I don't see what keeping any of the cookie attributes secret
buys you. I only see the need to make sure the integrity of
the packet is the same, and to authenticate that the data came
from the server.
Hiya Aaron,
You are absolutely correct; it is all about the integrity of the data.
The goal of the exercise is to ensure that the transaction data can not
be tampered with. Some data is user defined (i.e. ItemQuantity) and
some is not (i.e. ItemPrice etc). If the latter values only ever exist
server side, and are then simply displayed to the client, you avoid the
tampering problem entirely. In fact, once you look closely, there is no
logical requirement to place any data at all in a cookie and pass it
between the client and server. Aside from the need to protect its
integrity, it is also a performance hog; thousands of extra useless
bytes that get shifted between client and server with every request.
In the example you have given, most of the inputs into the MAC are
available to the user, and the remaining ones are available to the
developers/sysadmins. Anyone who has the full set, can roll their own
cookies and buy items for whatever they please. KAPOW! BIFF!
This particular problem is completely avoided by keeping all the
transaction data server side, in a session table.
As an aside, the subject of tying sessions to server/client addresses
etc has been covered off many times before; it suits some environments,
but many apps that try and implement this break in the real world. Many
corporates/ISPs use transparent proxy clusters that load balance clients
and change the source IP dynamically mid-session. Many Hosting
environments use web server clusters, which also have the same issue but
at the opposite end.
Martin...
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