an example of Format Preserving Encryption in Ruby using RC4 and pack. The important thing is to keep key length to be a power of 2

fp_ruby_rc4.rb

require 'openssl'
c = OpenSSL::Cipher::Cipher.new("rc4-40")

serial = 4000987
puts " *********** SERIAL NUMBER = #{serial}"
c.encrypt
c.key = "fdkjgfdoi4tu45fkgkljfg9485439tkjfgnjdshfghwhe54350gfgklkgje34324nkjdfhk45845843543k5kdjfk3454958439kkfkglgkttpoeoidsad49584305ldkfsdjf42jkndfl"

padded_serial = "%015d" % serial 
padded_serial_array = [padded_serial.slice(0,10).to_i, padded_serial.slice(10,15).to_i]
padded_serial_packed = padded_serial_array.pack("LS")
puts " #{ padded_serial.slice(0,10).to_i} and #{padded_serial.slice(10,15).to_i} " 
puts "padded_serial_packed  #{padded_serial_packed.unpack("LS").inspect} array = #{padded_serial_array.inspect} " 

padded_serial_packed_encrypted = c.update(padded_serial_packed) + c.final

order_number_pre = padded_serial_packed_encrypted.unpack("LS") 

puts "order_number_pre= #{ order_number_pre.inspect}"

order_number_pre_padded = [  ("%010d" % order_number_pre[0]).to_s, ("%05d" %  order_number_pre[1]).to_s ] 

#http://blog.anidear.com/2011/11/luhn-algorithm-implementation-in-ruby.html
def luhn(input)
  10 - input.split(//).collect(&:to_i).zip((1..input.length).to_a).map{|v,i| i%2==0?(v*2<10?v*2:v*2-9):v}.inject(&:+)%10
end

order_number = [luhn( order_number_pre_padded[0]).to_s +  order_number_pre_padded[0], luhn( order_number_pre_padded[1]).to_s +   order_number_pre_padded[1] ]
puts " ************  ORDER NUMBER = #{order_number.inspect} or #{order_number} **************"

d = OpenSSL::Cipher::Cipher.new("rc4-40")
d.decrypt
d.key = "fdkjgfdoi4tu45fkgkljfg9485439tkjfgnjdshfghwhe54350gfgklkgje34324nkjdfhk45845843543k5kdjfk3454958439kkfkglgkttpoeoidsad49584305ldkfsdjf42jkndfl"


order_number_pre_checksum = [ order_number[0].slice(1,11).to_i, order_number[1].slice(1,5).to_i ] 
puts "order_number_pre_checksum= #{order_number_pre_checksum.inspect}"
order_number_pre_checksum_packed = order_number_pre_checksum.pack("LS") 

padded_serial_packed_decrypted =  d.update(order_number_pre_checksum_packed ) + d.final
padded_serial_packed_decrypted_unpacked = padded_serial_packed_decrypted.unpack("LS") 
padded_serial_packed_decrypted_unpacked_reformatted = ["%010d" %padded_serial_packed_decrypted_unpacked[0], "%05d" %padded_serial_packed_decrypted_unpacked[1] ]

puts " padded_serial_packed_decrypted = #{padded_serial_packed_decrypted_unpacked_reformatted.inspect}"