Simple Cipher

cipher.rb

class Cipher
  VALUES = [('a'..'z').to_a, (0..25).to_a].transpose.to_h
  attr_reader :key
  
  def initialize(key = new_key)
    @key = key
    raise_error_conditions
  end
  
  def encode(msg)
    secure(msg)
  end
 
  def decode(msg)
    secure(msg, nil)
  end
 
  private
    def new_key
      100.times.map { ('a'..'z').to_a.sample }.join
    end
    
    def raise_error_conditions
      invalid = ('A'..'Z').to_a + ('0'..'9').to_a
      if invalid.any? { |char| key.include? char } || key.empty?
        raise ArgumentError, "Key can't be empty or contain invalid characters."
      end
    end
    
    def secure(msg, encode_flag=1)
      message = ''
      index = 0
      while index < msg.length
        distance = encode_or_decode_msg(msg, index, encode_flag)
        letter = distance % VALUES.keys.count
        message += VALUES.key(letter)
        index += 1
      end
      message
    end
  
    def encode_or_decode_msg(msg, index, encode_flag)
      if encode_flag
        VALUES[msg[index]].to_i + VALUES[key[index]].to_i
      else
        VALUES[msg[index]].to_i - VALUES[key[index]].to_i
      end
    end
end