poly1305.rst

.. 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.

.. 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.

>>> from cryptography.hazmat.primitives import poly1305
>>> 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:

>>> 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.