[U-Boot] Secure booting using u-boot 2013

Hello, I am trying to perform secure booting on nitrogen 6X SOM using u-boot 2013. I tried to pad and sign the u-boot.imx image for authentication.But the size of both u-boot.imx as well as u-boot-signed-padded.imx remains same. Technically signed and padded image should have more size than normal .imx image. Can anyone inform where and what changes should I make in the source code of u-boot so that I can solve the problem?

Thanks in Advance!