Validate login receipt expiration + level.

feature/registration/src/main/java/org/signal/registration/NetworkController.kt

@@ -148,8 +148,8 @@ interface NetworkController {
    *
    * Retries for the same [purchaseIdentifier] must reuse the same [receiptCredentialRequest].
    *
-   * Implementations must apply the same validations to the returned credential as are applied to one-time donation
-   * receipts, and the expected receipt expiration is `purchaseDate + 5 * 366` days, plus padding for clock skew.
+   * The expected receipt expiration is `purchaseDate + 5 * 366` days. [RegistrationRepository] validates the level
+   * and expiration of the credential this issues.
    *
    * `POST /v1/login-purchase/receipt_credentials`
    */

feature/registration/src/main/java/org/signal/registration/RegistrationRepository.kt

@@ -108,6 +108,8 @@ import javax.crypto.Cipher
 import javax.crypto.spec.GCMParameterSpec
 import javax.crypto.spec.SecretKeySpec
 import kotlin.time.Duration
+import kotlin.time.Duration.Companion.days
+import kotlin.time.Duration.Companion.milliseconds
 import kotlin.time.Duration.Companion.seconds
 
 class RegistrationRepository(
@@ -137,6 +139,10 @@ class RegistrationRepository(
     private val TAG = Log.tag(RegistrationRepository::class)
     private val json = Json { ignoreUnknownKeys = true }
 
+    private val SIGNAL_LOGIN_RECEIPT_LIFESPAN = (5 * 366).days
+    private val SIGNAL_LOGIN_RECEIPT_CLOCK_SKEW_BUFFER = 2.days
+    private val SIGNAL_LOGIN_RECEIPT_MAX_LIFESPAN = SIGNAL_LOGIN_RECEIPT_LIFESPAN + SIGNAL_LOGIN_RECEIPT_CLOCK_SKEW_BUFFER
+
     /** Builds a repository from the module's injected [RegistrationDependencies]. */
     fun create(context: Context): RegistrationRepository {
       val dependencies = RegistrationDependencies.get()
@@ -685,6 +691,17 @@ class RegistrationRepository(
       }
     }
 
+    val configuration = fetchSignalLoginConfiguration()
+    if (configuration == null) {
+      Log.w(TAG, "[redeemSignalLoginPurchaseAndRegister] No Signal Login configuration, so we cannot validate the issued credential.")
+      return SignalLoginPurchaseResult.NetworkError
+    }
+
+    if (!isSignalLoginReceiptCredentialValid(credential, configuration.level)) {
+      Log.w(TAG, "[redeemSignalLoginPurchaseAndRegister] The service issued a credential that failed validation.")
+      return SignalLoginPurchaseResult.UnknownError
+    }
+
     val presentation = when (val built = networkController.createReceiptCredentialPresentation(credential)) {
       is ReceiptCredentialResult.Success -> built.value
       ReceiptCredentialResult.VerificationFailed -> {
@@ -714,6 +731,27 @@ class RegistrationRepository(
     }
   }
 
+  /**
+   * Guards against the service tagging a credential with a distinctive level or expiration that would let it link the
+   * purchase to the account that redeems it.
+   */
+  private fun isSignalLoginReceiptCredentialValid(credential: ReceiptCredential, expectedLevel: Long): Boolean {
+    val now = System.currentTimeMillis().milliseconds
+    val maxExpirationTime = now + SIGNAL_LOGIN_RECEIPT_MAX_LIFESPAN
+    val isCorrectLevel = credential.receiptLevel == expectedLevel
+    val isExpiration86400 = credential.receiptExpirationTime % 86400 == 0L
+    val isExpirationInTheFuture = credential.receiptExpirationTime.seconds > now
+    val isExpirationWithinMax = credential.receiptExpirationTime.seconds <= maxExpirationTime
+
+    Log.i(
+      TAG,
+      "[isSignalLoginReceiptCredentialValid] isCorrectLevel: $isCorrectLevel (actual: ${credential.receiptLevel}, expected: $expectedLevel), " +
+        "isExpiration86400: $isExpiration86400, isExpirationInTheFuture: $isExpirationInTheFuture, isExpirationWithinMax: $isExpirationWithinMax"
+    )
+
+    return isCorrectLevel && isExpiration86400 && isExpirationInTheFuture && isExpirationWithinMax
+  }
+
   /**
    * The service rejects a retry that redeems the same purchase with a different request, so we reuse the context we
    * persisted for this purchase if there is one.

feature/registration/src/test/java/org/signal/registration/SignalLoginPurchaseTest.kt

@@ -38,6 +38,7 @@ import org.signal.registration.fakes.FakeNetworkController
 import org.signal.registration.fakes.FakeOneTimePurchaseApi
 import org.signal.registration.fakes.FakeStorageController
 import org.signal.registration.fakes.SystemOutLogger
+import kotlin.time.Duration.Companion.days
 import kotlin.time.Duration.Companion.seconds
 
 /**
@@ -249,6 +250,52 @@ class SignalLoginPurchaseTest {
     assertThat(purchaseApi.consumedTokens).isEmpty()
   }
 
+  // ==================== credential validation ====================
+
+  @Test
+  fun `a credential with the wrong level is rejected`() = runTest {
+    networkController.onCreateLoginPurchaseReceiptCredential = { request ->
+      RequestResult.Success(networkController.issueLoginReceiptCredential(request, level = FakeNetworkController.LOGIN_RECEIPT_LEVEL + 1))
+    }
+
+    assertCredentialRejected()
+  }
+
+  @Test
+  fun `a credential whose expiration is not day aligned is rejected`() = runTest {
+    networkController.onCreateLoginPurchaseReceiptCredential = { request ->
+      RequestResult.Success(networkController.issueLoginReceiptCredential(request, expirationSeconds = networkController.defaultReceiptExpirationSeconds() + 1))
+    }
+
+    assertCredentialRejected()
+  }
+
+  @Test
+  fun `a credential that expires too far in the future is rejected`() = runTest {
+    networkController.onCreateLoginPurchaseReceiptCredential = { request ->
+      RequestResult.Success(networkController.issueLoginReceiptCredential(request, expirationSeconds = networkController.defaultReceiptExpirationSeconds() + 7.days.inWholeSeconds))
+    }
+
+    assertCredentialRejected()
+  }
+
+  @Test
+  fun `a credential that has already expired is rejected`() = runTest {
+    networkController.onCreateLoginPurchaseReceiptCredential = { request ->
+      RequestResult.Success(networkController.issueLoginReceiptCredential(request, expirationSeconds = networkController.defaultReceiptExpirationSeconds() - FakeNetworkController.LOGIN_RECEIPT_LIFESPAN.inWholeSeconds))
+    }
+
+    assertCredentialRejected()
+  }
+
+  private suspend fun assertCredentialRejected() {
+    val result = purchaseAndRegister()
+
+    assertThat(result).isInstanceOf(SignalLoginPurchaseResult.UnknownError::class)
+    assertThat(networkController.lastRegisterAccountRequest).isNull()
+    assertThat(purchaseApi.consumedTokens).isEmpty()
+  }
+
   // ==================== pending payment ====================
 
   @Test