Algorithm: Rails crypt integer to random short string, can be used as link shortener (by directly crypting databse ids). 1296 possibility for each Int (for each crypting key). String generated are not linear and also include a validity check (hashsum), an

crypt_id.rb

##
 # MonkeyPatch crypt_id
 #
 # Provide an efficient way to crypt an id, with 
 # different possibility for each id.
 #
 # Three provided methods are:
 # Int.crypt_id(key='')
 # String.decrypt_id(key='')
 # String.change_charset(charset_from, charset_to)
 #
 # Each generated ID will be randomly different (1296 possibility), during
 # the decryption if the input string doesn't correspond
 # to an id, nil will be returned.
 #
 # Use (A-Z_) for the crypting key
 # 123456.crypt_id('MY_KEY') # return 'fk7qCo' (1296 different output possibility for each input number with the same crypting key)
 # 123456.crypt_id('MY_KEY') # return 'piThR2'
 # 123456.crypt_id('MY_KEY') # return 'kEANeq'
 # 'kEANeq'.decrypt_id('MY_KEY') # return 123456
 #
 # 123456.crypt_id('MY_KEY').decrypt_id('WRONG_KEY') # return nil
 # 123456.crypt_id('MY_KEY').decrypt_id('MY_KEY') # return 123456
 #
 # Using a LCG pseudo random generator, not using the builtin one 
 # because if it ever change one day, no previously generated 
 # crypted string will be able to be decrypted anymore.
 #
 # It's strongly advised to changed @a and @c variable of
 # the LCGRandom generator in a production environment, it
 # increase the security. Look at this article to choose
 # those two values correctly:
 # http://en.wikipedia.org/wiki/Linear_congruential_generator
 #
 # @author Jeremy Trufier <[email protected]>
 # @url https://gist.github.com/gists/3333894
 ##

# create a pseudorandom function easy to reproduce 
# in case we change langage or the suite change 
# one day in ruby
# http://en.wikipedia.org/wiki/Linear_congruential_generator
class LCGRandom
  def initialize(n)
    @seed = n || Time.now.usec
    @a = 1103515245 # same as GCC
    @c = 12345 # same as GCC
    @m = 2**31 # same as GCC
  end

  def srand(n)
    @seed = n
    self
  end

  def rand(l = nil)
    @seed = (@a * @seed + @c) % @m
    return @seed.to_f / @m if l == nil
    @seed % l
  end
end

class Integer

  def crypt_id(key = '')
    lcgr = LCGRandom.new(Time.now.usec)
    id = self

    a = (('1'..'9').to_a + ('a'..'z').to_a + ('A'..'Z').to_a + ['$','_']).join
    k = (('0'..'9').to_a + ('a'..'z').to_a).sample(2).join

    id = (id + 1).to_s(32)

    lcgr.srand k.to_i(36) * key.to_i(36)
    p = lcgr.rand
    a.size.times do |i|
      r = i + lcgr.rand(a.size - i)
      a[i], a[r] = a[r], a[i]
    end
    a = '0' + a

    h = 0
    id.length.times{|i| h += id[i].to_i(32)}
    h = h % 32
    
    id += h.to_s(32)
    l = id.length
    p = (p * l).floor
    id[p], id[l - 1] = id[l - 1], id[p]

    k[1] + id.change_charset(('0'..'9').to_a + ('a'..'v').to_a, a) + k[0]
  end
end

class String
  def change_charset(f, t)
    def bin_fix_length(b, s)
      b = '0' + b while b.length < s
      b
    end

    str = self.clone

    l = (f.length - 1).to_s(2).length
    str_c = ''
    str_c += bin_fix_length(f.index(str.slice!(0)).to_s(2), l) until str == ''
    str_c.slice!(0) while str_c.slice(0) == '0'

    l = t.length.to_s(2).length - 1
    s = (str_c.length.to_f / l).ceil * l
    str_c = bin_fix_length(str_c, s)
    str = t[str_c.slice!(-l,l).to_i(2)] + str until str_c == ''

    str
  end

  def decrypt_id(key = '')
    id = self.clone
    return nil if id.length <= 2

    a = (('1'..'9').to_a + ('a'..'z').to_a + ('A'..'Z').to_a + ['$','_']).join
    k = id.slice!(- 1, 1) + id.slice!(0)

    lcgr = LCGRandom.new(k.to_i(36) * key.to_i(36))
    p = lcgr.rand

    a.size.times do |i|
      r = i + lcgr.rand(a.size - i)
      a[i], a[r] = a[r], a[i]
    end
    a = '0' + a

    id = id.change_charset(a, ('0'..'9').to_a + ('a'..'v').to_a)

    l = id.length
    p = (p * l).floor
    id[p], id[l - 1] = id[l - 1], id[p]
    
    h = id.slice!(-1,1).to_i(32)
    h2 = 0
    id.length.times{|i| h2 += id[i].to_i(32)}
    return nil if (h2 % 32) != h

    id.to_i(32) - 1
  end
end