▁the ▁key ▁repe ats , ▁and ▁yet ▁it ▁still ▁only ▁requires ▁you ▁to ▁communicate ▁a ▁key ▁of ▁two ▁letters / numbers ▁to ▁set ▁things ▁up . ▁If ▁a ▁key ▁of ▁ 6 7 6 ▁length ▁is ▁not ▁long ▁enough , ▁another ▁rot or ▁can ▁be ▁added , ▁resulting ▁in ▁a ▁period ▁ 1 7 , 5 7 6 ▁letters ▁long . ▁ ▁In ▁order ▁to ▁be ▁as ▁easy ▁to ▁de ci pher ▁as ▁en ci pher , ▁some ▁rot or ▁machines , ▁most ▁not ably ▁the ▁En igma ▁machine , ▁were ▁designed ▁to ▁be ▁symmet rical , ▁i . e ., ▁encrypt ing ▁twice ▁with ▁the ▁same ▁settings ▁re co vers ▁the ▁original ▁message ▁( see ▁in volution ). ▁ ▁History ▁ ▁In vention ▁ ▁The ▁concept ▁of ▁a ▁rot or ▁machine ▁occurred ▁to ▁a ▁number ▁of ▁invent ors ▁independently ▁at ▁a ▁similar ▁time . ▁ ▁In ▁ 2 0 0 3 , ▁it ▁emer ged ▁that ▁the ▁first ▁invent ors ▁were ▁two ▁Dutch ▁naval ▁officers , ▁The o ▁A . ▁van ▁H eng el ▁( 1 8 7 5 – 1 9 3 9 ) ▁and ▁R . ▁P . ▁C . ▁Sp eng ler ▁( 1 8 7 5 – 1 9 5 5 ) ▁in ▁ 1 9 1 5 ▁( De ▁Lee uw , ▁ 2 0 0 3 ). ▁Pre viously , ▁the ▁in vention ▁had ▁been ▁as cribed ▁to ▁four ▁invent ors ▁working ▁independently ▁and ▁at ▁much ▁the ▁same ▁time : ▁Edward ▁He bern , ▁Ar vid