URLcrypt.rb

require 'openssl'
require 'base64'
require 'cgi'

module URLcrypt
  # avoid vowels to not generate four-letter words, etc.
  # this is important because those words can trigger spam 
  # filters when URLs are used in emails
  TABLE = "1bcd2fgh3jklmn4pqrstAvwxyz567890".freeze

  def self.key=(key)
    @key = key
  end

  def self.key
    @key
  end

  class Chunk
    def initialize(bytes)
      @bytes = bytes
    end

    def decode
      bytes = @bytes.take_while {|c| c != 61} # strip padding
      bytes = bytes.find_all{|b| !TABLE.index(b.chr).nil? } # remove invalid characters
      n = (bytes.length * 5.0 / 8.0).floor
      p = bytes.length < 8 ? 5 - (n * 8) % 5 : 0
      c = bytes.inject(0) {|m,o| (m << 5) + TABLE.index(o.chr)} >> p
      (0..n-1).to_a.reverse.collect {|i| ((c >> i * 8) & 0xff).chr}
    end

    def encode
      n = (@bytes.length * 8.0 / 5.0).ceil
      p = n < 8 ? 5 - (@bytes.length * 8) % 5 : 0
      c = @bytes.inject(0) {|m,o| (m << 8) + o} << p
      [(0..n-1).to_a.reverse.collect {|i| TABLE[(c >> i * 5) & 0x1f].chr},
        ("=" * (8-n))] # TODO: remove '=' padding generation
    end
  end

  class BaseCoder
    def initialize(options = {})
      @key = options[:key] || URLcrypt.key
      @data = options[:data]
    end

    # strip '=' padding, because we don't need it
    def encode(d = nil)
      d ||= @data
      chunks(d, 5).collect(&:encode).flatten.join.tr('=','')
    end

    def decode(d = nil)
      d ||= @data
      chunks(d, 8).collect(&:decode).flatten.join
    end

    def encrypt(d = nil)
      d ||= @data
      crypter = cipher(:encrypt)
      crypter.iv = iv = crypter.random_iv
      join_parts encode(iv), encode(crypter.update(d) + crypter.final)
    end

    def split_parts str
      str.split('Z')
    end

    def join_parts *args
      args.join("Z")
    end

    def decrypt(d = nil)
      d ||= @data
      iv, encrypted = split_parts(d).map{|part| decode(part)}
      fail DecryptError, "not a valid string to decrypt" unless iv && encrypted
      decrypter = cipher(:decrypt)
      decrypter.iv = iv
      decrypter.update(encrypted) + decrypter.final 
    end

    def cipher(mode)
      cipher = OpenSSL::Cipher.new('aes-256-cbc')
      cipher.send(mode)
      cipher.key = @key
      cipher
    end

    def chunks(str, size)
      result = []
      bytes = str.bytes
      while bytes.any? do
        result << Chunk.new(bytes.take(size))
        bytes = bytes.drop(size)
      end
      result
    end
  end

  class Base64Coder < BaseCoder
    def encode(d = nil)
      d ||= @data
      Base64.urlsafe_encode64(d)
    end

    def decode(d = nil)
      d ||= @data
      Base64.urlsafe_decode64(d)
    end

    def split_parts str
      str.split(':')
    end

    def join_parts *args
      args.join(":")
    end
  end

  class CGIBase64Coder < BaseCoder
    def encode(d = nil)
      d ||= @data
      CGI.escape super(d)
    end

    def decode(d = nil)
      d ||= @data
      super CGI.unescape(d)
    end
  end

  def self.default_coder
    @default_coder || BaseCoder
  end

  def self.default_coder= val
    @default_coder = val
  end

  def self.encode(data)
    default_coder.new(data: data).encode
  end

  def self.decode(data)
    default_coder.new(data: data).decode
  end

  def self.decrypt(data)
    default_coder.new(data: data).decrypt
  end

  def self.encrypt(data)
    default_coder.new(data: data).encrypt
  end
  
  class DecryptError < ::ArgumentError; end
end