diff --git a/net/AssetRoots.java b/net/AssetRoots.java
index 2b8d0d2..b8d4df9 100644
--- a/net/AssetRoots.java
+++ b/net/AssetRoots.java
@@ -4,35 +4,47 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 
 import android.content.Context;
 
-/** Mozilla roots shipped as assets/roots/<sha1(subject)>-<n>.der */
+/**
+ * Mozilla roots shipped as assets/roots/<sha1(subject)>-<n>.der
+ *
+ * Roots are read from the APK once and kept: an app installed on the SD card loses its APK while the
+ * card is shared over USB, and a failed read must not leave every server "untrusted" until a restart.
+ */
 public final class AssetRoots implements CertVerifier.RootStore {
     private final Context ctx;
-    private Set<String> names;
+    private Set<String> names;                                                  // asset file names, once listed
+    private final Map<String, List<byte[]>> found = new HashMap<String, List<byte[]>>(); // by subject hash; null = none
 
     public AssetRoots(Context c) { ctx = c.getApplicationContext(); }
 
     public synchronized List<byte[]> find(byte[] subjectDer) {
+        String h = CertVerifier.sha1Hex(subjectDer);
+        if (found.containsKey(h)) return found.get(h);
         try {
             if (names == null) {
-                names = new HashSet<String>();
                 String[] l = ctx.getAssets().list("roots");
-                if (l != null) for (int i = 0; i < l.length; i++) names.add(l[i]);
+                if (l == null || l.length == 0) return null; // APK unreadable right now: try again next time
+                Set<String> n = new HashSet<String>();
+                for (int i = 0; i < l.length; i++) n.add(l[i]);
+                names = n;
             }
-            String h = CertVerifier.sha1Hex(subjectDer);
             List<byte[]> out = null;
             for (int i = 0; names.contains(h + "-" + i + ".der"); i++) {
                 if (out == null) out = new ArrayList<byte[]>();
                 out.add(readAll(ctx.getAssets().open("roots/" + h + "-" + i + ".der")));
             }
+            found.put(h, out);
             return out;
         } catch (IOException e) {
-            return null;
+            return null; // not cached: the next handshake reads again
         }
     }