Launch School challenge - crypto square

crypto_square.rb

class Crypto
  def initialize(str)
    @input = str.downcase
                .chars
                .keep_if { |c| c =~ /[0-9a-z]/ }
  end

  def size
    Math.sqrt(@input.length).ceil
  end

  def normalize_plaintext
    @input.join
  end

  def plaintext_segments
    @input.each_slice(self.size).map(&:join)
  end

  def normalize_ciphertext
    rows = self.plaintext_segments.map(&:chars)

    rows.last << " " until rows.last.length == rows.first.length

    rows.transpose
        .map(&:join)
        .map(&:strip)
        .join(" ")
  end

  def ciphertext
    self.normalize_ciphertext
        .split(" ")
        .join
  end
end