4 0 s ▁( B abb age ) ▁was ▁any ▁technique ▁known ▁which ▁could ▁reli ably ▁break ▁any ▁of ▁the ▁pol yal ph ab etic ▁ci ph ers . ▁His ▁technique ▁also ▁looked ▁for ▁repeating ▁patterns ▁in ▁the ▁ci pher text , ▁which ▁provide ▁cl ues ▁about ▁the ▁length ▁of ▁the ▁key . ▁Once ▁this ▁is ▁known , ▁the ▁message ▁essentially ▁becomes ▁a ▁series ▁of ▁messages , ▁each ▁as ▁long ▁as ▁the ▁length ▁of ▁the ▁key , ▁to ▁which ▁normal ▁frequency ▁analysis ▁can ▁be ▁applied . ▁Charles ▁B abb age , ▁Friedrich ▁Kas is ki , ▁and ▁William ▁F . ▁Fried man ▁are ▁among ▁those ▁who ▁did ▁most ▁to ▁develop ▁these ▁techniques . ▁ ▁Ci pher ▁design ers ▁tried ▁to ▁get ▁users ▁to ▁use ▁a ▁different ▁substitution ▁for ▁every ▁letter , ▁but ▁this ▁usually ▁meant ▁a ▁very ▁long ▁key , ▁which ▁was ▁a ▁problem ▁in ▁several ▁ways . ▁A ▁long ▁key ▁takes ▁longer ▁to ▁convey ▁( secure ly ) ▁to ▁the ▁parties ▁who ▁need ▁it , ▁and ▁so ▁mistakes ▁are ▁more ▁likely ▁in ▁key ▁distribution . ▁Also , ▁many ▁users ▁do ▁not ▁have ▁the ▁p atience ▁to ▁carry ▁out ▁length y , ▁letter - per fect ▁evolution s , ▁and ▁certainly ▁not ▁under ▁time ▁pressure ▁or ▁battle field ▁stress . ▁The ▁' ult imate ' ▁ci pher ▁of ▁this ▁type ▁would ▁be ▁one ▁in ▁which ▁such ▁a ▁' long ' ▁key ▁could ▁be ▁generated ▁from ▁a ▁simple ▁pattern ▁( ide ally ▁automatically ), ▁producing ▁a ▁ci pher ▁in ▁which ▁there ▁are ▁so ▁many