.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/structures/transactions/plot_01_malleability.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_transactions_plot_01_malleability.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_transactions_plot_01_malleability.py:


Transaction malleability and segregated witness (2014-2017)
===========================================================

Bitcoin's transaction id hashed the whole transaction, signatures included.
Anyone relaying a transaction could alter its signature encoding without
invalidating it, changing the id. In 2014 the Mt. Gox exchange blamed
malleability for lost withdrawals; Decker and Wattenhofer found it explained
only a small fraction. Segregated witness (2017) moved signatures out of
the id, which also made payment channels such as Lightning safe to build.

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

The same payment, signed twice, has two txids: a child transaction that
named the first id would point at nothing. The unsigned id, like SegWit's
txid, is the same for every valid signature.

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

.. GENERATED FROM PYTHON SOURCE LINES 23-25

One payment, two signatures, two txids
--------------------------------------

.. GENERATED FROM PYTHON SOURCE LINES 25-41

.. code-block:: Python

    import matplotlib.pyplot as plt

    import blockchainkit as bk

    alice_key = 7
    payment = bk.structures.Transaction(
        bk.crypto.public_key(alice_key), bk.structures.address(bk.crypto.public_key(11)), 25, 0
    )
    first = payment.signed(alice_key, signing_nonce=1001)
    second = payment.signed(alice_key, signing_nonce=2002)  # Same payment, re-signed.
    assert first.is_valid() and second.is_valid()
    assert first.txid != second.txid
    assert first.unsigned_id == second.unsigned_id
    print("txid:       ", first.txid.hex()[:16], "vs", second.txid.hex()[:16])
    print("unsigned id:", first.unsigned_id.hex()[:16], "for both")





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

 .. code-block:: none

    txid:        fa96fe8ee56adcbe vs df5d5b6e70155532
    unsigned id: bf31ad29acf982f8 for both




.. GENERATED FROM PYTHON SOURCE LINES 42-44

A child that names its parent by txid breaks
--------------------------------------------

.. GENERATED FROM PYTHON SOURCE LINES 44-49

.. code-block:: Python

    child_refers_to = first.txid
    confirmed = second  # A miner confirmed the re-signed version.
    assert child_refers_to != confirmed.txid
    assert first.unsigned_id == confirmed.unsigned_id  # A SegWit-style reference still works.








.. GENERATED FROM PYTHON SOURCE LINES 50-63

.. code-block:: Python

    ids = {f"signature {n}": payment.signed(alice_key, signing_nonce=n) for n in (11, 22, 33, 44)}
    fig, ax = plt.subplots(figsize=(8, 3))
    for row, (label, tx) in enumerate(ids.items()):
        ax.text(0.02, row, label, va="center")
        ax.text(0.30, row, tx.txid.hex()[:20], va="center", family="monospace", color="#dc2626")
        ax.text(0.68, row, tx.unsigned_id.hex()[:20], va="center", family="monospace", color="#16a34a")
    ax.text(0.30, len(ids), "txid (covers signature)", weight="bold")
    ax.text(0.68, len(ids), "unsigned id (SegWit)", weight="bold")
    ax.set(xlim=(0, 1), ylim=(-0.7, len(ids) + 0.5), title="Re-signing changes one id, not the other")
    ax.axis("off")
    ax.invert_yaxis()
    fig.tight_layout()




.. image-sg:: /api/gallery/structures/transactions/images/sphx_glr_plot_01_malleability_001.png
   :alt: Re-signing changes one id, not the other
   :srcset: /api/gallery/structures/transactions/images/sphx_glr_plot_01_malleability_001.png
   :class: sphx-glr-single-img





.. GENERATED FROM PYTHON SOURCE LINES 64-69

Exercise
--------
In Lightning, two parties pre-sign a refund transaction that spends a
funding transaction before it is broadcast. Explain why the refund needs
an id for its parent that cannot change.


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

   **Total running time of the script:** (0 minutes 0.159 seconds)


.. _sphx_glr_download_api_gallery_structures_transactions_plot_01_malleability.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/transactions/plot_01_malleability.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

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

      :download:`Download Jupyter notebook: plot_01_malleability.ipynb <plot_01_malleability.ipynb>`

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

      :download:`Download Python source code: plot_01_malleability.py <plot_01_malleability.py>`

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

      :download:`Download zipped: plot_01_malleability.zip <plot_01_malleability.zip>`


.. only:: html

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

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