▁current ▁D RY AD ▁page , ▁since ▁the ▁page ▁is ▁getting ▁twice ▁as ▁much ▁use ▁as ▁a ▁single - authentication ▁scheme . ▁ ▁Enc ryption ▁The ▁second ▁mode ▁is ▁used ▁to ▁encrypt ▁short ▁numeric ▁information ▁( such ▁as ▁map ▁coordinates ▁or ▁a ▁new ▁radio ▁frequency ). ▁The ▁c oder ▁arbitr arily ▁selects ▁two ▁letters ▁at ▁random . ▁The ▁first ▁letter ▁design ates ▁a ▁row ▁in ▁the ▁current ▁active ▁page . ▁The ▁second ▁one ▁is ▁used ▁similarly ▁to ▁the ▁authentication ▁mode , ▁except ▁that ▁the ▁letter ▁immediately ▁to ▁the ▁right ▁is ▁selected . ▁This ▁selected ▁letter ▁is ▁called ▁the ▁" SET ▁L ET TER ." ▁ ▁Num bers ▁are ▁en ci pher ed ▁one ▁digit ▁at ▁a ▁time . ▁A ▁ci pher text ▁letter ▁is ▁chosen ▁from ▁the ▁selected ▁row ▁( the ▁row ▁designated ▁by ▁the ▁SET ▁L ET TER ) ▁in ▁the ▁column ▁under ▁the ▁plain ▁text ▁digit . ▁If ▁the ▁digit ▁occurs ▁more ▁than ▁once ▁in ▁the ▁number , ▁the ▁c oder ▁is ▁instruct ed ▁to ▁choose ▁a ▁different ▁letter ▁in ▁the ▁same ▁column . ▁ ▁All ▁the ▁digits ▁in ▁a ▁single ▁plain text ▁number ▁are ▁encoded ▁from ▁the ▁same ▁row . ▁( There ▁is ▁also ▁a ▁provision ▁for ▁encoding ▁letters ▁associated ▁with ▁map ▁grid ▁coordinates .) ▁ ▁See ▁also ▁ ▁B AT CO ▁— ▁similar ▁paper ▁based ▁tact ical ▁ci pher ▁used ▁by ▁the ▁British ▁forces ▁ ▁M - 9 4 ▁— ▁tact ical ▁ci pher ▁used ▁in ▁World ▁War ▁II ▁ ▁D ry ad ▁— ▁original ▁meaning ▁in ▁myth ology