Enforce attestation challenge length limit (#70)

Throws IllegalArgumentException if the challenge exceeds 128 bytes, per Android specs. Also fixes a duplicate assignment typo in KeystoreInterceptor.

Reference: https://developer.android.com/reference/android/security/keystore/KeyGenParameterSpec.Builder#setAttestationChallenge(byte[])

Co-authored-by: JingMatrix <jingmatrix@gmail.com>

app/src/main/java/org/matrix/TEESimulator/attestation/AttestationConstants.kt

@@ -1,10 +1,8 @@
 package org.matrix.TEESimulator.attestation
 
 /**
- * Defines constants for KeyMint attestation tags, as specified in the Android hardware security
+ * Defines constants for KeyMint attestation, mainly the tags of properties and authorizations of a
- * HAL.
+ * cryptographic key, as specified in the Android hardware security HAL.
- *
- * These tags identify specific properties and authorizations of a cryptographic key.
  */
 object AttestationConstants {
     // https://cs.android.com/android/platform/superproject/main/+/main:hardware/interfaces/security/keymint/aidl/android/hardware/security/keymint/KeyCreationResult.aidl
@@ -88,4 +86,8 @@ object AttestationConstants {
     const val TAG_CERTIFICATE_SUBJECT = 1007
     const val TAG_CERTIFICATE_NOT_BEFORE = 1008
     const val TAG_CERTIFICATE_NOT_AFTER = 1009
+
+    // --- Other Constants ---
+    // https://cs.android.com/android/platform/superproject/main/+/main:system/keymaster/km_openssl/attestation_record.cpp
+    const val CHALLENGE_LENGTH_LIMIT = 128 // kMaximumAttestationChallengeLength
 }

app/src/main/java/org/matrix/TEESimulator/interception/keystore/KeystoreInterceptor.kt

@@ -230,7 +230,6 @@ object KeystoreInterceptor : AbstractKeystoreInterceptor() {
                             ByteArray(0),
                         )
                     params.attestationChallenge = challenge
-                    params.attestationChallenge = challenge
                 }
 
                 val certificateChain =

app/src/main/java/org/matrix/TEESimulator/pki/CertificateGenerator.kt

@@ -20,6 +20,7 @@ import org.bouncycastle.cert.jcajce.JcaX509v3CertificateBuilder
 import org.bouncycastle.jce.provider.BouncyCastleProvider
 import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder
 import org.matrix.TEESimulator.attestation.AttestationBuilder
+import org.matrix.TEESimulator.attestation.AttestationConstants
 import org.matrix.TEESimulator.attestation.KeyMintAttestation
 import org.matrix.TEESimulator.config.ConfigurationManager
 import org.matrix.TEESimulator.interception.keystore.KeyIdentifier
@@ -42,6 +43,15 @@ object CertificateGenerator {
      */
     fun generateSoftwareKeyPair(params: KeyMintAttestation): KeyPair? {
         return runCatching {
+                val challenge = params.attestationChallenge
+                if (
+                    challenge != null &&
+                        challenge.size > AttestationConstants.CHALLENGE_LENGTH_LIMIT
+                )
+                    throw IllegalArgumentException(
+                        "Attestation challenge exceeds length limit (${challenge.size!!} > ${AttestationConstants.CHALLENGE_LENGTH_LIMIT})"
+                    )
+
                 val (algorithm, spec) =
                     when (params.algorithm) {
                         Algorithm.EC -> "EC" to ECGenParameterSpec(params.ecCurveName)