
         }
 
@@ -242,9 +254,26 @@ class ApplicationSupportState(
         return "$message.$signature"
     }
 
+    private suspend fun getCloudSecureAreaTrustedRootKeys(
+        env: FlowEnvironment
+    ): CloudSecureAreaTrustedRootKeys {
+        return env.cache(CloudSecureAreaTrustedRootKeys::class) { configuration, resources ->
+            val certificateName = configuration.getValue("csa.certificate")
+                ?: "cloud_secure_area/certificate.pem"
+            val certificate = X509Cert.fromPem(resources.getStringResource(certificateName)!!)
+            CloudSecureAreaTrustedRootKeys(
+                trustedKeys = setOf(ByteString(certificate.javaX509Certificate.publicKey.encoded))
+            )
+        }
+    }
+
     internal data class AttestationData(
         val certificate: X509Cert,
         val privateKey: EcPrivateKey,
         val clientId: String
     )
+
+    internal data class CloudSecureAreaTrustedRootKeys(
+        val trustedKeys: Set<ByteString>
+    )
 }
diff --git a/server/src/main/java/com/android/identity/wallet/server/CloudSecureAreaServlet.kt b/server/src/main/java/com/android/identity/wallet/server/CloudSecureAreaServlet.kt
@@ -13,6 +13,7 @@ import com.android.identity.crypto.javaX509Certificate
 import com.android.identity.flow.handler.FlowNotifications
 import com.android.identity.flow.server.Configuration
 import com.android.identity.flow.server.FlowEnvironment
+import com.android.identity.flow.server.Resources
 import com.android.identity.flow.server.Storage
 import com.android.identity.issuance.WalletServerSettings
 import com.android.identity.securearea.cloud.CloudSecureAreaServer
@@ -80,25 +81,38 @@ class CloudSecureAreaServlet : BaseHttpServlet() {
                 )
             }
 
-            fun createKeyMaterial(): KeyMaterial {
+            fun createKeyMaterial(serverEnvironment: FlowEnvironment): KeyMaterial {
                 val serverSecureAreaBoundKey = Random.Default.nextBytes(32)
 
                 val now = Clock.System.now()
                 val validFrom = now
                 val validUntil = now.plus(DateTimePeriod(years = 10), TimeZone.currentSystemDefault())
 
-                // Create Attestation Root w/ self-signed certificate.
+                // Load attestation root
+                val configuration = serverEnvironment.getInterface(Configuration::class)!!
+                val resources = serverEnvironment.getInterface(Resources::class)!!
+                val certificateName = configuration.getValue("csa.certificate")
+                    ?: "cloud_secure_area/certificate.pem"
+                val rootCertificate = X509Cert.fromPem(resources.getStringResource(certificateName)!!)
+                val privateKeyName = configuration.getValue("csa.privateKey")
+                    ?: "cloud_secure_area/private_key.pem"
+                val rootPrivateKey = EcPrivateKey.fromPem(
+                    resources.getStringResource(privateKeyName)!!,
+                    rootCertificate.ecPublicKey
+                )
+
+                // Create instance-specific intermediate certificate.
                 val attestationKey = Crypto.createEcPrivateKey(EcCurve.P256)
                 val attestationKeySignatureAlgorithm = Algorithm.ES256
                 val attestationKeySubject = "CN=Cloud Secure Area Attestation Root"
                 val attestationKeyCertificate = X509Cert.create(
                     attestationKey.publicKey,
-                    attestationKey,
+                    rootPrivateKey,
                     null,
                     attestationKeySignatureAlgorithm,
                     "1",
                     attestationKeySubject,
-                    attestationKeySubject,
+                    rootCertificate.javaX509Certificate.issuerX500Principal.name,
                     validFrom,
                     validUntil,
                     setOf(),
@@ -126,7 +140,7 @@ class CloudSecureAreaServlet : BaseHttpServlet() {
                 return KeyMaterial(
                     serverSecureAreaBoundKey,
                     attestationKey,
-                    X509CertChain(listOf(attestationKeyCertificate)),
+                    X509CertChain(listOf(attestationKeyCertificate, rootCertificate)),
                     attestationKeySignatureAlgorithm,
                     attestationKeySubject,
                     cloudBindingKeyAttestationKey,
@@ -150,7 +164,7 @@ class CloudSecureAreaServlet : BaseHttpServlet() {
             val keyMaterialBlob = runBlocking {
                 storage.get("RootState", "", "cloudSecureAreaKeyMaterial")?.toByteArray()
                     ?: let {
-                        val blob = KeyMaterial.createKeyMaterial().toCbor()
+                        val blob = KeyMaterial.createKeyMaterial(serverEnvironment).toCbor()
                         storage.insert(
                             "RootState",
                             "",

diff --git a/server/src/main/resources/resources/cloud_secure_area/certificate.pem b/server/src/main/resources/resources/cloud_secure_area/certificate.pem
@@ -0,0 +1,10 @@
+-----BEGIN CERTIFICATE-----
+MIIBUzCB+qADAgECAgkAjXAZuiLhFG8wCgYIKoZIzj0EAwIwFzEVMBMGA1UEAwwM
+Y3NhX2Rldl9yb290MB4XDTI0MTExMzAwMDcwM1oXDTM0MTEyMTAwMDcwM1owFzEV
+MBMGA1UEAwwMY3NhX2Rldl9yb290MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE
+SwFeYEYGbPrayIwwCFeOgNkgfuJZViqnX0GHAyJ2aAtA4Pm88Txy4gKWkIeAw12v
+JEfq75Y4WVBTU3mDppPTxKMvMC0wHQYDVR0OBBYEFEqc1iDkhWpfhozT8rxG49A6
+ClfbMAwGA1UdEwQFMAMBAf8wCgYIKoZIzj0EAwIDSAAwRQIgM75KRzV6I4gARk1I
+BcCX7n+0r2OEXWfF67N0lHcKNjYCIQDQssA1bu0juNn+GXQNmc0CVhdSAF2JbRc+
+71j/NYLI4w==
+-----END CERTIFICATE-----
diff --git a/server/src/main/resources/resources/cloud_secure_area/private_key.pem b/server/src/main/resources/resources/cloud_secure_area/private_key.pem
@@ -0,0 +1,4 @@
+-----BEGIN PRIVATE KEY-----
+MEACAQAwEwYHKoZIzj0CAQYIKoZIzj0DAQcEJjAkAgEBBB9Uhppta7eVZr6gnN4T
+92WEy2+QoQQy/JdDT/MUlXL/
+-----END PRIVATE KEY-----