rot13.rb

def rot13(string)
  a                 = 'a'.ord
  z                 = 'z'.ord
  capital_a         = 'A'.ord
  capital_z         = 'Z'.ord
  decyphered_string = ""

  string.unpack('c*').each do |char|
    if (a..z).cover?(char)
      modified_char = (((char - a) + 13) % 26) + a
      decyphered_string << [modified_char].pack('c*')
    elsif (capital_a..capital_z).cover?(char)
      modified_char = (((char - capital_a) + 13) % 26) + capital_a
      decyphered_string << [modified_char].pack('c*')
    else
      decyphered_string << char
    end
  end

  decyphered_string
end