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.
Key derivation functions derive bytes suitable for cryptographic operations from passwords or other data sources using a pseudo-random function (PRF). Different KDFs are suitable for different tasks such as:
Cryptographic key derivation
Deriving a key suitable for use as input to an encryption algorithm. Typically this means taking a password and running it through an algorithm such as PBKDF2HMAC or HKDF. This process is typically known as key stretching.
Password storage
When storing passwords you want to use an algorithm that is computationally intensive. Legitimate users will only need to compute it once (for example, taking the user’s password, running it through the KDF, then comparing it to the stored value), while attackers will need to do it billions of times. Ideal password storage KDFs will be demanding on both computational and memory resources.
New in version 0.2.
PBKDF2 (Password Based Key Derivation Function 2) is typically used for deriving a cryptographic key from a password. It may also be used for key storage, but an alternate key storage KDF such as Scrypt is generally considered a better solution.
This class conforms to the KeyDerivationFunction interface.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> # Salts should be randomly generated
>>> salt = os.urandom(16)
>>> # derive
>>> kdf = PBKDF2HMAC(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... iterations=100000,
... backend=backend
... )
>>> key = kdf.derive(b"my great password")
>>> # verify
>>> kdf = PBKDF2HMAC(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... iterations=100000,
... backend=backend
... )
>>> kdf.verify(b"my great password", key)
Parameters: |
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Parameters: | key_material (bytes) – The input key material. For PBKDF2 this should be a password. |
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Return bytes: | the derived key. |
Raises: |
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This generates and returns a new key from the supplied password.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match. This can be used for checking whether the password a user provides matches the stored derived key.
New in version 0.2.
HKDF (HMAC-based Extract-and-Expand Key Derivation Function) is suitable for deriving keys of a fixed size used for other cryptographic operations.
Warning
HKDF should not be used for password storage.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDF
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> salt = os.urandom(16)
>>> info = b"hkdf-example"
>>> hkdf = HKDF(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... info=info,
... backend=backend
... )
>>> key = hkdf.derive(b"input key")
>>> hkdf = HKDF(
... algorithm=hashes.SHA256(),
... length=32,
... salt=salt,
... info=info,
... backend=backend
... )
>>> hkdf.verify(b"input key", key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises TypeError: | |
This exception is raised if key_material is not bytes. |
Derives a new key from the input key material by performing both the extract and expand operations.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
New in version 0.5.
HKDF consists of two stages, extract and expand. This class exposes an expand only version of HKDF that is suitable when the key material is already cryptographically strong.
Warning
HKDFExpand should only be used if the key material is cryptographically strong. You should use HKDF if you are unsure.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.hkdf import HKDFExpand
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> info = b"hkdf-example"
>>> key_material = os.urandom(16)
>>> hkdf = HKDFExpand(
... algorithm=hashes.SHA256(),
... length=32,
... info=info,
... backend=backend
... )
>>> key = hkdf.derive(key_material)
>>> hkdf = HKDFExpand(
... algorithm=hashes.SHA256(),
... length=32,
... info=info,
... backend=backend
... )
>>> hkdf.verify(key_material, key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises: |
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Derives a new key from the input key material by performing both the extract and expand operations.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
New in version 1.0.
ConcatKDFHash (Concatenation Key Derivation Function) is defined by the NIST Special Publication NIST SP 800-56Ar2 document, to be used to derive keys for use after a Key Exchange negotiation operation.
Warning
ConcatKDFHash should not be used for password storage.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.concatkdf import ConcatKDFHash
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> otherinfo = b"concatkdf-example"
>>> ckdf = ConcatKDFHash(
... algorithm=hashes.SHA256(),
... length=256,
... otherinfo=otherinfo,
... backend=backend
... )
>>> key = ckdf.derive(b"input key")
>>> ckdf = ConcatKDFHash(
... algorithm=hashes.SHA256(),
... length=256,
... otherinfo=otherinfo,
... backend=backend
... )
>>> ckdf.verify(b"input key", key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises TypeError: | |
This exception is raised if key_material is not bytes. |
Derives a new key from the input key material.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
New in version 1.0.
Similar to ConcatKFDHash but uses an HMAC function instead.
Warning
ConcatKDFHMAC should not be used for password storage.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.concatkdf import ConcatKDFHMAC
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> salt = os.urandom(16)
>>> otherinfo = b"concatkdf-example"
>>> ckdf = ConcatKDFHMAC(
... algorithm=hashes.SHA256(),
... length=256,
... salt=salt,
... otherinfo=otherinfo,
... backend=backend
... )
>>> key = ckdf.derive(b"input key")
>>> ckdf = ConcatKDFHMAC(
... algorithm=hashes.SHA256(),
... length=256,
... salt=salt,
... otherinfo=otherinfo,
... backend=backend
... )
>>> ckdf.verify(b"input key", key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises TypeError: | |
This exception is raised if key_material is not bytes. |
Derives a new key from the input key material.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
New in version 1.1.
