paper 15 - anonymity CROWDS


crowd members are termed as jondos. when a jondo receives a request, it
flips a biased coin to determine whether or not to forward this request to
another jondo. each packet has a path identifier to enable jondos to take
independent actions since it might occupy multiple positions on the path.
a member of the crowd may submit requests initiated by others users.

SECURITY ANALYSIS
sender anonymity is not offered against a local eavesdropper.
receiver anonymity is offered beyond suspicion.

increasing the probability of forwarding a request increases the number of
collaborating jondos that can be tolerated (since forwarding paths will be
longer)however system performance decreases.

even crowd members cannot identify the initiator of the request.

crowd based systems can fail in case of malicious members which may drop
packets or forward incorrect ones.

HTML TIMING ATTACKS :
when a jondo receives a HTML page as response, it might request for
additional URLs from that page. The immediate nature of these requests
opens opportunities for timing attacks by collaborating jondos. Timing
attacks have been eliminated by the last jondo on the path requesting
these URLs and sending them on the same path as the original request.
Thus, the initiator waits for these URLs instead of explicitly issuing
requests.



P5

P5 allows users to trade-off the degree of anonymity for communication
efficiency ( you can decrease the amount of anonymity but not increase it)
packets are encrypted using the receiver's public key. every message is
hop-by-hop encrypted (each node acts as a mix).

a broadcast hierarchy is created to make the broadcast mechanism scalable.
the hierarchy consists of a binary tree which is constructed using the
public key of the users. each node of the tree contains a bitstring and a
bitmask (b/m). the bitmask specifies as to how many of the msb of the
bitstring are valid.

a message sent on (b/m) is forwarded to:
1) all members in (b/m) group
2) all members of the group at nodes higher up ( which match some prefix
of the bitstring b)
3) all members of the subtree below this node (nodes whose bitstring has
prefix b)


each user in the system can join a set of broadcast groups dependent upon
the hash of the user's public key. the user can also select the bitmask
value thus determining the specific group that he wants to join.

noise:

each user generates a fixed amount of traffic at all times to prevent
local eavesdropping.
message dropping algorithms must be used to control queue sizes (which
increase easily due to broadcast mechanism)


ANONYMITY ANALYSIS :
sender and receiver anonymity depend upon group sizes.

ATTACKS :

Correlation attacks will be avoided due to noise packets.

Difference and Intersection attacks map the user to a smaller subset of
the group based upon the knowledge of his presence in more than 1 group.





HERBIVORE FS:

based on DC-nets
 DC nets have the following properties
  1) only 1 sender can send at a time
  2) requires a mediator
  3) even if 1 member in the clique is malicious the scheme fails beyond
repair


herbivore FS takes care of 1) and 2)
 1) to prevent multiple messages from being garbled - a concept of
transmission slots and reservation protocols exist
 2) the mediator is replaced by a broadcast ( or another center node which
acts a mediator for each round)


 3) the paper talks about a strike table mechanism to remove malicious
node. However, a malicious node could still target a smaller set of users
without being striked out.


each node is forced to choose a random ID to prevent targeting a
particular user.

multiple instances of the protocol with different mediators help in
achieving higher throughputs.


INSERTION ATTACKS :
each node reserves space for two sets of files A and B - A for the files
that it introduced and B for the files introduced by the clique.
files placed on the A-list are transferred to the B-list after response to
the first query. the B-list is managed as a LRU cache. This caching
mechanism hides the identity of the node as the node does not insert a
file into the network more than once(each member of the clique then has a
copy of it in the

B-list)
dropping of files seems to be a valid tradeoff of availability against
security.

















