.. 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
>>> 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: Secret key as ``bytes``.
: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.
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