encrypt.rb

def encrypt(text, key = -3)
  alphabet = ("A".."Z").to_a
  letters = text.chars
  letters.map do |letter|
    letter_index = alphabet.index(letter)
    letter_index.nil? ? letter : alphabet[(letter_index + key) % alphabet.size]
  end.join
end

def decrypt(text, key = -3)
  encrypt(text, key * -1)
end

secret_text = "FK ZOVMQLDOXMEV, X ZXBPXO ZFMEBO, XIPL HKLTK XP ZXBPXO'P ZFMEBO, QEB PEFCQ ZFMEBO, ZXBPXO'P ZLAB LO ZXBPXO PEFCQ, FP LKB LC QEB PFJMIBPQ XKA JLPQ TFABIV HKLTK BKZOVMQFLK QBZEKFNRBP. FQ FP X QVMB LC PRYPQFQRQFLK ZFMEBO FK TEFZE BXZE IBQQBO FK QEB MIXFKQBUQ FP OBMIXZBA YV X IBQQBO PLJB CFUBA KRJYBO LC MLPFQFLKP ALTK QEB XIMEXYBQ. CLO BUXJMIB, TFQE X IBCQ PEFCQ LC 3, A TLRIA YB OBMIXZBA YV X, B TLRIA YBZLJB Y, XKA PL LK. QEB JBQELA FP KXJBA XCQBO GRIFRP ZXBPXO, TEL RPBA FQ FK EFP MOFSXQB ZLOOBPMLKABKZB."
puts decrypt(secret_text, -3)