▁the ▁row ▁of ▁the ▁first ▁chosen ▁letter . ▁The ▁station ▁being ▁challeng ed ▁would ▁then ▁authentic ate ▁by ▁pick ing ▁the ▁letter ▁directly ▁below ▁the ▁row ▁and ▁position ▁of ▁the ▁second ▁letter ▁selected . ▁ ▁For ▁example , ▁using ▁the ▁example ▁ci pher ▁sheet ▁to ▁the ▁right ▁and ▁the ▁NAT O ▁ph on etic ▁alphabet , ▁Jason ▁could ▁challenge ▁Peg gy ▁by ▁transm itting ▁" authentic ate ▁Al pha ▁Bra vo ". ▁Peg gy ' s ▁correct ▁response ▁would ▁then ▁be ▁" authentic ate ▁Yan kee ". ▁ ▁Another ▁form ▁used ▁involves ▁selecting ▁the ▁third ▁letter ▁to ▁the ▁right ▁of ▁the ▁second ▁letter ▁chosen ▁by ▁the ▁challeng ing ▁station ▁( J ason ' s ▁" Bra vo " ▁letter ). ▁Both ▁the ▁direction al ▁offset ▁( up , ▁down , ▁left ▁or ▁right ) ▁and ▁numer al ▁offset ▁can ▁be ▁values ▁different ▁from ▁the ▁examples ▁given ▁here , ▁but ▁must ▁be ▁agreed ▁upon ▁and ▁understood ▁by ▁both ▁parties ▁before ▁authentication . ▁ ▁One ▁problem ▁presented ▁is ▁that ▁an ▁enemy ▁imp erson ator ▁has ▁a ▁one ▁in ▁ 2 5 ▁chance ▁of ▁guessing ▁the ▁correct ▁response ▁( one ▁in ▁ 2 4 ▁if ▁a ▁letter ▁is ▁selected ▁from ▁the ▁same ▁row ). ▁A ▁solution ▁to ▁this ▁is ▁for ▁Jason ▁to ▁require ▁Peg gy ▁to ▁authentic ate ▁twice ; ▁lower ing ▁the ▁imp erson ator ' s ▁odd s ▁of ▁guessing ▁the ▁correct ▁response ▁to ▁one ▁in ▁ 6 2 5 . ▁The ▁down side ▁to ▁this ▁method ▁is ▁reduced ▁lon ge vity ▁of ▁the