Switch to CBC mode with a derived IV.

1) Since we're not CPU or space constrained (and are in fact
   padding), and since keystream reuse would be more catastrophic
   than IV reuse without chosen plaintext.

library/androidTest/java/org/whispersystems/textsecure/push/PushTransportDetailsTest.java

@@ -0,0 +1,57 @@
+package org.whispersystems.textsecure.push;
+
+import android.test.AndroidTestCase;
+import android.util.Base64;
+
+import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
+import java.util.Arrays;
+
+public class PushTransportDetailsTest extends AndroidTestCase {
+
+  private final PushTransportDetails transportV2 = new PushTransportDetails(2);
+  private final PushTransportDetails transportV3 = new PushTransportDetails(3);
+
+  public void testV3Padding() {
+    for (int i=0;i<159;i++) {
+      byte[] message = new byte[i];
+      assertEquals(transportV3.getPaddedMessageBody(message).length, 159);
+    }
+
+    for (int i=159;i<319;i++) {
+      byte[] message = new byte[i];
+      assertEquals(transportV3.getPaddedMessageBody(message).length, 319);
+    }
+
+    for (int i=319;i<479;i++) {
+      byte[] message = new byte[i];
+      assertEquals(transportV3.getPaddedMessageBody(message).length, 479);
+    }
+  }
+
+  public void testV2Padding() {
+    for (int i=0;i<480;i++) {
+      byte[] message = new byte[i];
+      assertTrue(transportV2.getPaddedMessageBody(message).length == message.length);
+    }
+  }
+
+  public void testV3Encoding() throws NoSuchAlgorithmException {
+    byte[] message = new byte[501];
+    SecureRandom.getInstance("SHA1PRNG").nextBytes(message);
+
+    byte[] padded = transportV3.getEncodedMessage(message);
+
+    assertTrue(Arrays.equals(padded, message));
+  }
+
+  public void testV2Encoding() throws NoSuchAlgorithmException {
+    byte[] message = new byte[501];
+    SecureRandom.getInstance("SHA1PRNG").nextBytes(message);
+
+    byte[] padded = transportV2.getEncodedMessage(message);
+
+    assertTrue(Arrays.equals(padded, message));
+  }
+
+}