If the client is on Mac OS X, why do they need to create a zip file at all? You can use something like DropDMG <http://c-command.com/dropdmg/>; to create an encrypted .dmg file by dropping files/folders on it. Alternatively, if the client is on Windows, have them use a Windows ZIP utility, and then unzip on the Mac side with The Unarchiver <http://www.apple.com/downloads/macosx/system_disk_utilities/....