.. hazmat::

Elliptic curve cryptography
===========================

.. module:: cryptography.hazmat.primitives.asymmetric.ec


.. function:: generate_private_key(curve, backend)

    .. versionadded:: 0.5

    Generate a new private key on ``curve`` for use with ``backend``.

    :param curve: A :class:`EllipticCurve` provider.

    :param backend: A
        :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`
        provider.

    :returns: A new instance of a :class:`EllipticCurvePrivateKey` provider.


Elliptic Curve Signature Algorithms
-----------------------------------

.. class:: ECDSA(algorithm)

    .. versionadded:: 0.5

    The ECDSA signature algorithm first standardized in NIST publication
    `FIPS 186-3`_, and later in `FIPS 186-4`_.

    :param algorithm: An instance of a
        :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`
        provider.

    .. doctest::

        >>> from cryptography.hazmat.backends import default_backend
        >>> from cryptography.hazmat.primitives import hashes
        >>> from cryptography.hazmat.primitives.asymmetric import ec
        >>> private_key = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... )
        >>> signer = private_key.signer(ec.ECDSA(hashes.SHA256()))
        >>> signer.update(b"this is some data I'd like")
        >>> signer.update(b" to sign")
        >>> signature = signer.finalize()

    The ``signature`` is a ``bytes`` object, whose contents is DER encoded as
    described in :rfc:`3279`. This can be decoded using
    :func:`~cryptography.hazmat.primitives.asymmetric.utils.decode_dss_signature`.



.. class:: EllipticCurvePrivateNumbers(private_value, public_numbers)

    .. versionadded:: 0.5

    The collection of integers that make up an EC private key.

    .. attribute:: public_numbers

        :type: :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurvePublicNumbers`

        The :class:`EllipticCurvePublicNumbers` which makes up the EC public
        key associated with this EC private key.

    .. attribute:: private_value

        :type: int

        The private value.

    .. method:: private_key(backend)

        Convert a collection of numbers into a private key suitable for doing
        actual cryptographic operations.

        :param backend: A
            :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`
            provider.

        :returns: A new instance of a :class:`EllipticCurvePrivateKey`
            provider.


.. class:: EllipticCurvePublicNumbers(x, y, curve)

    .. versionadded:: 0.5

    The collection of integers that make up an EC public key.

     .. attribute:: curve

        :type: :class:`EllipticCurve`

        The elliptic curve for this key.

    .. attribute:: x

        :type: int

        The affine x component of the public point used for verifying.

    .. attribute:: y

        :type: int

        The affine y component of the public point used for verifying.

    .. method:: public_key(backend)

        Convert a collection of numbers into a public key suitable for doing
        actual cryptographic operations.

        :param backend: A
            :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`
            provider.

        :returns: A new instance of a :class:`EllipticCurvePublicKey`
            provider.

    .. method:: encode_point()

        .. versionadded:: 1.1

        Encodes an elliptic curve point to a byte string as described in
        `SEC 1 v2.0`_ section 2.3.3. This method only supports uncompressed
        points.

        :return bytes: The encoded point.

    .. classmethod:: from_encoded_point(curve, data)

        .. versionadded:: 1.1

        Decodes a byte string as described in `SEC 1 v2.0`_ section 2.3.3 and
        returns an :class:`EllipticCurvePublicNumbers`. This method only
        supports uncompressed points.

        :param curve: An
            :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurve`
            instance.

        :param bytes data: The serialized point byte string.

        :returns: An :class:`EllipticCurvePublicNumbers` instance.

        :raises ValueError: Raised on invalid point type or data length.

        :raises TypeError: Raised when curve is not an
            :class:`~cryptography.hazmat.primitives.asymmetric.ec.EllipticCurve`.

Elliptic Curve Key Exchange algorithm
-------------------------------------

.. class:: ECDH()

    .. versionadded:: 1.1

    The Elliptic Curve Diffie-Hellman Key Exchange algorithm first standardized
    in NIST publication `800-56A`_, and later in `800-56Ar2`_.

    For most applications the ``shared_key`` should be passed to a key
    derivation function.

    .. doctest::

        >>> from cryptography.hazmat.backends import default_backend
        >>> from cryptography.hazmat.primitives.asymmetric import ec
        >>> private_key = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... )
        >>> peer_public_key = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... ).public_key()
        >>> shared_key = private_key.exchange(ec.ECDH(), peer_public_key)

    ECDHE (or EECDH), the ephemeral form of this exchange, is **strongly
    preferred** over simple ECDH and provides `forward secrecy`_ when used.
    You must generate a new private key using :func:`generate_private_key` for
    each :meth:`~EllipticCurvePrivateKey.exchange` when performing an ECDHE key
    exchange.

