validate_cpf.rb

class Cpf
  def verifica_cpf(cpf = nil)
    return false if cpf.nil?
    if int_cpf(c[0.8], 1) != c[9] || int_cpf(c[0..9], 0) != c[10]
      false
    else
      true
    end
  end 
  
  def int_cpf(c, pos)
    som = 0
    for n in c.to_s.split('').collect {|i| i.to_i }
      pos += 1
      som += n * pos
    end
    som % 11
  end
  
end

module ActiveRecord
  module Validations
    module ClassMethods
      def validate_cpf(*attr_names)
        cpf_validator = Cpf.new
        configuration = {
          :message => 'invalido', 
          :on => :save 
        }  
        validates_each(attr_names, configuration) do |record, attr_name,value|
         if !cpf_validator.verifica_cpf(value)
           record.errors.add(attr_name, configuration[:message])
         end
        end
      end
    end
  end
end