path: root/docs/hazmat/primitives/asymmetric/ec.rst

.. 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: An instance of :class:`EllipticCurve`.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`.

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


.. function:: derive_private_key(private_value, curve, backend)

    .. versionadded:: 1.6

    Derive a private key from ``private_value`` on ``curve`` for use with
    ``backend``.

    :param int private_value: The secret scalar value.

    :param curve: An instance of :class:`EllipticCurve`.

    :param backend: An instance of
        :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`.

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


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

    .. 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()
        ... )
        >>> data = b"this is some data I'd like to sign"
        >>> signature = private_key.sign(
        ...     data,
        ...     ec.ECDSA(hashes.SHA256())
        ... )

    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`.

    If your data is too large to be passed in a single call, you can hash it
    separately and pass that value using
    :class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`.

    .. doctest::

        >>> from cryptography.hazmat.primitives.asymmetric import utils
        >>> chosen_hash = hashes.SHA256()
        >>> hasher = hashes.Hash(chosen_hash, default_backend())
        >>> hasher.update(b"data & ")
        >>> hasher.update(b"more data")
        >>> digest = hasher.finalize()
        >>> sig = private_key.sign(
        ...     digest,
        ...     ec.ECDSA(utils.Prehashed(chosen_hash))
        ... )


    Verification requires the public key, the signature itself, the signed
    data, and knowledge of the hashing algorithm that was used when producing
    the signature:

    >>> public_key = private_key.public_key()
    >>> public_key.verify(signature, data, ec.ECDSA(hashes.SHA256()))

    If the signature is not valid, an
    :class:`~cryptography.exceptions.InvalidSignature` exception will be raised.

    If your data is too large to be passed in a single call, you can hash it
    separately and pass that value using
    :class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`.

    .. doctest::

        >>> chosen_hash = hashes.SHA256()
        >>> hasher = hashes.Hash(chosen_hash, default_backend())
        >>> hasher.update(b"data & ")
        >>> hasher.update(b"more data")
        >>> digest = hasher.finalize()
        >>> public_key.verify(
        ...     sig,
        ...     digest,
        ...     ec.ECDSA(utils.Prehashed(chosen_hash))
        ... )

    .. note::
        Although in this case the public key was derived from the private one,
        in a typical setting you will not possess the private key. The
        `Key loading`_ section explains how to load the public key from other
        sources.


.. 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: An instance of
            :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`.

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


.. 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: An instance of
            :class:`~cryptography.hazmat.backends.interfaces.EllipticCurveBackend`.

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

    .. 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
        >>> # Generate a private key for use in the exchange.
        >>> private_key = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... )
        >>> # In a real handshake the peer_public_key will be received from the
        >>> # other party. For this example we'll generate another private key
        >>> # and get a public key from that.
        >>> peer_public_key = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... ).public_key()
        >>> shared_key = private_key.exchange(ec.ECDH(), peer_public_key)
        >>> # For the next handshake we MUST generate another private key.
        >>> private_key_2 = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... )
        >>> peer_public_key_2 = ec.generate_private_key(
        ...     ec.SECP384R1(), default_backend()
        ... ).public_key()
        >>> shared_key_2 = private_key_2.exchange(ec.ECDH(), peer_public_key_2)

    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 instances 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:: SECP224R1

    .. 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

        Size (in bits) of a secret scalar for the curve (as generated by
        :func:`generate_private_key`).


.. class:: EllipticCurveSignatureAlgorithm

    .. versionadded:: 0.5
    .. versionchanged:: 1.6
        :class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`
        can now be used as an ``algorithm``.

    A signature algorithm for use with elliptic curve keys.

    .. attribute:: algorithm

        :type: :class:`~cryptography.hazmat.primitives.hashes.HashAlgorithm` or
            :class:`~cryptography.hazmat.primitives.asymmetric.utils.Prehashed`

        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`_. An elliptic curve private key that is not an
    :term:`opaque key` also implements
    :class:`EllipticCurvePrivateKeyWithSerialization` to provide serialization
    methods.

