Google Android encrypted application packages (.asec) are used in Android operating system to protect applications installed on SD card, inserted in the smart phone (Settings ► Applications ► click on application ► click on Move to SD Card option).
If you transport the application back to the Android phone or tablet, it will be decrypted back to standard APK format (Settings ► Applications ► click on application ► click on Move from SD Card option).
This operation essentially represents asec to apk conversion and there is really no other way how to do it.