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()

See also the DHBackend API for additional functionality.

Group parameters

cryptography.hazmat.primitives.asymmetric.dh.generate_parameters(generator, key_size, backend=None)[source]

New in version 1.7.

Generate a new DH parameter group for use with backend.

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.

  • backend – An optional DHBackend instance.

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[source]

New in version 1.7.

generate_private_key()[source]

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()[source]

Return the numbers that make up this set of parameters.

Returns

A DHParameterNumbers.

parameter_bytes(encoding, format)[source]

New 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.

class cryptography.hazmat.primitives.asymmetric.dh.DHParametersWithSerialization[source]

New in version 1.7.

Alias for DHParameters.

Key interfaces

class cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey[source]

New in version 1.7.

key_size

The bit length of the prime modulus.

public_key()[source]

Return the public key associated with this private key.

Returns

A DHPublicKey.

parameters()[source]

Return the parameters associated with this private key.

Returns

A DHParameters.

exchange(peer_public_key)[source]

New 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()[source]

Return the numbers that make up this private key.

Returns

A DHPrivateNumbers.

private_bytes(encoding, format, encryption_algorithm)[source]

New 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
Return bytes

Serialized key.

class cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKeyWithSerialization[source]

New in version 1.7.

Alias for DHPrivateKey.

class cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey[source]

New in version 1.7.

key_size

The bit length of the prime modulus.

parameters()[source]

Return the parameters associated with this private key.

Returns

A DHParameters.

public_numbers()[source]

Return the numbers that make up this public key.

Returns

A DHPublicNumbers.

public_bytes(encoding, format)[source]

New 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
Return bytes

Serialized key.

class cryptography.hazmat.primitives.asymmetric.dh.DHPublicKeyWithSerialization[source]

New in version 1.7.

Alias for DHPublicKey.

Numbers

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

New 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

New in version 1.8.

Type

int

p subgroup order value.

parameters(backend=None)[source]

New in version 1.7.

Parameters

backend – An optional instance of DHBackend.

Returns

A new instance of DHParameters.

class cryptography.hazmat.primitives.asymmetric.dh.DHPrivateNumbers(x, public_numbers)[source]

New 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(backend=None)[source]

New in version 1.7.

Parameters

backend – An optional instance of DHBackend.

Returns

A new instance of DHPrivateKey.

class cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers(y, parameter_numbers)[source]

New 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(backend=None)[source]

New in version 1.7.

Parameters

backend – An optional instance of DHBackend.

Returns

A new instance of DHPublicKey.