▁The ▁most ▁famous ▁example ▁is ▁the ▁German ▁En igma ▁machine , ▁the ▁output ▁of ▁which ▁was ▁de ci pher ed ▁by ▁the ▁Al lies ▁during ▁World ▁War ▁II , ▁ ▁producing ▁intelligence ▁code - named ▁Ult ra . ▁ ▁Description ▁The ▁primary ▁component ▁is ▁a ▁set ▁of ▁rot ors , ▁also ▁term ed ▁whe els ▁or ▁drums , ▁which ▁are ▁rot ating ▁dis ks ▁with ▁an ▁array ▁of ▁elect rical ▁contacts ▁on ▁either ▁side . ▁The ▁w iring ▁between ▁the ▁contacts ▁implements ▁a ▁fixed ▁substitution ▁of ▁letters , ▁replacing ▁them ▁in ▁some ▁complex ▁fashion . ▁On ▁its ▁own , ▁this ▁would ▁offer ▁little ▁security ; ▁however , ▁after ▁encrypt ing ▁each ▁letter , ▁the ▁rot ors ▁advance ▁positions , ▁changing ▁the ▁substitution . ▁By ▁this ▁means , ▁a ▁rot or ▁machine ▁produces ▁a ▁complex ▁pol yal ph ab etic ▁substitution ▁ci pher , ▁which ▁changes ▁with ▁every ▁key press . ▁ ▁Background ▁ ▁In ▁classical ▁crypt ography , ▁one ▁of ▁the ▁earliest ▁encryption ▁methods ▁was ▁the ▁simple ▁substitution ▁ci pher , ▁where ▁letters ▁in ▁a ▁message ▁were ▁system atically ▁replaced ▁using ▁some ▁secret ▁scheme . ▁Mon o alph ab etic ▁substitution ▁ci ph ers ▁used ▁only ▁a ▁single ▁replacement ▁scheme ▁— ▁sometimes ▁term ed ▁an ▁" al phabet "; ▁this ▁could ▁be ▁easily ▁broken , ▁for ▁example , ▁by ▁using ▁frequency ▁analysis . ▁Some what ▁more ▁secure ▁were ▁schemes ▁involving ▁multiple ▁al ph ab ets , ▁pol yal ph ab etic ▁ci ph ers . ▁Because ▁such ▁schemes ▁were ▁implemented ▁by ▁hand ,