columnar_transposition.rb

PLAINTEXT = ARGV[0].gsub(/[^A-Za-z]/, '')
KEY       = ARGV[1]

p KEY
  .chars
  .cycle
  .take(PLAINTEXT.size)
  .zip(PLAINTEXT.chars)
  .reduce(Hash.new { [] }) { |acc, (key, char)| acc.merge(key => acc[key] + [char]) }
  .to_a
  .sort
  .flat_map(&:last)
  .join