rot13_digest.rb

require 'zlib'

module (Module.new)::Digest
  ROT13 = 'a'.upto('z').zip('a'.upto('z').lazy.cycle.drop(13)).then { |r| r.concat r.map { |a| a.map(&:upcase) } }.to_h
  def rot13_digest(s)
    String.new(encoding: Encoding::BINARY).tap { |buffer|
      Zlib
        .deflate(s, Zlib::NO_COMPRESSION)
        .each_char
        .map { |c| ROT13[c] }
        .lazy
        .cycle
        .then { |block|
          block
            .zip(block.drop(13))
            .cycle
            .lazy
        }.tap { |stream|
          buffer << stream.next.join until buffer.length == 32
        }
    }
  end
  prepend_features Kernel
end

rot13_digest File.read(__FILE__)
#=> "krmywvoerdhzvbeqrhyrmyvZobqhyzrb"