From:      William Casarin <jb55@jb55.com>
Date:      2019-06-07
Subject:   OpenTimestamps


OpenTimestamps is a protocol for time-stamping data into blockchains
(mainly Bitcoin, but it's built to be blockchain agnostic). This post
is a quick overview of the OTS proof file format, to get an idea of
how it works.


Here's what an open timestamp looks like:


$ otsprint timestamp.ots

  version 1
  file_hash sha256 7ff0c7d02a0dcaef63105b9f5d99c93adc2f501f4f204dcd5918c4a0acd10b55
  append 5065d9a0de5a108dcbd4714467709574
  sha256
  |
  \--> append 755b5dd0fa86ec516ddc26d0e765835a
  |    sha256
  |    append fd8f1b61b316d80b79de2607d9207c2a
  |    sha256
  |    prepend 5bf9ca01
  |    append 8cb3e5188471af08
  |    |
  |    \--> attestation calendar https://finney.calendar.eternitywall.com
  |    sha256
  |    append a6cc7120ab7d7a607a9e24221aa15fb7558444364205acd6ab61ad370839b3fe
  |    sha256
  |    prepend 1b16150ba8327ea18cc82ba6781814ef28fec0abf8b423eb2bf385c1bad5d1ea
  |    sha256
  |    [..]
  |    append a8c1513167c876dd119c3f33b705c2aecd6ea28f893e54b4c7d491ab7b528938
  |    sha256
  |    sha256
  |    append ab90169d97f24b64b726f894277754d09c8ca027a0b1f6054c2525a1ddfcce91
  |    sha256
  |    sha256
  |    attestation bitcoin height 551387
  |
  \--> append 7c7730e6fb81c3ed44d6d672a88d5b42
  |    sha256
  |    prepend 5bf9ca00
  |    append 727477c1c5d31991
  |    attestation calendar https://alice.btc.calendar.opentimestamps.org
  |
  \--> append 8e1e19d95e45043c6972dc717880fc23
  |    sha256
  |    prepend 5bf9ca01
  |    append e1c083a949c95ece
  |    attestation calendar https://bob.btc.calendar.opentimestamps.org
  append aa0d7770962773c4970da6bdc8210dd5
  sha256
  prepend 5bf9ca01
  append ca471b3db3ecc5b2
  attestation calendar https://btc.calendar.catallaxy.com


As we can see, it's a merkle tree of various operations such as
appending, prepending, and hashing data. These are mainly used when
combining other merkle trees together. This is the role of a calendar
server, allowing you to timestamp millions or more documents together
into a single tree.

The merkle root eventually makes it's way into a Bitcoin transaction.
The OTS proof is a merkle path to this root. With a fully upgraded OTS
proof, you can verify the existence of some document before a point in
time. This can be done completely independently by downloading the
blockchain and checking it yourself with the command `ots verify`