Basic support for encrypted push-based attachments.

1) Move the attachment structures into the encrypted message body.

2) Encrypt attachments with symmetric keys transmitted in the
   encryptd attachment pointer structure.

3) Correctly handle asynchronous decryption and categorization of
   encrypted push messages.

TODO: Correct notification process and network/interruption
      retries.

library/src/org/whispersystems/textsecure/util/Util.java

@@ -45,6 +45,33 @@ public class Util {
 
   }
 
+  public static byte[][] split(byte[] input, int firstLength, int secondLength) {
+    byte[][] parts = new byte[2][];
+
+    parts[0] = new byte[firstLength];
+    System.arraycopy(input, 0, parts[0], 0, firstLength);
+
+    parts[1] = new byte[secondLength];
+    System.arraycopy(input, firstLength, parts[1], 0, secondLength);
+
+    return parts;
+  }
+
+  public static byte[][] split(byte[] input, int firstLength, int secondLength, int thirdLength) {
+    byte[][] parts = new byte[3][];
+
+    parts[0] = new byte[firstLength];
+    System.arraycopy(input, 0, parts[0], 0, firstLength);
+
+    parts[1] = new byte[secondLength];
+    System.arraycopy(input, firstLength, parts[1], 0, secondLength);
+
+    parts[2] = new byte[thirdLength];
+    System.arraycopy(input, firstLength + secondLength, parts[2], 0, thirdLength);
+
+    return parts;
+  }
+
   public static boolean isEmpty(String value) {
     return value == null || value.trim().length() == 0;
   }
@@ -94,6 +121,18 @@ public class Util {
     return new String(bout.toByteArray());
   }
 
+  public static void readFully(InputStream in, byte[] buffer) throws IOException {
+    int offset = 0;
+
+    for (;;) {
+      int read = in.read(buffer, offset, buffer.length - offset);
+
+      if (read + offset < buffer.length) offset += read;
+      else                		           return;
+    }
+  }
+
+
   public static void copy(InputStream in, OutputStream out) throws IOException {
     byte[] buffer = new byte[4096];
     int read;