From: Brian Candler <B.Candler@...>
Date: 2003-08-07T19:38:47+09:00
Subject: Re: More on DRB & OpenSSL

On Thu, Aug 07, 2003 at 06:49:32PM +0900, Hugh Sasse Staff Elec Eng wrote:
> It doesn't encrypt the message, but does a checksum with data that
> is never transmitted.  Thus you can only forge the checksum if you
> have that data, so you can trust it.  From the comments I wrote:
> 
> # A nonce is a word that is used only once (according to concise
> # Oxford Dictionary.)  The purpose is that it is generated, and a
> # password is added to it, and the hash of the whole string is
> # generated.  Thus a hash is passed across the network so that the
> # password can be checked against this hash without having to send
> # the password across the network.

I'm not sure why you want a nonce here; just a hash of (message + shared
secret) will do. But if you're paranoid you'll sign your objects with a
timestamp as well.

Try the code below. You can store session objects in a HTML input field like
this, or if the objects are small enough they can be sent to the browser as
a cookie!

Regards,

Brian.


  class SecureMarshall
    def initialize(secret, lifetime = 3600)
      require 'digest/md5'
      unless secret.is_a? String and secret.size >= 12
        raise "SecureMarshall secret must be at least 12 characters" 
      end
      @secret = secret
      @lifetime = lifetime
    end

    def encode(obj)
      out = Marshal.dump([obj, Time.now.to_i + @lifetime])
      [Marshal.dump([out, Digest::MD5::digest(out + @secret)])].  \
		pack("m").gsub(/\n/,'')   # base64 encode
    end

    def decode(key)
      raise TypeError, "Input must be a String" unless key.is_a? String
      out, hash = Marshal.load(key.unpack("m")[0])
      if Digest::MD5::digest(out + @secret) != hash
        raise ArgumentError, "Invalid signature! Tampering with objects is forbidden"
      end
      obj, expiry = Marshal.load(out)
      if Time.now.to_i > expiry
        raise "Object expired"
      end
      obj
    end

    def freshen(key)
      encode(decode(key))   # re-sign the object with a new lifetime
    end
  end

  if __FILE__ == $0
    require 'test/unit'
    class MarshallTest < Test::Unit::TestCase
      def setup
        @crypt = SecureMarshall.new("wibbly bibbly", 2)
        @obj = ["fred", "jim"]
      end

      def test_decode
        a = @crypt.encode(@obj)
        b = @crypt.decode(a)
        assert_equal(@obj,b)
      end

      def test_wrong_key
        a = @crypt.encode(@obj)
        crypt2 = SecureMarshall.new("another secret", 2)
        assert_raises(ArgumentError) { crypt2.decode(a) }
      end

      def test_tamper
        a = @crypt.encode(@obj)
        a[25] += 1  # in the middle of the MD5 checksum
        assert_raises(ArgumentError) { @crypt.decode(a) }
      end

      def test_lifetime
        a = @crypt.encode(@obj)
        sleep 4
        assert_raises(RuntimeError) { @crypt.decode(a) }
      end

      def test_freshen
        a = @crypt.encode(@obj)
        4.times do
          sleep 1
          a = @crypt.freshen(a)
        end
        b = @crypt.decode(a)
        assert_equal(@obj,b)
      end
    end
  end