g . ▁ 6 ▁hours ), ▁called ▁a ▁crypt oper iod . ▁ ▁A ▁D RY AD ▁ci pher ▁sheet ▁contains ▁ 2 5 ▁lines ▁or ▁rows ▁of ▁sc ram bled ▁letters . ▁Each ▁line ▁is ▁l abeled ▁by ▁the ▁letters ▁A ▁to ▁Y ▁in ▁a ▁column ▁on ▁the ▁left ▁of ▁the ▁page . ▁Each ▁row ▁contains ▁a ▁random ▁permut ation ▁of ▁the ▁letters ▁A ▁through ▁Y . ▁The ▁letters ▁in ▁each ▁row ▁are ▁grouped ▁into ▁ 1 0 ▁columns ▁l abeled ▁ 0 ▁through ▁ 9 . ▁ ▁The ▁columns ▁under ▁ 0 , ▁ 1 , ▁ 2 ▁and ▁ 5 ▁have ▁more ▁letters ▁than ▁the ▁other ▁digits , ▁which ▁have ▁just ▁two ▁each . ▁ ▁While ▁cru de , ▁the ▁D RY AD ▁N umer al ▁Ci pher / Authentication ▁System ▁has ▁the ▁advantage ▁of ▁being ▁fast , ▁relatively ▁easy ▁and ▁requires ▁no ▁extra ▁equipment ▁( such ▁as ▁a ▁p enc il ). ▁The ▁presence ▁of ▁more ▁ci pher - text ▁columns ▁under ▁the ▁digits ▁ 0 , ▁ 1 , ▁ 2 ▁and ▁ 5 , ▁is ▁apparently ▁intended ▁to ▁make ▁ci pher text ▁frequency ▁analysis ▁more ▁difficult . ▁But ▁much ▁of ▁the ▁security ▁comes ▁from ▁keeping ▁the ▁crypt oper iod ▁short . ▁ ▁D RY AD ▁can ▁be ▁used ▁in ▁two ▁modes , ▁authentication ▁or ▁encryption . ▁ ▁Authentication ▁ ▁For ▁authentication , ▁a ▁challeng ing ▁station ▁selects ▁a ▁letter ▁at ▁random ▁from ▁the ▁left ▁most ▁column ▁followed ▁by ▁a ▁second ▁( random ly ▁selected ) ▁letter ▁in