encrypter.rb

def encrypter(sentence)
  # Create an array of the alphabet
  alphabet = ("A".."Z").to_a
  # Break all the sentences
  sentence_letters = sentence.split("")
  # Set our encription key
  key = -3
  # Use map to create one sentence
  sentence_letters.map do |letter|
    letter_index = alphabet.index(letter)

    letter_index.nil? ? letter : alphabet[(letter_index + key) % alphabet.size]
    # Join back
  end.join
end

puts encrypter("THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG")

encrypter_specs.rb

require_relative './encrypter.rb'

describe '#encrypter' do
  it 'returns an encrypted sentence' do
    sentence = "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG"
    expected = "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD"
    actual   = encrypter(sentence)
    expect(actual).to eq(expected)
  end
end