a_laise.rb

key = ['f', 'c', 's', 'c']

message = "Gqfltwj emgj clgfv ! Aqltj rjqhjsksg ekxuaqs, ua xtwk n'feuguvwb gkwp xwj, ujts f'npxkqvjgw nw tjuwcz ugwygjtfkf qz uw efezg sqk gspwonu. Jgsfwb-aqmu f Pspygk nj 29 cntnn hqzt dg igtwy fw xtvjg rkkunqf.".downcase.chars
alphabetic_chars = ('a'..'z').to_a

index = 0
clear_message = ""
message.each do |char|
  if alphabetic_chars.include?(char)
    key_index = index % key.count
    key_letter = key[key_index]
    clear_index = char.ord-key_letter.ord
    clear_letter = alphabetic_chars[clear_index]
    clear_message += clear_letter.chr
    index += 1
  else
    clear_message += char
  end
end

puts clear_message