  Bitcoin provides freshness properties by forming a blockchain where each
block is associated with its timestamp and the previous block. Due to these
properties, the Bitcoin protocol is being used as a decentralized, trusted, and
secure timestamping service. Although Bitcoin participants which create new
blocks cannot modify their order, they can manipulate timestamps almost
undetected. This undermines the Bitcoin protocol as a reliable timestamping
service. In particular, a newcomer that synchronizes the entire blockchain has
a little guarantee about timestamps of all blocks.
  In this paper, we present a simple yet powerful mechanism that increases the
reliability of Bitcoin timestamps. Our protocol can provide evidence that a
block was created within a certain time range. The protocol is efficient,
backward compatible, and surprisingly, currently deployed SSL/TLS servers can
act as reference time sources. The protocol has many applications and can be
used for detecting various attacks against the Bitcoin protocol.
