_pay_form.erb.html

<form action="<%= @pay_desc['mrh_url'] %>" method="post">
  <input type=hidden name=MrchLogin value="<%= @pay_desc['mrh_login'] %>">
  <input type=hidden name=OutSum value="<%= @pay_desc['out_summ'] %>">
  <input type=hidden name=InvId value="<%= @pay_desc['inv_id'] %>">
  <input type=hidden name=Desc value="<%= @pay_desc['inv_desc'] %>">
  <input type=hidden name=SignatureValue value="<%= @pay_desc['crc'] %>">
  <input type=hidden name=Shp_item value="<%= @pay_desc['shp_item'] %>">
  <input type=hidden name=IncCurrLabel value="<%= @pay_desc['in_curr'] %>">
  <input type=hidden name=Culture value="<%= @pay_desc['culture'] %>">
  <input type=submit value='Оплатить'>
</form>

controller.rb

require 'digest/md5'

class PaymentsController < ApplicationController
  
  def pay
    @order = Order.where(:id => params[:id]).first
    unless @order.blank? && @order.payment.blank?
      @pay_desc = Hash.new
      @pay_desc['mrh_url']   = Payment::MERCHANT_URL
      @pay_desc['mrh_login'] = Payment::MERCHANT_LOGIN
      @pay_desc['mrh_pass1'] = Payment::MERCHANT_PASS_1
      @pay_desc['inv_id']    = 0
      @pay_desc['inv_desc']  = @order.payment.desc
      @pay_desc['out_summ']  = @order.payment.price.to_s
      @pay_desc['shp_item']  = @order.id
      @pay_desc['in_curr']   = "WMRM"
      @pay_desc['culture']   = "ru"
      @pay_desc['encoding']  = "utf-8"
      @pay_desc['crc'] = Payment::get_hash(@pay_desc['mrh_login'], 
                                           @pay_desc['out_summ'],
                                           @pay_desc['inv_id'], 
                                           @pay_desc['mrh_pass1'], 
                                           "Shp_item=#{@pay_desc['shp_item']}")     
    end
  end

  def result
    crc = Payment::get_hash(params['OutSum'], 
                            params['InvId'], 
                            Payment::MERCHANT_PASS_2,
                            "Shp_item=#{params['Shp_item']}")
    @result = "FAIL"
    1.times do |x|
      break if params['SignatureValue'].blank? || crc.casecmp(params['SignatureValue']) != 0
      @order = Order.where(:id => params['Shp_item']).first
      break if @order.blank? || @order.payment.price != params['OutSum'].to_f
      @order.payment.invid = params['InvId'].to_i
      @order.payment.status = Payment::STATUS_OK
      @order.payment.save
      # ...
      @result = "OK#{params['InvId']}"
    end
  end

  def success
  end

  def fail
  end
end

model.rb

require 'open-uri'
require 'digest/md5'
  
class Payment
  include Mongoid::Document # I use mongoid
  include Mongoid::Timestamps
  
  field :status, :type => Integer
  field :invid,  :type => Integer
  field :price,  :type => Float
  field :desc
  # ...
  
  MERCHANT_URL    = 'https://merchant.roboxchange.com/Index.aspx' 
  # test interface: http://test.robokassa.ru/Index.aspx
  SERVICES_URL    = 'https://merchant.roboxchange.com/WebService/Service.asmx' 
  #test interface: http://test.robokassa.ru/Webservice/Service.asmx
  MERCHANT_LOGIN  = 'login'
  MERCHANT_PASS_1 = 'your_pass_1'
  MERCHANT_PASS_2 = 'your_pass_2'
  
  def self.get_currencies(lang = "ru")
    svc_url = "#{SERVICES_URL}/GetCurrencies?MerchantLogin=#{MERCHANT_LOGIN}&Language=#{lang}"
    doc = Nokogiri::XML(open(svc_url))
    doc.xpath("//xmlns:Group").map {|g|{
      'code' => g['Code'],
      'desc' => g['Description'],
      'items' => g.xpath('.//xmlns:Currency').map {|c| {
        'label' => c['Label'], 
        'name' => c['Name']
      }}
    }} if doc.xpath("//xmlns:Result/xmlns:Code").text.to_i == 0
  end
  
  def self.get_payment_methods(lang = "ru")
    svc_url = "#{SERVICES_URL}/GetPaymentMethods?MerchantLogin=#{MERCHANT_LOGIN}&Language=#{lang}"
    doc = Nokogiri::XML(open(svc_url)) 
    doc.xpath("//xmlns:Method").map {|g| {
      'code' => g['Code'],
      'desc' => g['Description']
    }} if doc.xpath("//xmlns:Result/xmlns:Code").text.to_i == 0
  end
  
  def self.get_rates(sum = 1, curr = '', lang="ru")
    svc_url = "#{SERVICES_URL}/GetRates?MerchantLogin=#{MERCHANT_LOGIN}&IncCurrLabel=#{curr}&OutSum=#{sum}&Language=#{lang}"
    doc = Nokogiri::XML(open(svc_url))
    doc.xpath("//xmlns:Group").map {|g| {
      'code' => g['Code'],
      'desc' => g['Description'],
      'items' => g.xpath('.//xmlns:Currency').map {|c| {
        'label' => c['Label'], 
        'name' => c['Name'],
        'rate' => c.xpath('./xmlns:Rate')[0]['IncSum']
      }}
    }} if doc.xpath("//xmlns:Result/xmlns:Code").text.to_i == 0
  end
  
  def self.operation_state(id)
    crc = get_hash(MERCHANT_LOGIN, id.to_s, MERCHANT_PASS_2)
    svc_url = "#{SERVICES_URL}/OpState?MerchantLogin=#{MERCHANT_LOGIN}&InvoiceID=#{id}&Signature=#{crc}&StateCode=80"

    doc = Nokogiri::XML(open(svc_url))
    
    return nil unless doc.xpath("//xmlns:Result/xmlns:Code").text.to_i == 0
    
    state_desc = {
      1   => 'Информация об операции с таким InvoiceID не найдена',
      5   => 'Только инициирована, деньги не получены',
      10  => 'Деньги не были получены, операция отменена',
      50  => 'Деньги получены, ожидание решение пользователя о платеже',
      60  => 'Деньги после получения были возвращены пользователю',
      80  => 'Исполнение операции приостановлено',
      100 => 'Операция завершена успешно',
    }
    
    s = doc.xpath("//xmlns:State")[0]
    code = s.xpath('./xmlns:Code').text.to_i
    state = {
      'code' => code,
      'desc' => state_desc[code],
      'request_date' => s.xpath('./xmlns:RequestDate').text,
      'state_date' => s.xpath('./xmlns:StateDate').text
    }
  end
  
  def self.get_hash(*s)
    Digest::MD5.hexdigest(s.join(':'))
  end
  
end

routes.rb

  match 'payment/result'  => "payments#result"
  match 'payment/success' => "payments#success"
  match 'payment/fail'    => "payments#fail"
  match 'payment'         => "payments#pay"