.rb

require 'digest/md5'
string = 'MerchantId=5000&OrderId=214545&Amount=500.00&Currency=RUB&PayRequsites={
     "PayRequsites" : {
         "ReceiverName" : "УФК по МО (УГИБДД ГУ МВД России по Московской области)",
        "ReceiverInn" : "7702300872",
        "ReceiverKpp" : "770201001",
        "Bic" : "044583001",
        "BankName" : "Отделение №1 МГТУ Банка России г.Москва 705",
        "Account" : "40101810600000010102",
        "CorrAccount" : "40101810600000010102",
        "PersAccount" : "40101810600000010102",
        "PayPurpose" : "Оплата штрафа",
        "Okato" : "46241000000",
        "Kbk" : "18811630020016000140",
        "Uin" : "18810QМ50АК402195ZZ7"
    },
    "Payer" : {
        "Name" : "ИВАН",
        "MiddleName" : "ИВАНОВИЧ",
        "Surname" : "ИВАНОВ",
        "Zip" : "127001",
        "Address" : "Москва, ул 8 марта, дом 1",
        "Inn" : "40101810600000010102",
    }
}&PrivateSecurityKey=E3551A87-8475-467A-AFBC-1EF086431ECC'

Digest::MD5.hexdigest string # => "ee60dd8b2d7cfe301e0acd328847f592"