Elliptic Curves
---------------

Elliptic curves provide equivalent security at much smaller key sizes than
other asymmetric cryptography systems such as RSA or DSA. For many operations
elliptic curves are also significantly faster; `elliptic curve diffie-hellman
is faster than diffie-hellman`_.

.. note::
    Curves with a size of `less than 224 bits`_ should not be used. You should
    strongly consider using curves of at least 224 bits.

Generally the NIST prime field ("P") curves are significantly faster than the
other types suggested by NIST at both signing and verifying with ECDSA.

Prime fields also `minimize the number of security concerns for elliptic-curve
cryptography`_. However, there is `some concern`_ that both the prime field and
binary field ("B") NIST curves may have been weakened during their generation.

Currently `cryptography` only supports NIST curves, none of which are
considered "safe" by the `SafeCurves`_ project run by Daniel J. Bernstein and
Tanja Lange.

All named curves are providers of :class:`EllipticCurve`.

.. class:: SECT571K1

    .. versionadded:: 0.5

    SECG curve ``sect571k1``. Also called NIST K-571.


.. class:: SECT409K1

    .. versionadded:: 0.5

    SECG curve ``sect409k1``. Also called NIST K-409.


.. class:: SECT283K1

    .. versionadded:: 0.5

    SECG curve ``sect283k1``. Also called NIST K-283.


.. class:: SECT233K1

    .. versionadded:: 0.5

    SECG curve ``sect233k1``. Also called NIST K-233.


.. class:: SECT163K1

    .. versionadded:: 0.5

    SECG curve ``sect163k1``. Also called NIST K-163.


.. class:: SECT571R1

    .. versionadded:: 0.5

    SECG curve ``sect571r1``. Also called NIST B-571.


.. class:: SECT409R1

    .. versionadded:: 0.5

    SECG curve ``sect409r1``. Also called NIST B-409.


.. class:: SECT283R1

    .. versionadded:: 0.5

    SECG curve ``sect283r1``. Also called NIST B-283.


.. class:: SECT233R1

    .. versionadded:: 0.5

    SECG curve ``sect233r1``. Also called NIST B-233.


.. class:: SECT163R2

    .. versionadded:: 0.5

    SECG curve ``sect163r2``. Also called NIST B-163.


.. class:: SECP521R1

    .. versionadded:: 0.5

    SECG curve ``secp521r1``. Also called NIST P-521.


.. class:: SECP384R1

    .. versionadded:: 0.5

    SECG curve ``secp384r1``. Also called NIST P-384.


.. class:: SECP256R1

    .. versionadded:: 0.5

    SECG curve ``secp256r1``. Also called NIST P-256.


.. class:: SECT224R1

    .. versionadded:: 0.5

    SECG curve ``secp224r1``. Also called NIST P-224.


.. class:: SECP192R1

    .. versionadded:: 0.5

    SECG curve ``secp192r1``. Also called NIST P-192.


.. class:: SECP256K1

    .. versionadded:: 0.9

    SECG curve ``secp256k1``.


