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Add X.509 certificate extensions for RFC 5280 compliance

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- BasicConstraints: CA=false (critical)
- SubjectKeyIdentifier via SHA-1 hash
- AuthorityKeyIdentifier linked to issuer cert
This commit is contained in:
Enginex0
2026-02-06 00:47:32 +01:00
committed by GKI Builder
parent c367aa5efc
commit a11a5e41a2
1 changed files with 50 additions and 46 deletions
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app/src/main/java/org/matrix/TEESimulator/pki/CertificateGenerator.kt
+50 -46
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@@ -13,10 +13,12 @@ import java.security.spec.ECGenParameterSpec
import java.security.spec.RSAKeyGenParameterSpec
import java.util.Date
import org.bouncycastle.asn1.x500.X500Name
import org.bouncycastle.asn1.x509.BasicConstraints
import org.bouncycastle.asn1.x509.Extension
import org.bouncycastle.asn1.x509.KeyUsage
import org.bouncycastle.cert.X509CertificateHolder
import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter
import org.bouncycastle.cert.jcajce.JcaX509ExtensionUtils
import org.bouncycastle.cert.jcajce.JcaX509v3CertificateBuilder
import org.bouncycastle.jce.provider.BouncyCastleProvider
import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder
@@ -28,20 +30,8 @@ import org.matrix.TEESimulator.interception.keystore.KeyIdentifier
import org.matrix.TEESimulator.interception.keystore.shim.KeyMintSecurityLevelInterceptor
import org.matrix.TEESimulator.logging.SystemLogger
/**
* Responsible for generating new cryptographic key pairs and X.509 certificate chains.
*
* This object simulates the behavior of the Android KeyMint/Keymaster HAL by creating certificates
* that include a fully-featured, simulated attestation extension.
*/
object CertificateGenerator {
/**
* Generates a software-based cryptographic key pair.
*
* @param params The parameters specifying the key's algorithm, size, and other properties.
* @return A new [KeyPair], or `null` on failure.
*/
fun generateSoftwareKeyPair(params: KeyMintAttestation): KeyPair? {
return runCatching {
val (algorithm, spec) =
@@ -64,17 +54,6 @@ object CertificateGenerator {
.getOrNull()
}
/**
* Generates a certificate chain for a given key pair. This is the primary function for creating
* attested certificates.
*
* @param uid The UID of the application requesting the key.
* @param subjectKeyPair The key pair for which the certificate will be generated.
* @param attestKeyAlias Optional alias of a key to use for attestation signing.
* @param params The parameters for the new key and its attestation.
* @param securityLevel The security level to embed in the attestation.
* @return A [List] of [Certificate] forming the new chain, or `null` on failure.
*/
fun generateCertificateChain(
uid: Int,
subjectKeyPair: KeyPair,
@@ -91,22 +70,26 @@ object CertificateGenerator {
return runCatching {
val keybox = getKeyboxForAlgorithm(uid, params.algorithm)
// Determine the signing key and issuer. If an attestKey is provided, use it.
// Otherwise, fall back to the root key from the keybox.
val (signingKey, issuer) =
val (signingKey, issuer, issuerCert) =
if (attestKeyAlias != null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
getAttestationKeyInfo(uid, attestKeyAlias)?.let { it.first to it.second }
?: (keybox.keyPair to getIssuerFromKeybox(keybox))
getAttestationKeyInfo(uid, attestKeyAlias)?.let {
Triple(it.first, it.second, null as X509Certificate?)
} ?: Triple(
keybox.keyPair,
getIssuerFromKeybox(keybox),
keybox.certificates.firstOrNull() as? X509Certificate
)
} else {
keybox.keyPair to getIssuerFromKeybox(keybox)
Triple(
keybox.keyPair,
getIssuerFromKeybox(keybox),
keybox.certificates.firstOrNull() as? X509Certificate
)
}
// Build the new leaf certificate with the simulated attestation.
val leafCert =
buildCertificate(subjectKeyPair, signingKey, issuer, params, uid, securityLevel)
buildCertificate(subjectKeyPair, signingKey, issuer, issuerCert, params, uid, securityLevel)
// If not self-attesting, the chain is just the leaf. Otherwise, append the keybox
// chain.
if (attestKeyAlias != null) {
listOf(leafCert)
} else {
@@ -117,10 +100,6 @@ object CertificateGenerator {
.getOrNull()
}
/**
* A convenience function that combines key pair generation and certificate chain generation.
