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.

X25519 key exchange

X25519 is an elliptic curve Diffie-Hellman key exchange using Curve25519. It 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.

>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> # Generate a private key for use in the exchange.
>>> private_key = X25519PrivateKey.generate()
>>> # 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 = X25519PrivateKey.generate().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',
...     backend=default_backend()
... ).derive(shared_key)
>>> # For the next handshake we MUST generate another private key.
>>> private_key_2 = X25519PrivateKey.generate()
>>> peer_public_key_2 = X25519PrivateKey.generate().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',
...     backend=default_backend()
... ).derive(shared_key_2)

Key interfaces

class cryptography.hazmat.primitives.asymmetric.x25519.X25519PrivateKey

New in version 2.0.

classmethod generate()

Generate an X25519 private key.

Returns:X25519PrivateKey

public_key()

Returns:X25519PublicKey

exchange(peer_public_key)

Parameters:peer_public_key (X25519PublicKey) – The public key for the peer. Returns bytes:A shared key.

class cryptography.hazmat.primitives.asymmetric.x25519.X25519PublicKey

New in version 2.0.

classmethod from_public_bytes(data)

Parameters:data (bytes) – 32 byte public key. Returns:X25519PublicKey

>>> from cryptography.hazmat.primitives.asymmetric import x25519
>>> private_key = x25519.X25519PrivateKey.generate()
>>> public_key = private_key.public_key()
>>> public_bytes = public_key.public_bytes()
>>> loaded_public_key = x25519.X25519PublicKey.from_public_bytes(public_bytes)

public_bytes()

Returns bytes:The raw bytes of the public key.