.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/structures/timestamps/plot_01_linked_timestamps.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_api_gallery_structures_timestamps_plot_01_linked_timestamps.py>`
        to download the full example code or to run this example in your browser via JupyterLite.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_api_gallery_structures_timestamps_plot_01_linked_timestamps.py:


Linked timestamps: tamper evidence by hashing the past (Haber and Stornetta 1991)
=================================================================================

Haber and Stornetta asked how to timestamp a digital document so that not
even the timestamping service could backdate it. Their answer: each
certificate includes the hash of the previous one. Changing any old record
changes its hash, which breaks every later link. A blockchain is this
chain of linked timestamps, with blocks as the records.

What to look for
----------------

Edit one record in the middle and every link after it fails. Rewriting
history requires recomputing every later record, and anyone holding a
recent hash detects the change.

The history behind this experiment: :doc:`/history/structures_breakthroughs`.

.. GENERATED FROM PYTHON SOURCE LINES 22-24

Link ten records
----------------

.. GENERATED FROM PYTHON SOURCE LINES 24-45

.. code-block:: Python

    from dataclasses import replace

    import matplotlib.pyplot as plt

    import blockchainkit as bk

    documents = [f"lab notebook, page {page}".encode() for page in range(10)]
    records = []
    previous = bytes(32)
    for time, document in enumerate(documents):
        header = bk.structures.BlockHeader(previous, bk.crypto.sha256(document), time, time, 0, 0)
        records.append(header)
        previous = header.hash


    def broken_links(chain):
        return [i for i in range(1, len(chain)) if chain[i].previous_hash != chain[i - 1].hash]


    assert broken_links(records) == []








.. GENERATED FROM PYTHON SOURCE LINES 46-48

Backdate page 4
---------------

.. GENERATED FROM PYTHON SOURCE LINES 48-53

.. code-block:: Python

    tampered = list(records)
    tampered[4] = replace(records[4], merkle_root=bk.crypto.sha256(b"a better result"))
    print("broken links after editing page 4:", broken_links(tampered))
    assert broken_links(tampered) == [5]





.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    broken links after editing page 4: [5]




.. GENERATED FROM PYTHON SOURCE LINES 54-56

Repairing the break means rewriting everything after it
-------------------------------------------------------

.. GENERATED FROM PYTHON SOURCE LINES 56-62

.. code-block:: Python

    rewritten = list(tampered)
    for i in range(5, len(rewritten)):
        rewritten[i] = replace(rewritten[i], previous_hash=rewritten[i - 1].hash)
    assert broken_links(rewritten) == []
    assert rewritten[-1].hash != records[-1].hash  # Anyone holding the latest hash notices.








.. GENERATED FROM PYTHON SOURCE LINES 63-74

.. code-block:: Python

    fig, ax = plt.subplots(figsize=(8, 2.6))
    status = ["#16a34a"] * 4 + ["#dc2626"] + ["#f59e0b"] * 5
    ax.bar(range(10), [1] * 10, color=status)
    ax.set(
        xticks=range(10),
        yticks=[],
        xlabel="record",
        title="Edited (red) and every record that must be rewritten (amber)",
    )
    fig.tight_layout()




.. image-sg:: /api/gallery/structures/timestamps/images/sphx_glr_plot_01_linked_timestamps_001.png
   :alt: Edited (red) and every record that must be rewritten (amber)
   :srcset: /api/gallery/structures/timestamps/images/sphx_glr_plot_01_linked_timestamps_001.png
   :class: sphx-glr-single-img





.. GENERATED FROM PYTHON SOURCE LINES 75-80

Exercise
--------
Haber and Stornetta also proposed publishing the latest hash in a newspaper.
Why does one widely witnessed hash protect every earlier record? What plays
the newspaper's role in Bitcoin?


.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 0.087 seconds)


.. _sphx_glr_download_api_gallery_structures_timestamps_plot_01_linked_timestamps.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: lite-badge

      .. image:: images/jupyterlite_badge_logo.svg
        :target: ../../../../lite/lab/index.html?path=api/gallery/structures/timestamps/plot_01_linked_timestamps.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: plot_01_linked_timestamps.ipynb <plot_01_linked_timestamps.ipynb>`

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: plot_01_linked_timestamps.py <plot_01_linked_timestamps.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: plot_01_linked_timestamps.zip <plot_01_linked_timestamps.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_