=== otc

Oblivious transfer (OT) communications protocol message/response functionality implementations based on Curve25519 <https://cr.yp.to/ecdh.html>__ and the Ristretto <https://ristretto.group>__ group.

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Purpose

This library provides data structures and methods for a basic oblivious transfer (OT) <https://en.wikipedia.org/wiki/Oblivious_transfer>__ communications protocol defined in work by Chou and Orlandi <https://eprint.iacr.org/2015/267>. Thanks to the underlying oblivious <https://pypi.org/project/oblivious> library, method implementations rely on cryptographic primitives found within the libsodium <https://github.com/jedisct1/libsodium>__ library.

For more information and background about the underlying mathematical structures and primitives, consult materials about Curve25519 <https://cr.yp.to/ecdh.html>, the Ristretto <https://ristretto.group> group, and the related Ed25519 <https://ed25519.cr.yp.to>__ system.

Installation and Usage

This library is available as a package on PyPI <https://pypi.org/project/otc>__::

python -m pip install otc

The library can be imported in the usual manner::

import otc
from otc import *

Example ^^^^^^^

.. |send| replace:: send .. _send: https://otc.readthedocs.io/en/4.0.0/_source/otc.html#otc.otc.send

.. |reply| replace:: reply .. _reply: https://otc.readthedocs.io/en/4.0.0/_source/otc.html#otc.otc.send.reply

.. |receive| replace:: receive .. _receive: https://otc.readthedocs.io/en/4.0.0/_source/otc.html#otc.otc.receive

.. |query| replace:: query .. _query: https://otc.readthedocs.io/en/4.0.0/_source/otc.html#otc.otc.receive.query

.. |elect| replace:: elect .. _elect: https://otc.readthedocs.io/en/4.0.0/_source/otc.html#otc.otc.receive.elect

Suppose that a sender wants to send exactly one of two payloads to a receiver (such as one of two decryption keys). Furthermore, the receiver does not want to reveal to the sender which of the two payloads they chose to receive. To begin, the sender can create a |send|_ object s. The sender can then send the public key s.public to the receiver (via some communication channel)::

>>> import otc
>>> s = otc.send()
>>> s.public.to_base64()
'LJKN1DmNwGiCGWl4O5DsJMIEnlJm6yhb1o2hYS8A4Hg='

The receiver can create a |receive|_ object. Once they receive s.public from the sender (via some communications channel), they can use the |receive|_ object's |query|_ method to create an encrypted selection that the sender cannot decrypt. In the example below, the receiver is choosing the second message by supplying the integer 1 (where the two valid options are 0 or 1)::

>>> r = otc.receive()
>>> selection = r.query(s.public, 1)

The sender can then use its |send|_ object's |reply|_ method to create two encrypted replies based on the receiver's selection; the receiver will only be able to decrypt the pre-selected payload, and the sender does not know which of the two payloads can be decrypted by the receiver::

>>> replies = s.reply(selection, bytes([0] * 16), bytes([255] * 16))

Finally, the receiver can decrypt their chosen payload using the |receive|_ object's |elect|_ method. The example below confirms that the receiver has indeed received an encrypted version of the second message::

>>> r.elect(s.public, 1, *replies) == bytes([255] * 16)
True

See the article Privacy-Preserving Information Exchange Using Python <https://medium.com/nthparty/privacy-preserving-information-exchange-using-python-1a4a11bed3d5>__ for a more detailed presentation of this example.

Development

All installation and development dependencies are fully specified in pyproject.toml. The project.optional-dependencies object is used to specify optional requirements <https://peps.python.org/pep-0621>__ for various development tasks. This makes it possible to specify additional options (such as docs, lint, and so on) when performing installation using pip <https://pypi.org/project/pip>__::

python -m pip install .[docs,lint]

Documentation ^^^^^^^^^^^^^ The documentation can be generated automatically from the source files using Sphinx <https://www.sphinx-doc.org>__::

python -m pip install .[docs]
cd docs
sphinx-apidoc -f -E --templatedir=_templates -o _source .. && make html

Testing and Conventions ^^^^^^^^^^^^^^^^^^^^^^^ All unit tests are executed and their coverage is measured when using pytest <https://docs.pytest.org>__ (see the pyproject.toml file for configuration details)::

python -m pip install .[test]
python -m pytest

Alternatively, all unit tests are included in the module itself and can be executed using doctest <https://docs.python.org/3/library/doctest.html>__::

python src/otc/otc.py -v

Style conventions are enforced using Pylint <https://pylint.pycqa.org>__::

python -m pip install .[lint]
python -m pylint src/otc

Contributions ^^^^^^^^^^^^^ In order to contribute to the source code, open an issue or submit a pull request on the GitHub page <https://github.com/nthparty/otc>__ for this library.

Versioning ^^^^^^^^^^ The version number format for this library and the changes to the library associated with version number increments conform with Semantic Versioning 2.0.0 <https://semver.org/#semantic-versioning-200>__.

Publishing ^^^^^^^^^^ This library can be published as a package on PyPI <https://pypi.org/project/otc>__ by a package maintainer. First, install the dependencies required for packaging and publishing::

python -m pip install .[publish]

Ensure that the correct version number appears in pyproject.toml, and that any links in this README document to the Read the Docs documentation of this package (or its dependencies) have appropriate version numbers. Also ensure that the Read the Docs project for this library has an automation rule <https://docs.readthedocs.io/en/stable/automation-rules.html>__ that activates and sets as the default all tagged versions. Create and push a tag for this version (replacing ?.?.? with the version number)::

git tag ?.?.?
git push origin ?.?.?

Remove any old build/distribution files. Then, package the source into a distribution archive::

rm -rf build dist src/*.egg-info
python -m build --sdist --wheel .

Finally, upload the package distribution archive to PyPI <https://pypi.org>__::

python -m twine upload dist/*