Database Reference
In-Depth Information
reader to stall. The idea is that for each query by the reader, the blocker
tag
always
sends
both
and 0 and a 1. This means that the reader would
have to recursively traverse the entire tree, an undesirable situation,
which could cause the reader to stall. The blocker tag can be modified
to block only those tags with certain prefixes, a process which is referred
to as
selective blocking
. For example, the blocker tag may respond to
the reader only for those prefixes which correspond to the left subtree
of the root. This means that all RFIDs which start with a “0” are now
protected or “blocked” from scanning. Thus, by carefully assigning pre-
fixes to different products, it is possible to selectively block only certain
kinds of products for the purposes of privacy protection. Alternatively,
it may be possible to reset the prefix in a tag at check-out time, so as to
move the tag from the unprotected zone to the protected zone, once it
has been bought by the consumer. A blocker tag may either be carried
by a consumer on their person (through active acquisition), or may be
provided by a supermarket in a grocery bag, so as to prevent undesired
scanning of the items bought by a consumer. Once the items are removed
from the bag (for example, when food items are placed in a refrigerator),
the tags can become usable again, since they have been removed from
the vicinity of the blocker tag. The main drawback of blocker tags is
that they only provide an “opt-out” mechanism, in which tags are active
by default, and consumers must take the step of acquiring blockers in
order to protect their privacy. Such opt-out mechanisms are very useful,
when the tags only need to be blocked at certain times, places, or in the
possession of certain people.
We further note that a
polite blocking
protocol can be implemented,
which allows the readers to query the blocker tags, which tells them the
portions of the tree that they should not traverse [41]. Thus, the blocker
tag is being “polite” to the reader in telling it, which portions of the
tree it is blocking. The tree-walking protocol can then be modified in
order to not query those portions of the tree which are being blocked.
Polite blocking is useful, when the environment may contain legitimate
readers, which should not be made to inadvertently stall by the use of
blocker tags. Since authorized readers are likely to follow the proper
protocol, they will not be affected by the blocker tag. Furthermore,
even if unauthorized readers use the proper protocol, they will be unable
to access the tags of items with protected prefixes. This is the entire
purpose of blocking.
The blocking approach can be considered a kind of passive jamming,
and can be used both for privacy protection or for malicious purposes.
When used for malicious purposes, it can be considered equivalent to a
