.. DO NOT EDIT.
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.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/vm/ethereum/plot_04_integer_overflow.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_vm_ethereum_plot_04_integer_overflow.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_vm_ethereum_plot_04_integer_overflow.py:


Integer overflow: minting tokens from nothing (BeautyChain, 2018)
=================================================================

EVM words wrap modulo 2**256. In April 2018 an attacker called the
BeautyChain (BEC) token's ``batchTransfer`` with two recipients and a value of
2**255. The contract computed the total as ``2 * 2**255``, which wraps to 0,
checked that the sender had at least 0 tokens, and credited each recipient
2**255 tokens. Exchanges suspended the token (CVE-2018-10299). The SafeMath
library's checked arithmetic, and later Solidity 0.8's default overflow
checks, close the hole.

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

The unchecked contract accepts the call from a sender with no tokens, and
the total supply jumps by 2**256: tokens appear from nowhere.
The checked version reverts, and both versions agree on every ordinary
transfer.

The history behind this experiment: :doc:`/history/vm_breakthroughs`.
See :doc:`/exercises/vm` for a worked solution to the exercise.

.. GENERATED FROM PYTHON SOURCE LINES 26-28

The attack
----------

.. GENERATED FROM PYTHON SOURCE LINES 28-46

.. code-block:: Python

    import matplotlib.pyplot as plt

    import blockchainkit as bk

    ATTACKER, RECIPIENT_A, RECIPIENT_B = 1, 2, 3
    balances = {ATTACKER: 0, 4: 1_000_000}
    unchecked = bk.vm.batch_transfer(ATTACKER, [RECIPIENT_A, RECIPIENT_B])
    checked = bk.vm.batch_transfer(ATTACKER, [RECIPIENT_A, RECIPIENT_B], checked=True)
    value = 2**255
    assert 2 * value % 2**256 == 0
    after = bk.vm.execute(unchecked, arguments=(value,), storage=balances).storage
    print("recipient A now holds", after[RECIPIENT_A])
    assert after[RECIPIENT_A] == after[RECIPIENT_B] == value and after[ATTACKER] == 0
    try:
        bk.vm.execute(checked, arguments=(value,), storage=balances)
    except bk.vm.VMError as error:
        print("checked version:", error)





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

 .. code-block:: none

    recipient A now holds 57896044618658097711785492504343953926634992332820282019728792003956564819968
    checked version: reverted




.. GENERATED FROM PYTHON SOURCE LINES 47-49

Ordinary transfers are unaffected
---------------------------------

.. GENERATED FROM PYTHON SOURCE LINES 49-61

.. code-block:: Python

    funded = {ATTACKER: 500}
    for v in (1, 10, 100, 250):
        a = bk.vm.execute(unchecked, arguments=(v,), storage=funded).storage
        b = bk.vm.execute(checked, arguments=(v,), storage=funded).storage
        assert a == b and sum(a.values()) == 500

    supply = {"before": sum(balances.values()), "after attack": sum(after.values())}
    fig, ax = plt.subplots(figsize=(6, 3.5))
    ax.bar(supply.keys(), [s / 2**255 for s in supply.values()], color=["#64748b", "#dc2626"])
    ax.set(ylabel="total supply / 2**255", title="Tokens from a wrapped multiplication")
    fig.tight_layout()




.. image-sg:: /api/gallery/vm/ethereum/images/sphx_glr_plot_04_integer_overflow_001.png
   :alt: Tokens from a wrapped multiplication
   :srcset: /api/gallery/vm/ethereum/images/sphx_glr_plot_04_integer_overflow_001.png
   :class: sphx-glr-single-img





.. GENERATED FROM PYTHON SOURCE LINES 62-67

Exercise
--------
The checked version tests ``amount / count == value``. Why does that detect
every overflow of ``count * value`` for ``count >= 1``? Could the check
``amount >= value`` replace it?


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

   **Total running time of the script:** (0 minutes 0.091 seconds)


.. _sphx_glr_download_api_gallery_vm_ethereum_plot_04_integer_overflow.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/vm/ethereum/plot_04_integer_overflow.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

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

      :download:`Download Jupyter notebook: plot_04_integer_overflow.ipynb <plot_04_integer_overflow.ipynb>`

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

      :download:`Download Python source code: plot_04_integer_overflow.py <plot_04_integer_overflow.py>`

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

      :download:`Download zipped: plot_04_integer_overflow.zip <plot_04_integer_overflow.zip>`


.. only:: html

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

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