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5
Super-
Peer
Partially
signed
tokens
Super-
Peer
7
6
New
tokens
Super-
Peer
4
Peer
1
Super-
Peer
Collected (foreign) tokens
3
New
(unsigned)
tokens
IDs
2
Avoid double
spending
Account Holder
FIGURE 5.1: A super-peer-based token accounting system for P2P file shar-
ing [Hausheer et al., 2003].
Upon receipt of these unsigned tokens, the file owner provides the requested
files to the file consumer. When the files are successfully received, the file
consumer sends the signed version of the tokens to the file owner. In this
manner, Hauscheer et al. argued that there is no incentive for the peers to
cheat.
Yang and Garcia-Molina [Yang and Garcia-Molina, 2003] proposed the
PPay micropayment system in which each peer can buy a coin from a broker.
The peer then becomes the “owner” of the coin and can spend it to some other
peer. An important feature is that even after the coin is spent, the original
owner still has the responsibility to check the subsequent usage of the coin.
For example, suppose A is the owner of a coin which is spent to B. If B wants
to spend the coin in turn to C, the original owner A needs to check whether
such a transaction is valid (e.g., to avoid double spending of the same coin).
If A is o
ine (e.g., temporarily departed the P2P system), then the broker is
responsible to perform such checking.
Although the PPay system described above is a useful tool for supporting
P2P sharing, Jia et al. [Jia et al., 2005] observed that PPay can be further
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