Generate "prekeys" at push registration time.

This generates a large number of key exchange messages and
registers them with the server during signup.

src/org/thoughtcrime/securesms/crypto/PreKeyUtil.java

@@ -0,0 +1,113 @@
+package org.thoughtcrime.securesms.crypto;
+
+import android.content.Context;
+import android.util.Log;
+
+import com.google.protobuf.ByteString;
+import org.thoughtcrime.securesms.database.keys.InvalidKeyIdException;
+import org.thoughtcrime.securesms.database.keys.PreKeyRecord;
+import org.thoughtcrime.securesms.encoded.PreKeyProtos.PreKeyEntity;
+import org.whispersystems.textsecure.push.PreKeyList;
+import org.whispersystems.textsecure.util.Base64;
+
+import java.io.File;
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.LinkedList;
+import java.util.List;
+
+public class PreKeyUtil {
+
+  public static final int BATCH_SIZE = 70;
+
+  public static List<PreKeyRecord> generatePreKeys(Context context, MasterSecret masterSecret) {
+    List<PreKeyRecord> records        = new LinkedList<PreKeyRecord>();
+    long               preKeyIdOffset = getNextPreKeyId(context);
+
+    for (int i=0;i<BATCH_SIZE;i++) {
+      Log.w("PreKeyUtil", "Generating PreKey: " + (preKeyIdOffset + i));
+      PreKeyPair   keyPair = new PreKeyPair(masterSecret, KeyUtil.generateKeyPair());
+      PreKeyRecord record  = new PreKeyRecord(context, masterSecret, preKeyIdOffset + i, keyPair);
+
+      record.save();
+      records.add(record);
+    }
+
+    return records;
+  }
+
+  public static List<PreKeyRecord> getPreKeys(Context context, MasterSecret masterSecret) {
+    List<PreKeyRecord> records      = new LinkedList<PreKeyRecord>();
+    File               directory    = getPreKeysDirectory(context);
+    String[]           keyRecordIds = directory.list();
+
+    for (String keyRecordId : keyRecordIds) {
+      try {
+        records.add(new PreKeyRecord(context, masterSecret, Long.parseLong(keyRecordId)));
+      } catch (InvalidKeyIdException e) {
+        Log.w("PreKeyUtil", e);
+        new File(getPreKeysDirectory(context), keyRecordId).delete();
+      } catch (NumberFormatException nfe) {
+        Log.w("PreKeyUtil", nfe);
+        new File(getPreKeysDirectory(context), keyRecordId).delete();
+      }
+    }
+
+    return records;
+  }
+
+  public static void clearPreKeys(Context context) {
+    File     directory  = getPreKeysDirectory(context);
+    String[] keyRecords = directory.list();
+
+    for (String keyRecord : keyRecords) {
+      new File(directory, keyRecord).delete();
+    }
+  }
+
+  public static PreKeyList toJson(List<PreKeyRecord> records) {
+    List<String> encoded = new LinkedList<String>();
+
+    for (PreKeyRecord record : records) {
+      PreKeyEntity entity  = PreKeyEntity.newBuilder().setId(record.getId())
+                                         .setKey(ByteString.copyFrom(KeyUtil.encodePoint(record.getKeyPair().getPublicKey().getQ())))
+                                         .build();
+
+      String encodedEntity = Base64.encodeBytesWithoutPadding(entity.toByteArray());
+
+      encoded.add(encodedEntity);
+    }
+
+    return new PreKeyList(encoded);
+  }
+
+  private static long getNextPreKeyId(Context context) {
+    try {
+      File     directory    = getPreKeysDirectory(context);
+      String[] keyRecordIds = directory.list();
+      long     nextPreKeyId = 0;
+
+      for (String keyRecordId : keyRecordIds) {
+        if (Long.parseLong(keyRecordId) > nextPreKeyId)
+          nextPreKeyId = Long.parseLong(keyRecordId);
+      }
+
+      if (nextPreKeyId == 0)
+        nextPreKeyId = SecureRandom.getInstance("SHA1PRNG").nextInt(Integer.MAX_VALUE/2);
+
+      return nextPreKeyId;
+    } catch (NoSuchAlgorithmException e) {
+      throw new AssertionError(e);
+    }
+  }
+
+  private static File getPreKeysDirectory(Context context) {
+    File directory = new File(context.getFilesDir(), PreKeyRecord.PREKEY_DIRECTORY);
+
+    if (!directory.exists())
+      directory.mkdirs();
+
+    return directory;
+  }
+
+}