This is a quote from the book "B-Money" by Wei Dai
...ney are kept bya subset of the participants (called servers from now on) instead ofeveryone. These servers are linked by a Usenet-style broadcast channel.The format of transaction messages broad
casted on this channel remain thesame as in the first protocol, but the affected participants of eachtransaction should verify that the message has been received andsuccessfully processed by a r
andomly selected subset of the servers.
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Since the servers must be trusted to a degree, some mechanism is needed tokeep them honest. Each server is required to deposit a certain amou...