We　investigate　the　physical　layer　security　of　a　multiple-input　single-output　(MISO)　channel　in　the　presence　of　a　multi-antenna　passive　eavesdropper.　Traditionally,　artificial　noise　(AN)　scheme　is　an　efficient　security　strategy　without　the　channel　state　information　(CSI)　of　the　eavesdropper.　In　this　paper,　we　propose　and　analyze　an　efficient　security　scheme　called　the　artificial　fast　fading　(AFF)　scheme.　The　basic　idea　is　to　randomly　weight　the　information　symbols　at　different　transmit　antennas　in　a　special　manner　so　that　the　channel　of　the　intended　receiver　is　an　AWGN　channel　while　that　of　the　eavesdropper　is　a　fast　fading　channel.　This　prevents　the　eavesdropper　obtaining　the　CSI　using　the　blind　channel　estimation　techniques　so　that　it　can　only　detect　the　information　symbols　in　a　non-coherent　way,　which　greatly　reduces　the　amount　of　information　intercepted.　More　importantly,　we　derive　the　achievable　secrecy　rate　of　the　sheme　and　provide　a　low　bound　of　it,　which　can　be　numerically　calculated.　Compared　with　the　exact　secrecy　rate　of　the　AN　scheme,　even　the　low　bound　of　the　AFF　scheme　is　larger,　when　the　eavesdropper　has　more　antennas　than　the　transmitter.