Here's one of Nabokov
's methods for a secret code. Note the charming detail that "one-letter words
For their correspondence in the first period of separation, Van and Ada had invented a code ... One-letter words remained undisguised. In any longer word each letter was replaced by the one succeeding it in the alphabet at such an ordinal point–second, third, fourth, and so forth–which corresponded to the number of letters in that word. Thus "love", a four-letter word, became "pszi" ("p" being the fourth letter after "l" in the alphabetic series, "s" the fourth after "o" et cetera), whilst, say, "lovely" (in which the longer stretch made it necessary, in two instances, to resume the alphabet after exhausting it) became "ruBkrE", where the letters overflowing into the new alphabetic series were capitalized.
—V. Nabokov, Ada or Ardor: A Family Chronicle
Via Gretel und Hänsel