Key Interfaces
~~~~~~~~~~~~~~

.. class:: EllipticCurve

    .. versionadded:: 0.5

    A named elliptic curve.

    .. attribute:: name

        :type: string

        The name of the curve. Usually the name used for the ASN.1 OID such as
        ``secp256k1``.

    .. attribute:: key_size

        :type: int

        The bit length of the curve's base point.


.. class:: EllipticCurveSignatureAlgorithm

    .. versionadded:: 0.5

    A signature algorithm for use with elliptic curve keys.

    .. attribute:: algorithm

        :type: :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm`

        The digest algorithm to be used with the signature scheme.


.. class:: EllipticCurvePrivateKey

    .. versionadded:: 0.5

    An elliptic curve private key for use with an algorithm such as `ECDSA`_ or
    `EdDSA`_.

    .. method:: signer(signature_algorithm)

        Sign data which can be verified later by others using the public key.
        The signature is formatted as DER-encoded bytes, as specified in
        :rfc:`3279`.

        :param signature_algorithm: An instance of a
            :class:`EllipticCurveSignatureAlgorithm` provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.asymmetric.AsymmetricSignatureContext`

    .. method:: exchange(algorithm, peer_public_key)

        .. versionadded:: 1.1

        Perform's a key exchange operation using the provided algorithm with
        the peer's public key.

        For most applications the result should be passed to a key derivation
        function.

        :param algorithm: The key exchange algorithm, currently only
            :class:`~cryptography.hazmat.primitives.asymmetric.ec.ECDH` is
            supported.
        :param EllipticCurvePublicKey peer_public_key: The public key for the
            peer.

        :returns bytes: A shared key.

    .. method:: public_key()

        :return: :class:`EllipticCurvePublicKey`

        The EllipticCurvePublicKey object for this private key.


.. class:: EllipticCurvePrivateKeyWithSerialization

    .. versionadded:: 0.8

    Extends :class:`EllipticCurvePrivateKey`.

    .. method:: private_numbers()

        Create a :class:`EllipticCurvePrivateNumbers` object.

        :returns: An :class:`EllipticCurvePrivateNumbers` instance.

    .. method:: private_bytes(encoding, format, encryption_algorithm)

        Allows serialization of the key to bytes. Encoding (
        :attr:`~cryptography.hazmat.primitives.serialization.Encoding.PEM` or
        :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`),
        format (
        :attr:`~cryptography.hazmat.primitives.serialization.PrivateFormat.TraditionalOpenSSL`
        or
        :attr:`~cryptography.hazmat.primitives.serialization.PrivateFormat.PKCS8`)
        and encryption algorithm (such as
        :class:`~cryptography.hazmat.primitives.serialization.BestAvailableEncryption`
        or :class:`~cryptography.hazmat.primitives.serialization.NoEncryption`)
        are chosen to define the exact serialization.

        :param encoding: A value from the
            :class:`~cryptography.hazmat.primitives.serialization.Encoding` enum.

        :param format: A value from the
            :class:`~cryptography.hazmat.primitives.serialization.PrivateFormat` enum.

        :param encryption_algorithm: An instance of an object conforming to the
            :class:`~cryptography.hazmat.primitives.serialization.KeySerializationEncryption`
            interface.

        :return bytes: Serialized key.


.. class:: EllipticCurvePublicKey

    .. versionadded:: 0.5

    An elliptic curve public key.

    .. method:: verifier(signature, signature_algorithm)

        Verify data was signed by the private key associated with this public
        key.

        :param bytes signature: The signature to verify. DER encoded as
            specified in :rfc:`3279`.

        :param signature_algorithm: An instance of a
            :class:`EllipticCurveSignatureAlgorithm` provider.

        :returns:
            :class:`~cryptography.hazmat.primitives.asymmetric.AsymmetricVerificationContext`

     .. attribute:: curve

        :type: :class:`EllipticCurve`

        The elliptic curve for this key.

    .. method:: public_numbers()

        Create a :class:`EllipticCurvePublicNumbers` object.

        :returns: An :class:`EllipticCurvePublicNumbers` instance.

    .. method:: public_bytes(encoding, format)

        Allows serialization of the key to bytes. Encoding (
        :attr:`~cryptography.hazmat.primitives.serialization.Encoding.PEM` or
        :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`) and
        format (
        :attr:`~cryptography.hazmat.primitives.serialization.PublicFormat.SubjectPublicKeyInfo`)
        are chosen to define the exact serialization.

        :param encoding: A value from the
            :class:`~cryptography.hazmat.primitives.serialization.Encoding` enum.

        :param format: A value from the
            :class:`~cryptography.hazmat.primitives.serialization.PublicFormat` enum.

        :return bytes: Serialized key.


.. class:: EllipticCurvePublicKeyWithSerialization

    .. versionadded:: 0.6

    Alias for :class:`EllipticCurvePublicKey`.


.. _`FIPS 186-3`: http://csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf
.. _`FIPS 186-4`: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
.. _`800-56A`: http://csrc.nist.gov/publications/nistpubs/800-56A/SP800-56A_Revision1_Mar08-2007.pdf
.. _`800-56Ar2`: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-56Ar2.pdf
.. _`some concern`: https://crypto.stackexchange.com/questions/10263/should-we-trust-the-nist-recommended-ecc-parameters
.. _`less than 224 bits`: http://www.ecrypt.eu.org/ecrypt2/documents/D.SPA.20.pdf
.. _`elliptic curve diffie-hellman is faster than diffie-hellman`: http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1100&context=cseconfwork
.. _`minimize the number of security concerns for elliptic-curve cryptography`: https://cr.yp.to/ecdh/curve25519-20060209.pdf
.. _`SafeCurves`: http://safecurves.cr.yp.to/
.. _`ECDSA`: https://en.wikipedia.org/wiki/ECDSA
.. _`EdDSA`: https://en.wikipedia.org/wiki/EdDSA
.. _`forward secrecy`: https://en.wikipedia.org/wiki/Forward_secrecy
.. _`SEC 1 v2.0`: http://www.secg.org/sec1-v2.pdf
