Danger

This is a “Hazardous Materials” module. You should ONLY use it if you’re 100% absolutely sure that you know what you’re doing because this module is full of land mines, dragons, and dinosaurs with laser guns.

Diffie-Hellman key exchange

Note

For security and performance reasons we suggest using ECDH instead of DH where possible.

Diffie-Hellman key exchange (D–H) is a method that allows two parties to jointly agree on a shared secret using an insecure channel.

Exchange Algorithm

For most applications the shared_key should be passed to a key derivation function. This allows mixing of additional information into the key, derivation of multiple keys, and destroys any structure that may be present.

Warning

This example does not give forward secrecy and is only provided as a demonstration of the basic Diffie-Hellman construction. For real world applications always use the ephemeral form described after this example.

>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> # Generate some parameters. These can be reused.
>>> parameters = dh.generate_parameters(generator=2, key_size=2048)
>>> # Generate a private key for use in the exchange.
>>> server_private_key = parameters.generate_private_key()
>>> # In a real handshake the peer is a remote client. For this
>>> # example we'll generate another local private key though. Note that in
>>> # a DH handshake both peers must agree on a common set of parameters.
>>> peer_private_key = parameters.generate_private_key()
>>> shared_key = server_private_key.exchange(peer_private_key.public_key())
>>> # Perform key derivation.
>>> derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
... ).derive(shared_key)
>>> # And now we can demonstrate that the handshake performed in the
>>> # opposite direction gives the same final value
>>> same_shared_key = peer_private_key.exchange(
...     server_private_key.public_key()
... )
>>> same_derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
... ).derive(same_shared_key)
>>> derived_key == same_derived_key

DHE (or EDH), the ephemeral form of this exchange, is strongly preferred over simple DH and provides forward secrecy when used. You must generate a new private key using generate_private_key() for each exchange() when performing an DHE key exchange. An example of the ephemeral form:

>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> # Generate some parameters. These can be reused.
>>> parameters = dh.generate_parameters(generator=2, key_size=2048)
>>> # Generate a private key for use in the exchange.
>>> private_key = parameters.generate_private_key()
>>> # 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. Note that in a DH handshake both peers
>>> # must agree on a common set of parameters.
>>> peer_public_key = parameters.generate_private_key().public_key()
>>> shared_key = private_key.exchange(peer_public_key)
>>> # Perform key derivation.
>>> derived_key = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
... ).derive(shared_key)
>>> # For the next handshake we MUST generate another private key, but
>>> # we can reuse the parameters.
>>> private_key_2 = parameters.generate_private_key()
>>> peer_public_key_2 = parameters.generate_private_key().public_key()
>>> shared_key_2 = private_key_2.exchange(peer_public_key_2)
>>> derived_key_2 = HKDF(
...     algorithm=hashes.SHA256(),
...     length=32,
...     salt=None,
...     info=b'handshake data',
... ).derive(shared_key_2)

To assemble a DHParameters and a DHPublicKey from primitive integers, you must first create the DHParameterNumbers and DHPublicNumbers objects. For example, if p, g, and y are int objects received from a peer:

pn = dh.DHParameterNumbers(p, g)
parameters = pn.parameters()
peer_public_numbers = dh.DHPublicNumbers(y, pn)
peer_public_key = peer_public_numbers.public_key()

Group parameters

cryptography.hazmat.primitives.asymmetric.dh.generate_parameters(generator, key_size)

Added in version 1.7.

Generate a new DH parameter group.

Parameters:

generator – The int to use as a generator. Must be 2 or 5.
key_size – The bit length of the prime modulus to generate.

Returns:

DH parameters as a new instance of DHParameters.

Raises:

ValueError – If key_size is not at least 512.

class cryptography.hazmat.primitives.asymmetric.dh.DHParameters

Added in version 1.7.

generate_private_key()

Generate a DH private key. This method can be used to generate many new private keys from a single set of parameters.

Returns:: An instance of DHPrivateKey.

parameter_numbers()

Return the numbers that make up this set of parameters.

Returns:: A DHParameterNumbers.

parameter_bytes(encoding, format)

Added in version 2.0.

Allows serialization of the parameters to bytes. Encoding ( PEM or DER) and format ( PKCS3) are chosen to define the exact serialization.

Parameters:

encoding – A value from the Encoding enum.
format – A value from the ParameterFormat enum. At the moment only PKCS3 is supported.

Return bytes:

Serialized parameters.

Key interfaces

class cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey

Added in version 1.7.

key_size: The bit length of the prime modulus.

public_key()

Return the public key associated with this private key.

Returns:: A DHPublicKey.

parameters()

Return the parameters associated with this private key.

Returns:: A DHParameters.

exchange(peer_public_key)

Added in version 1.7.

Parameters:: peer_public_key (DHPublicKey) – The public key for the peer.
Return bytes:: The agreed key. The bytes are ordered in ‘big’ endian.

private_numbers()

Return the numbers that make up this private key.

Returns:: A DHPrivateNumbers.

private_bytes(encoding, format, encryption_algorithm)

Added in version 1.8.

Allows serialization of the key to bytes. Encoding ( PEM or DER), format ( PKCS8) and encryption algorithm (such as BestAvailableEncryption or NoEncryption) are chosen to define the exact serialization.

Parameters:

encoding – A value from the Encoding enum.
format – A value from the PrivateFormat enum.
encryption_algorithm – An instance of an object conforming to the KeySerializationEncryption interface.

Return bytes:

Serialized key.

class cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey

Added in version 1.7.

key_size: The bit length of the prime modulus.

parameters()

Return the parameters associated with this private key.

Returns:: A DHParameters.

public_numbers()

Return the numbers that make up this public key.

Returns:: A DHPublicNumbers.

public_bytes(encoding, format)

Added in version 1.8.

Allows serialization of the key to bytes. Encoding ( PEM or DER) and format ( SubjectPublicKeyInfo) are chosen to define the exact serialization.

Parameters:

encoding – A value from the Encoding enum.
format – A value from the PublicFormat enum.

Return bytes:

Serialized key.

Numbers

class cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers(p, g, q=None)

Added in version 0.8.

The collection of integers that define a Diffie-Hellman group.

p

Type:: int

The prime modulus value.

g

Type:: int

The generator value. Must be 2 or greater.

q

Added in version 1.8.

Type:: int

p subgroup order value.

parameters()

Added in version 1.7.

Returns:: A new instance of DHParameters.

class cryptography.hazmat.primitives.asymmetric.dh.DHPrivateNumbers(x, public_numbers)

Added in version 0.8.

The collection of integers that make up a Diffie-Hellman private key.

public_numbers

Type:: DHPublicNumbers

The DHPublicNumbers which makes up the DH public key associated with this DH private key.

x

Type:: int

The private value.

private_key()

Added in version 1.7.

Returns:: A new instance of DHPrivateKey.

class cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers(y, parameter_numbers)

Added in version 0.8.

The collection of integers that make up a Diffie-Hellman public key.

parameter_numbers

Type:

DHParameterNumbers

The parameters for this DH group.

y

Type:: int

The public value.

public_key()

Added in version 1.7.

Returns:: A new instance of DHPublicKey.