diff --git a/composer.json b/composer.json
index bf7d571..7cb3e22 100644
--- a/composer.json
+++ b/composer.json
@@ -25,6 +25,8 @@
   ],
   "require": {
     "php": "^5.5|^7",
+    "ext-curl": "*",
+    "ext-json": "*",
     "guzzlehttp/guzzle": "^6",
     "paragonie/constant_time_encoding": "^1|^2",
     "paragonie/sodium_compat": "^1.11"
diff --git a/src/Fetch.php b/src/Fetch.php
index 8ee7eea..bf5f692 100644
--- a/src/Fetch.php
+++ b/src/Fetch.php
@@ -35,6 +35,12 @@ class Fetch
      */
     protected $chroniclePublicKey = '';
 
+    /**
+     * List of bundles that have just been downloaded (e.g. RemoteFetch)
+     * @var array<int, string> $unverified
+     */
+    protected $unverified = [];
+
     /**
      * Fetch constructor.
      *
@@ -70,10 +76,13 @@ public function getLatestBundle($checkEd25519Signature = null, $checkChronicle =
         if (\is_null($checkEd25519Signature)) {
             $checkEd25519Signature = (bool) (static::CHECK_SIGNATURE_BY_DEFAULT && $sodiumCompatIsntSlow);
         }
-        if (\is_null($checkChronicle)) {
+        $conditionalChronicle = \is_null($checkChronicle);
+        if ($conditionalChronicle) {
             $checkChronicle = (bool) (static::CHECK_CHRONICLE_BY_DEFAULT && $sodiumCompatIsntSlow);
         }
 
+        /** @var int $bundleIndex */
+        $bundleIndex = 0;
         /** @var Bundle $bundle */
         foreach ($this->listBundles('', $this->trustChannel) as $bundle) {
             if ($bundle->hasCustom()) {
@@ -88,14 +97,33 @@ public function getLatestBundle($checkEd25519Signature = null, $checkChronicle =
                 $valid = true;
                 if ($checkEd25519Signature) {
                     $valid = $valid && $validator->checkEd25519Signature($bundle);
+                    if (!$valid) {
+                        $this->markBundleAsBad($bundleIndex, 'Ed25519 signature mismatch');
+                    }
                 }
-                if ($checkChronicle) {
+                if ($conditionalChronicle && $checkChronicle) {
+                    // Conditional Chronicle check (only on first brush):
+                    $index = array_search($bundle->getFilePath(), $this->unverified, true);
+                    if ($index !== false) {
+                        $validChronicle = $validator->checkChronicleHash($bundle);
+                        $valid = $valid && $validChronicle;
+                        if ($validChronicle) {
+                            unset($this->unverified[$index]);
+                        } else {
+                            $this->markBundleAsBad($bundleIndex, 'Chronicle');
+                        }
+                    }
+                } elseif ($checkChronicle) {
+                    // Always check Chronicle:
                     $valid = $valid && $validator->checkChronicleHash($bundle);
                 }
                 if ($valid) {
                     return $bundle;
                 }
+            } else {
+                $this->markBundleAsBad($bundleIndex, 'SHA256 mismatch');
             }
+            ++$bundleIndex;
         }
         throw new BundleException('No valid bundles were found in the data directory.');
     }
@@ -133,18 +161,31 @@ public function setChronicle($url, $publicKey)
     }
 
     /**
-     * List bundles
-     *
-     * @param string $customValidator Fully-qualified class name for Validator
-     * @param string $trustChannel
-     * @return array<int, Bundle>
-     *
-     * @throws CertaintyException
+     * @param int $index
+     * @param string $reason
+     * @return void
+     * @throws EncodingException
+     * @throws FilesystemException
      */
-    protected function listBundles(
-        $customValidator = '',
-        $trustChannel = Certainty::TRUST_DEFAULT
-    ) {
+    protected function markBundleAsBad($index = 0, $reason = '')
+    {
+        /** @var array<int, array<string, string>> $data */
+        $data = $this->loadCaCertsFile();
+        $now = (new \DateTime())->format(\DateTime::ATOM);
+        $data[$index]['bad-bundle'] = 'Marked bad on ' . $now . ' for reason: ' . $reason;
+        \file_put_contents(
+            $this->dataDirectory . '/ca-certs.json',
+            json_encode($data, JSON_PRETTY_PRINT)
+        );
+    }
+
+    /**
+     * @return array
+     * @throws EncodingException
+     * @throws FilesystemException
+     */
+    protected function loadCaCertsFile()
+    {
         if (!\file_exists($this->dataDirectory . '/ca-certs.json')) {
             throw new FilesystemException('ca-certs.json not found in data directory.');
         }
@@ -160,6 +201,23 @@ protected function listBundles(
         if (!\is_array($data)) {
             throw new EncodingException('ca-certs.json is not a valid JSON file.');
         }
+        return (array) $data;
+    }
+
+    /**
+     * List bundles
+     *
+     * @param string $customValidator Fully-qualified class name for Validator
+     * @param string $trustChannel
+     * @return array<int, Bundle>
+     *
+     * @throws CertaintyException
+     */
+    protected function listBundles(
+        $customValidator = '',
+        $trustChannel = Certainty::TRUST_DEFAULT
+    ) {
+        $data = $this->loadCaCertsFile();
         $bundles = [];
         /** @var array<string, string> $row */
         foreach ($data as $row) {
@@ -167,6 +225,14 @@ protected function listBundles(
                 // The necessary keys are not defined.
                 continue;
             }
+            if (!file_exists($this->dataDirectory . '/' . $row['file'])) {
+                // Skip nonexistent files
+                continue;
+            }
+            if (!empty($row['bad-bundle'])) {
+                // Bundle marked as "bad"
+                continue;
+            }
             if ($row['trust-channel'] !== $trustChannel) {
                 // Only include these.
                 continue;
diff --git a/src/RemoteFetch.php b/src/RemoteFetch.php
index 9cb9715..ac74c9d 100644
--- a/src/RemoteFetch.php
+++ b/src/RemoteFetch.php
@@ -185,6 +185,7 @@ protected function remoteFetchBundles()
                 /** @var string $body */
                 $body = (string) $request->getBody();
                 \file_put_contents($this->dataDirectory . '/' . $filename, $body);
+                $this->unverified []= $this->dataDirectory . '/' . $item['file'];
             }
         }
 
diff --git a/test/RemoteFetchTest.php b/test/RemoteFetchTest.php
index afec2f6..a745720 100644
--- a/test/RemoteFetchTest.php
+++ b/test/RemoteFetchTest.php
@@ -58,5 +58,13 @@ public function testRemoteFetch()
         );
         $fetch->setCacheTimeout(new \DateInterval('PT01M'));
         $this->assertTrue($fetch->cacheExpired());
+
+
+        $latest = $fetch->getLatestBundle();
+        file_put_contents($latest->getFilePath(), ' corrupt', FILE_APPEND);
+        (new RemoteFetch($this->dir))->getLatestBundle();
+
+        $cacerts = json_decode(file_get_contents($this->dir . '/ca-certs.json'), true);
+        $this->assertTrue(!empty($cacerts[0]['bad-bundle']));
     }
 }