1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
|
.. hazmat::
Diffie-Hellman key exchange
===========================
.. currentmodule:: cryptography.hazmat.primitives.asymmetric.dh
`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.
.. code-block:: pycon
>>> from cryptography.hazmat.backends import default_backend
>>> from cryptography.hazmat.primitives.asymmetric import dh
>>> parameters = dh.generate_parameters(generator=2, key_size=2048,
... backend=default_backend())
>>> private_key = parameters.generate_private_key()
>>> peer_public_key = parameters.generate_private_key().public_key()
>>> shared_key = private_key.exchange(peer_public_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 :func:`~DHParameters.generate_private_key` for
each :meth:`~DHPrivateKeyWithSerialization.exchange` when performing an DHE key
exchange.
To assemble a :class:`~DHParameters` and a :class:`~DHPublicKey` from
primitive integers, you must first create the
:class:`~DHParameterNumbers` and :class:`~DHPublicNumbers` objects. For
example if **p**, **g**, and **y** are :class:`int` objects received from a
peer::
pn = dh.DHParameterNumbers(p, g)
parameters = pn.parameters(default_backend())
peer_public_numbers = dh.DHPublicNumbers(y, pn)
peer_public_key = peer_public_numbers.public_key(default_backend())
See also the :class:`~cryptography.hazmat.backends.interfaces.DHBackend`
API for additional functionality.
Group parameters
~~~~~~~~~~~~~~~~
.. function:: generate_parameters(generator, key_size, backend)
.. versionadded:: 0.9
Generate a new DH parameter group for use with ``backend``.
:param generator: The :class:`int` to use as a generator. Must be
2 or 5.
:param key_size: The bit length of the prime modulus to generate.
:param backend: A
:class:`~cryptography.hazmat.backends.interfaces.DHBackend`
instance.
:returns: DH parameters as a new instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
:raises ValueError: If ``key_size`` is not at least 512.
.. class:: DHParameters
.. versionadded:: 0.9
.. method:: generate_private_key()
.. versionadded:: 0.9
Generate a DH private key. This method can be used to generate many
new private keys from a single set of parameters.
:return: An instance of
:class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey`.
.. class:: DHParametersWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. method:: parameter_numbers()
Return the numbers that make up this set of parameters.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers`.
Key interfaces
~~~~~~~~~~~~~~
.. class:: DHPrivateKey
.. versionadded:: 0.9
.. attribute:: key_size
The bit length of the prime modulus.
.. method:: public_key()
Return the public key associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey`.
.. method:: parameters()
Return the parameters associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. class:: DHPrivateKeyWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateKey`.
.. method:: private_numbers()
Return the numbers that make up this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPrivateNumbers`.
.. method:: exchange(peer_public_key)
.. versionadded:: 1.7
:param DHPublicKeyWithSerialization peer_public_key: The public key for the
peer.
:return bytes: The agreed key. The bytes are ordered in 'big' endian.
.. class:: DHPublicKey
.. versionadded:: 0.9
.. attribute:: key_size
The bit length of the prime modulus.
.. method:: parameters()
Return the parameters associated with this private key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameters`.
.. class:: DHPublicKeyWithSerialization
.. versionadded:: 0.9
Inherits from :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicKey`.
.. method:: public_numbers()
Return the numbers that make up this public key.
:return: A :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers`.
Numbers
~~~~~~~
.. class:: DHParameterNumbers(p, g)
.. versionadded:: 0.8
The collection of integers that define a Diffie-Hellman group.
.. attribute:: p
:type: int
The prime modulus value.
.. attribute:: g
:type: int
The generator value. Must be 2 or 5.
.. class:: DHPrivateNumbers(x, public_numbers)
.. versionadded:: 0.8
The collection of integers that make up a Diffie-Hellman private key.
.. attribute:: public_numbers
:type: :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHPublicNumbers`
The :class:`DHPublicNumbers` which makes up the DH public
key associated with this DH private key.
.. attribute:: x
:type: int
The private value.
.. class:: DHPublicNumbers(y, parameter_numbers)
.. versionadded:: 0.8
The collection of integers that make up a Diffie-Hellman public key.
.. attribute:: parameter_numbers
:type: :class:`~cryptography.hazmat.primitives.asymmetric.dh.DHParameterNumbers`
The parameters for this DH group.
.. attribute:: y
:type: int
The public value.
.. _`Diffie-Hellman key exchange`: https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange
.. _`forward secrecy`: https://en.wikipedia.org/wiki/Forward_secrecy
|
