Amazon FPS GetTransactionStatus

 class Transaction
      attr_accessor :transaction_id
      attr_accessor :url

      SIGNATURE_KEYNAME = "Signature"
      SIGNATURE_METHOD_KEYNAME = "SignatureMethod"
      SIGNATURE_VERSION_KEYNAME = "SignatureVersion"

      HMAC_SHA256_ALGORITHM = "HmacSHA256"
      HMAC_SHA1_ALGORITHM = "HmacSHA1"

      SERVICE_HOST = "fps.sandbox.amazonaws.com"

      class << self
        attr_accessor :access_key, :secret_key
      end

      def access_key
        self.class.access_key
      end

      def secret_key
        self.class.secret_key
      end

      def self.get transaction_id
        tran = new
        tran.transaction_id = transaction_id
        tran.get
      end

      def current_timestamp
        Time.now.utc.iso8601
      end

      def build_query_params
        {
          'Action'           => 'GetTransactionStatus',
          'AWSAccessKeyId'   => access_key,
          'Timestamp'        => current_timestamp,
          'TransactionId'    => transaction_id,
          'Version'          => '2008-09-17',
          SIGNATURE_METHOD_KEYNAME  => HMAC_SHA256_ALGORITHM,
          SIGNATURE_VERSION_KEYNAME => "2"
        }.sort.map.inject({}) do |initial, item| 
          initial.merge!({ item.first => urlencode(item.last) })
        end.map{|k,v| "#{k}=#{v}" }.join("&")
      end

      def get
        query = build_query_params
        sig = generate_signature(query)
        self.url = "https://#{SERVICE_HOST}/?#{query}&#{SIGNATURE_KEYNAME}=#{sig}"
        self.url
      end

      def urlencode(plaintext)
        CGI.escape(plaintext.to_s).gsub("+", "%20").gsub("%7E", "~")
      end

      def generate_signature(query)
        sign = "GET\n#{SERVICE_HOST}\n/\n#{query}"
        algorithm = 'sha256'
        digest = OpenSSL::Digest::Digest.new(algorithm)
        sig = Base64.encode64(OpenSSL::HMAC.digest(digest, secret_key, sign)).chomp
        urlencode(sig)
      end
    end