[LINK] E-Tag security?

[Paul McGowan]

Hi Linkers,

This is mainly idle curiosity, but I was wondering if anyone knew 
what sort of security safeguards are in place for E-Tags as used on 
Melbourne and Sydney (and no doubt many other) tollways?

Specifcally, I was wondering what sort of protections there are in 
the scanning protocol to ensure that things like replay attacks can't 
happen. 

The scenario I am pondering involves producing a scanner that can 
elicit a reply from a parked car where an E-tag is visible, then 
replaying this reply to the toll gantry as you pass under it.  If it 
worked (and I sincerely hope it wouldn't) it would make a very easy 
way to steal from innocent motorists and use the tollways free, not 
to mention anonymously.  I would imagine that collecting many 
different replies and playng a random one would make it even harder 
to detect.

Can anyone point me to information on the actual protocol(s)?

Best regards,

Paul McGowan