How could I known which asset bundle is built when I invoke BuildPipeline.BuildAssetBundles?

I build the asset bundle without BuildAssetBundleOptions.ForceRebuildAssetBundle, so each time I invoke the BuildPipeline.BuildAssetBundles only a part of asset bundle is actual build.
I add a post process to encrypt the built asset bundles. Each time I get the AssetBundleManifest from the BuildPipeline.BuildAssetBundles and encrypt all asset bundles.

But there has a problem, the old asset bundle that not built will encrypt twice, that will make I encrypt one file more than one times, and that make a incorrect result.

Is there any way to find out which asset bundle is new build when I invoke the BuildPipeline.BuildAssetBundles?

 

1) don't overwrite. that way your encrypted bundles have one layer of encryption, even if the source is processed multiple times
2a) you can check the file timestamp (from your os) and only process new files, or
2b) you can keep the old manifest and compare the hash of each bundle. different hash => bundle is rebuilt.