    .. method:: exchange(algorithm, peer_public_key)

        .. versionadded:: 1.1

        Performs 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.

    .. method:: sign(data, signature_algorithm)

        .. versionadded:: 1.5

        Sign one block of data which can be verified later by others using the
        public key.

        :param bytes data: The message string to sign.

        :param signature_algorithm: An instance of
            :class:`EllipticCurveSignatureAlgorithm`, such as :class:`ECDSA`.

        :return bytes: Signature.

    .. attribute:: key_size

        .. versionadded:: 1.9

        :type: int

        Size (in bits) of a secret scalar for the curve (as generated by
        :func:`generate_private_key`).


.. class:: EllipticCurvePrivateKeyWithSerialization

    .. versionadded:: 0.8

    This interface contains additional methods relating to serialization.
    Any object with this interface also has all the methods from
    :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.

     .. 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.

    .. method:: verify(signature, data, signature_algorithm)

        .. versionadded:: 1.5

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

        :param bytes signature: The signature to verify.

        :param bytes data: The message string that was signed.

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

        :raises cryptography.exceptions.InvalidSignature: If the signature does
            not validate.

    .. attribute:: key_size

        .. versionadded:: 1.9

        :type: int

        Size (in bits) of a secret scalar for the curve (as generated by
        :func:`generate_private_key`).


.. class:: EllipticCurvePublicKeyWithSerialization

    .. versionadded:: 0.6

    Alias for :class:`EllipticCurvePublicKey`.



Serialization
~~~~~~~~~~~~~

This sample demonstrates how to generate a private key and serialize it.


.. doctest::

    >>> from cryptography.hazmat.backends import default_backend
    >>> from cryptography.hazmat.primitives import hashes
    >>> from cryptography.hazmat.primitives.asymmetric import ec
    >>> from cryptography.hazmat.primitives import serialization

    >>> private_key = ec.generate_private_key(ec.SECP384R1(), default_backend())

    >>> serialized_private = private_key.private_bytes(
    ...     encoding=serialization.Encoding.PEM,
    ...     format=serialization.PrivateFormat.PKCS8,
    ...     encryption_algorithm=serialization.BestAvailableEncryption(b'testpassword')
    ... )
    >>> serialized_private.splitlines()[0]
    '-----BEGIN ENCRYPTED PRIVATE KEY-----'

You can also serialize the key without a password, by relying on
:class:`~cryptography.hazmat.primitives.serialization.NoEncryption`.

The public key is serialized as follows:


.. doctest::

    >>> public_key = private_key.public_key()
    >>> serialized_public = public_key.public_bytes(
    ...     encoding=serialization.Encoding.PEM,
    ...     format=serialization.PublicFormat.SubjectPublicKeyInfo
    ... )
    >>> serialized_public.splitlines()[0]
    '-----BEGIN PUBLIC KEY-----'

This is the part that you would normally share with the rest of the world.


Key loading
~~~~~~~~~~~

This extends the sample in the previous section, assuming that the variables
``serialized_private`` and ``serialized_public`` contain the respective keys
in PEM format.

.. doctest::

    >>> loaded_public_key = serialization.load_pem_public_key(
    ...     serialized_public,
    ...     backend=default_backend()
    ... )

    >>> loaded_private_key = serialization.load_pem_private_key(
    ...     serialized_private,
    ...     # or password=None, if in plain text
    ...     password=b'testpassword',
    ...     backend=default_backend()
    ... )


.. _`FIPS 186-3`: https://csrc.nist.gov/csrc/media/publications/fips/186/3/archive/2009-06-25/documents/fips_186-3.pdf
.. _`FIPS 186-4`: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
.. _`800-56A`: https://csrc.nist.gov/publications/detail/sp/800-56a/revised/archive/2007-03-14
.. _`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`: https://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