A Caesar cipher example in Ruby

caesar.rb

class Caesar
  def initialize(rot: 13, lower: [*'a'..'z'], upper: [*'A'..'Z'])
    @rot = rot
    @book = book(lower:, upper:)
  end

  def cipher(message) = message.gsub(/[a-zA-Z]/, @book)
  alias_method :call, :cipher

  private

  def book(lower:, upper:) = dict(lower).merge(dict(upper))
  def dict(chars) = chars.rotate(@rot).zip(chars).to_h
end

rot13 = Caesar.new
p rot13.cipher('Cicada')