dencrypt.rb

# frozen_string_literal: true

# LICENSE: MIT

#
# A very simple deterministic string encryption.
# Variation of a Caesar cipher.
#
# Intended to be used with strings,
# primary purpose of this approach is to merely make it *harder* for an eavesdropper
# to see the actual data contained in a string.
#
# This is in no way a replacement for a "proper" encryption, but it's fast as all hells,
# and, unlike "proper" encryption methods, increases string length only slightly
# (`Base64.urlsafe_encode64` overhead), which can sometimes be crucial.
#

#
# Usage:
#
#   require "dencrypt"
#
#   things = "Eggs, brains, signals, wires."
#   things.length                              #=> 29
#   mush = Dencrypt.scramble(things)           #=> "3tbFotPazpak3dnXu-CujojhmNHQ3N1U0Y6S0N0"
#   mash = Dencrypt.scramble(things)           #=> "3tbFotPazpak3dnXu-CujojhmNHQ3N1U0Y6S0N0"
#   mush == mash                               #=> true
#   mush.length                                #=> 39
#   neat = Dencrypt.unscramble(mush)           #=> "Eggs, brains, signals, wires."
#   neat == things                             #=> true
#
module Dencrypt
  class DencryptionError < StandardError; end
  class EncryptionError < DencryptionError; end
  class DecryptionError < DencryptionError; end

  DENCRYPTION_KEY = defined?(Rails) ? Rails.application.secret_key_base : ENV.fetch("DENCRYPTION_KEY")

  module_function def scramble(data)
    seed = Digest::MD5.hexdigest(DENCRYPTION_KEY.to_s).to_i(16) # deterministic seed integer for Random
    key = DENCRYPTION_KEY.bytes.cycle
    data.bytes.                                 # for each byte in the string stream,
    map{|byte| ((byte + key.next) % 255).chr }. # perform a simple byte shift
    shuffle(random: Random.new(seed)).          # then sprinkle with deterministic Array#shuffle on top,
    join.                                       # to make things interesting.
    then{ Base64.urlsafe_encode64 _1, padding: false }
  rescue
    raise EncryptionError.new
  end

  # Well, duh.
  module_function def unscramble(data)
    seed = Digest::MD5.hexdigest(DENCRYPTION_KEY.to_s).to_i(16) # same deterministic seed int
    key = DENCRYPTION_KEY.bytes.cycle
    Base64.urlsafe_decode64(data).bytes.
    unshuffle(random: Random.new(seed)).        # <- hero of the day!
    map{|byte| ((byte - key.next) % 255).chr }. # shift that byte back
    join
  rescue
    raise DecryptionError.new
  end
end