app.rb

require 'sinatra'
require 'httparty'
require 'openssl'
require 'pry'

class VixPayments < Sinatra::Base

  post '/checkout' do
    fields = params.select{|k,v| k.start_with? 'x_'}
    fields.delete('x_signature')
    message = fields.sort.join
    key = 'vix'

    signature = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), key, message)

    payload = {
        x_account_id: params['x_account_id'],
        x_reference: params['x_reference'],
        x_currency: params['x_currency'],
        x_test: params['x_test'],
        x_amount: params['x_amount'],
        x_gateway_reference: "VIX-" + "#{rand(10**15)}",
        x_timestamp: Time.now.utc,
        x_result: 'completed',
      }

    message_response = payload.sort.join
    payload['x_signature'] = OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), key, message_response)
    callback_url = params['x_url_callback']

    response = HTTParty.post(callback_url, body: payload)
    redirect params['x_url_complete']
  end
end

VixPayments.run!