Bitcoin and Cryptocurrency Technologies

the previous and following documents — then collectively the participants can ensure that the history
cannot be changed after the fact. In particular, the relative ordering of documents is preserved.
Figure 4: linked timestamping​. To create a certificate for a document, the timestamp server includes
a hash pointer to the previous document’s certificate, the current time, and signs these three data
elements together.
A later paper proposed an efficiency improvement: instead of linking documents individually, we can
collect them into blocks and link blocks together in a chain. Within each block, the documents would
again be linked together, but in a tree structure instead of linearly. This decreases the amount of
checking needed to verify that a particular document appears at a particular point in the history of the
system. Visually, this hybrid scheme looks like Figure 5.
Figure 5: efficient linked timestamping​. Arrows represent hash pointers and dotted vertical lines
indicate time intervals.
This data structure forms the skeleton of Bitcoin’s block chain, as we’ll see in Chapter 3. Bitcoin
refines it a subtle but important way: a Hashcash-esque protocol is used to delay how fast new blocks
are added to the chain. This modification has profound and favorable consequences for Bitcoin’s
security model. There is no longer the need for trusted servers; instead, events are recorded by a
collection of untrusted nodes called “miners”. Every miner keeps track of blocks, rather than having to
rely on regular users to do it. Anyone can become a miner by solving computational puzzles to create
blocks. Bitcoin also gets rid of signatures, relying only on hash pointers to ensure the integrity of the
data structure. Finally, the actual timestamps aren’t of much importance in Bitcoin, and the point of
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