code: static public BigInteger bytesToBigInteger(byte[] buffer, int index) {
          int length = bytesToInt(buffer, index);  // pull out the length of the big integer
          index += 4;
          byte[] bytes = new byte[length];
          System.arraycopy(buffer, index, bytes, 0, length);  // pull out the bytes for the big integer
          return new BigInteger(bytes);
      }
docs: This function converts the bytes in a byte array at the specified index to its
  corresponding big integer value.
  
  @param buffer The byte array containing the big integer.
  @param index The index for the first byte in the byte array.
  @return The corresponding big integer value.