.. hazmat::

Poly1305
========

.. currentmodule:: cryptography.hazmat.primitives.poly1305

.. testsetup::

    key = b"\x01" * 32

Poly1305 is an authenticator that takes a 32-byte key and a message and
produces a 16-byte tag. This tag is used to authenticate the message. Each key
**must** only be used once. Using the same key to generate tags for multiple
messages allows an attacker to forge tags. Poly1305 is described in
:rfc:`7539`.

.. class:: Poly1305(key)

    .. versionadded:: 2.7

    .. warning::

        Using the same key to generate tags for multiple messages allows an
        attacker to forge tags. Always generate a new key per message you want
        to authenticate. If you are using this as a MAC for
        symmetric encryption please use
        :class:`~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305`
        instead.

    .. doctest::

        >>> from cryptography.hazmat.primitives import poly1305
        >>> key = b"\x01" * 32 # A real key should come from os.urandom(32)
        >>> p = poly1305.Poly1305(key)
        >>> p.update(b"message to authenticate")
        >>> p.finalize()
        b'T\xae\xff3\xbdW\xef\xd5r\x01\xe2n=\xb7\xd2h'

    To check that a given tag is correct use the :meth:`verify` method.
    You will receive an exception if the tag is wrong:

    .. doctest::

        >>> p = poly1305.Poly1305(key)
        >>> p.update(b"message to authenticate")
        >>> p.verify(b"an incorrect tag")
        Traceback (most recent call last):
        ...
        cryptography.exceptions.InvalidSignature: Value did not match computed tag.

    :param key: The secret key.
    :type key: :term:`bytes-like`
    :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
        the version of OpenSSL ``cryptography`` is compiled against does not
        support this algorithm.

    .. method:: update(data)

        :param data: The bytes to hash and authenticate.
        :type data: :term:`bytes-like`
        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
        :raises TypeError: This exception is raised if ``data`` is not ``bytes``.

    .. method:: verify(tag)

        Finalize the current context and securely compare the MAC to
        ``tag``.

        :param bytes tag: The bytes to compare against.
        :raises cryptography.exceptions.AlreadyFinalized: See :meth:`finalize`
        :raises cryptography.exceptions.InvalidSignature: If tag does not
                                                          match.
        :raises TypeError: This exception is raised if ``tag`` is not
                           ``bytes``.

        .. method:: finalize()

        Finalize the current context and return the message authentication code
        as bytes.

        After ``finalize`` has been called this object can no longer be used
        and :meth:`update`, :meth:`verify`, and :meth:`finalize`
        will raise an :class:`~cryptography.exceptions.AlreadyFinalized`
        exception.

        :return bytes: The message authentication code as bytes.
        :raises cryptography.exceptions.AlreadyFinalized:

    .. classmethod:: generate_tag(key, data)

        A single step alternative to do sign operations. Returns the message
        authentication code as ``bytes`` for the given ``key`` and ``data``.

        :param key: Secret key as ``bytes``.
        :type key: :term:`bytes-like`
        :param data: The bytes to hash and authenticate.
        :type data: :term:`bytes-like`
        :return bytes: The message authentication code as bytes.
        :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
            the version of OpenSSL ``cryptography`` is compiled against does not
            support this algorithm.
        :raises TypeError: This exception is raised if ``key`` or ``data`` are
            not ``bytes``.

        .. doctest::

            >>> poly1305.Poly1305.generate_tag(key, b"message to authenticate")
            b'T\xae\xff3\xbdW\xef\xd5r\x01\xe2n=\xb7\xd2h'

    .. classmethod:: verify_tag(key, data, tag)

        A single step alternative to do verify operations. Securely compares the
        MAC to ``tag``, using the given ``key`` and ``data``.

        :param key: Secret key as ``bytes``.
        :type key: :term:`bytes-like`
        :param data: The bytes to hash and authenticate.
        :type data: :term:`bytes-like`
        :param bytes tag: The bytes to compare against.
        :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
            the version of OpenSSL ``cryptography`` is compiled against does not
            support this algorithm.
        :raises TypeError: This exception is raised if ``key``, ``data`` or
            ``tag`` are not ``bytes``.
        :raises cryptography.exceptions.InvalidSignature: If tag does not match.

        .. doctest::

            >>> poly1305.Poly1305.verify_tag(key, b"message to authenticate", b"an incorrect tag")
            Traceback (most recent call last):
            ...
            cryptography.exceptions.InvalidSignature: Value did not match computed tag.
