choose=: verb define
  sel=. 'Q' e. y
  alph=: (sel { 'JQ') -.~ a. {~ 65 + i.26
  norm=: [: dedouble alph restrict ('I' I.@:=&'J'@]} ])`(-.&'Q')@.sel@toupper
  ''
)

restrict=: ] -. -.~

splitDigraph=: ,`([,'X',])@.((= {.) *. 2 | #@])
dedouble=: splitDigraph/&.|.                     NB. progressively split digraphs in string

choose 'Q'

setkey=: verb define
  key=. ~.norm y,alph
  ref=: ,/ 2{."1 ~."1 (,"0/~ alph) ,"1 norm 'XQV'
  mode=. #. =/"2 inds=. 5 5#:key i. ref
  inds0=. (0 3,:2 1)&{@,"2 inds
  inds1=. 5|1 0 +"1 inds NB. same column
  inds2=. 5|0 1 +"1 inds NB. same row
  alt=: key {~ 5 #. mode {"_1 inds0 ,"2 3 inds1 ,:"2 inds2
  i. 0 0
)

pairs=: verb define
  2{."1 -.&' '"1 ~."1 (_2]\ norm y) ,"1 'XQV'
)

encrypt=: verb define
  ;:inv ;/ alt{~ref i. pairs y
)

decrypt=: verb define
  , ref{~alt i. pairs y
)