X963KDF (ANSI X9.63 Key Derivation Function) is defined by ANSI in the ANSI X9.63:2001 document, to be used to derive keys for use after a Key Exchange negotiation operation.
SECG in SEC 1 v2.0 recommends that ConcatKDFHash be used for new projects. This KDF should only be used for backwards compatibility with pre-existing protocols.
Warning
X963KDF should not be used for password storage.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.x963kdf import X963KDF
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> sharedinfo = b"ANSI X9.63 Example"
>>> xkdf = X963KDF(
... algorithm=hashes.SHA256(),
... length=256,
... sharedinfo=sharedinfo,
... backend=backend
... )
>>> key = xkdf.derive(b"input key")
>>> xkdf = X963KDF(
... algorithm=hashes.SHA256(),
... length=256,
... sharedinfo=sharedinfo,
... backend=backend
... )
>>> xkdf.verify(b"input key", key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises TypeError: | |
This exception is raised if key_material is not bytes. |
Derives a new key from the input key material.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
New in version 1.4.
KBKDF (Key Based Key Derivation Function) is defined by the NIST SP 800-108 document, to be used to derive additional keys from a key that has been established through an automated key-establishment scheme.
Warning
KBKDFHMAC should not be used for password storage.
>>> import os
>>> from cryptography.hazmat.primitives import hashes
>>> from cryptography.hazmat.primitives.kdf.kbkdf import (
... CounterLocation, KBKDFHMAC, Mode
... )
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> label = b"KBKDF HMAC Label"
>>> context = b"KBKDF HMAC Context"
>>> kdf = KBKDFHMAC(
... algorithm=hashes.SHA256(),
... mode=Mode.CounterMode,
... length=256,
... rlen=4,
... llen=4,
... location=CounterLocation.BeforeFixed,
... label=label,
... context=context,
... fixed=None,
... backend=backend
... )
>>> key = kdf.derive(b"input key")
>>> kdf = KBKDFHMAC(
... algorithm=hashes.SHA256(),
... mode=Mode.CounterMode,
... length=256,
... rlen=4,
... llen=4,
... location=CounterLocation.BeforeFixed,
... label=label,
... context=context,
... fixed=None,
... backend=backend
... )
>>> kdf.verify(b"input key", key)
Parameters: |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | The derived key. |
Raises TypeError: | |
This exception is raised if key_material is not bytes. |
Derives a new key from the input key material.
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match.
An enumeration for the key based key derivative modes.
The output of the PRF is computed with a counter as the iteration variable.
An enumeration for the key based key derivative counter location.
The counter iteration variable will be concatenated before the fixed input data.
The counter iteration variable will be concatenated after the fixed input data.
New in version 1.6.
Scrypt is a KDF designed for password storage by Colin Percival to be resistant against hardware-assisted attackers by having a tunable memory cost. It is described in RFC 7914.
This class conforms to the KeyDerivationFunction interface.
>>> import os
>>> from cryptography.hazmat.primitives.kdf.scrypt import Scrypt
>>> from cryptography.hazmat.backends import default_backend
>>> backend = default_backend()
>>> salt = os.urandom(16)
>>> # derive
>>> kdf = Scrypt(
... salt=salt,
... length=64,
... n=2**14,
... r=8,
... p=1,
... backend=backend
... )
>>> key = kdf.derive(b"my great password")
>>> # verify
>>> kdf = Scrypt(
... salt=salt,
... length=64,
... n=2**14,
... r=8,
... p=1,
... backend=backend
... )
>>> kdf.verify(b"my great password", key)
Parameters: |
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The computational and memory cost of Scrypt can be adjusted by manipulating the 3 parameters: n, r, and p. In general, the memory cost of Scrypt is affected by the values of both n and r, while n also determines the number of iterations performed. p increases the computational cost without affecting memory usage. A more in-depth explanation of the 3 parameters can be found here.
RFC 7914 recommends values of r=8 and p=1 while scaling n to a number appropriate for your system. The scrypt paper suggests a minimum value of n=2**14 for interactive logins (t < 100ms), or n=2**20 for more sensitive files (t < 5s).
Parameters: | backend – An instance of ScryptBackend. |
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Raises: |
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Parameters: | key_material (bytes) – The input key material. |
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Return bytes: | the derived key. |
Raises: |
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This generates and returns a new key from the supplied password.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match. This can be used for checking whether the password a user provides matches the stored derived key.
New in version 0.2.
Parameters: | key_material (bytes) – The input key material. Depending on what key derivation function you are using this could be either random bytes, or a user supplied password. |
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Returns: | The new key. |
Raises cryptography.exceptions.AlreadyFinalized: | |
This is raised when derive() or verify() is called more than once. |
This generates and returns a new key from the supplied key material.
Parameters: | |
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Raises: |
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This checks whether deriving a new key from the supplied key_material generates the same key as the expected_key, and raises an exception if they do not match. This can be used for something like checking whether a user’s password attempt matches the stored derived key.
[1] | See NIST SP 800-132. |