* Primarily used by the modern Keystore2 interceptor where generation is a single step.
*/
fun generateAttestedKeyPair(
uid: Int,
alias: String,
@@ -166,11 +145,9 @@ object CertificateGenerator {
?: throw Exception("Could not load keybox for UID $uid and algorithm $algorithmName")
}
/** Retrieves the key pair and issuer name for a given attestation key alias. */
private fun getAttestationKeyInfo(uid: Int, attestKeyAlias: String): Pair<KeyPair, X500Name>? {
SystemLogger.debug("Looking for attestation key: uid=$uid alias=$attestKeyAlias")
val keyId = KeyIdentifier(uid, attestKeyAlias)
// Access the public map of generated keys
val keyInfo = KeyMintSecurityLevelInterceptor.generatedKeys[keyId]
return if (keyInfo != null) {
val certChain = CertificateHelper.getCertificateChain(keyInfo.response)
@@ -188,7 +165,6 @@ object CertificateGenerator {
}
}
/** Maps KeyPurpose values to X.509 KeyUsage bits per KeyCreationResult.aidl spec */
private fun buildKeyUsageFromPurposes(purposes: List<Int>): Int {
var bits = 0
for (purpose in purposes) {
@@ -204,19 +180,22 @@ object CertificateGenerator {
return bits
}
/** Constructs a new X.509 certificate with a simulated attestation extension. */
private fun buildCertificate(
subjectKeyPair: KeyPair,
signingKeyPair: KeyPair,
issuer: X500Name,
issuerCert: X509Certificate?,
params: KeyMintAttestation,
uid: Int,
securityLevel: Int,
): Certificate {
val subject = params.certificateSubject ?: X500Name("CN=Android KeyStore Key")
val leafNotAfter =
(signingKeyPair.public as? X509Certificate)?.notAfter
?: Date(System.currentTimeMillis() + 31536000000L)
val subject = params.certificateSubject ?: X500Name("CN=Android Keystore Key")
SystemLogger.debug("[CertGen] Subject: $subject")
// Real TEEs use 25-30 year validity periods (per AOSP Beanpod KeyMaster observation)
val THIRTY_YEARS_MS = 30L * 365 * 24 * 60 * 60 * 1000
val leafNotAfter = issuerCert?.notAfter
?: Date(System.currentTimeMillis() + THIRTY_YEARS_MS)
SystemLogger.debug("[CertGen] Validity: ${params.certificateNotBefore ?: Date()} to $leafNotAfter")
val builder =
JcaX509v3CertificateBuilder(
@@ -232,8 +211,33 @@ object CertificateGenerator {
val keyUsageBits = buildKeyUsageFromPurposes(params.purpose)
if (keyUsageBits != 0) {
builder.addExtension(Extension.keyUsage, true, KeyUsage(keyUsageBits))
SystemLogger.debug("[CertGen] Added KeyUsage extension: bits=0x${keyUsageBits.toString(16)}")
}
// Add our custom, simulated attestation extension.
// RFC 5280 Section 4.2.1.9: end-entity cert must not act as CA
builder.addExtension(Extension.basicConstraints, true, BasicConstraints(false))
SystemLogger.debug("[CertGen] Added BasicConstraints: CA=false")
val extUtils = JcaX509ExtensionUtils()
// RFC 5280 Section 4.2.1.2: SHA-1 hash of subject public key
builder.addExtension(
Extension.subjectKeyIdentifier,
false,
extUtils.createSubjectKeyIdentifier(subjectKeyPair.public)
)
SystemLogger.debug("[CertGen] Added SubjectKeyIdentifier (SKI)")
// RFC 5280 Section 4.2.1.1: links certificate to issuer's signing key
if (issuerCert != null) {
builder.addExtension(
Extension.authorityKeyIdentifier,
false,
extUtils.createAuthorityKeyIdentifier(issuerCert)
)
SystemLogger.debug("[CertGen] Added AuthorityKeyIdentifier (AKI) from issuer")
}
builder.addExtension(
AttestationBuilder.buildAttestationExtension(params, uid, securityLevel)
)