key-derivation-functions.rst

Key derivation functions

cryptography.hazmat.primitives.kdf

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 ~cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC or ~cryptography.hazmat.primitives.kdf.hkdf.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.

Variable cost algorithms

PBKDF2

cryptography.hazmat.primitives.kdf.pbkdf2

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 ~cryptography.hazmat.primitives.kdf.scrypt.Scrypt is generally considered a better solution.

This class conforms to the ~cryptography.hazmat.primitives.kdf.KeyDerivationFunction interface.

>>> import os >>> from cryptography.hazmat.primitives import hashes >>> from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC >>> # Salts should be randomly generated >>> salt = os.urandom(16) >>> # derive >>> kdf = PBKDF2HMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... iterations=480000, ... ) >>> key = kdf.derive(b"my great password") >>> # verify >>> kdf = PBKDF2HMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... iterations=480000, ... ) >>> kdf.verify(b"my great password", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is (2³² - 1) * algorithm.digest_size.

param bytes salt

A salt. Secure values¹ are 128-bits (16 bytes) or longer and randomly generated.

param int iterations

The number of iterations to perform of the hash function. This can be used to control the length of time the operation takes. Higher numbers help mitigate brute force attacks against derived keys. A more detailed description can be consulted for additional information.

raises TypeError

This exception is raised if salt is not bytes.

derive(key_material)

param key_material

The input key material. For PBKDF2 this should be a password.

type key_material

bytes-like

return bytes

the derived key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

raises TypeError

This exception is raised if key_material is not bytes.

This generates and returns a new key from the supplied password.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

Scrypt

cryptography.hazmat.primitives.kdf.scrypt

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 7914.

This class conforms to the ~cryptography.hazmat.primitives.kdf.KeyDerivationFunction interface.

>>> import os >>> from cryptography.hazmat.primitives.kdf.scrypt import Scrypt >>> salt = os.urandom(16) >>> # derive >>> kdf = Scrypt( ... salt=salt, ... length=32, ... n=2*14, ... r=8, ... p=1, ... ) >>> key = kdf.derive(b"my great password") >>> # verify >>> kdf = Scrypt( ... salt=salt, ... length=32, ... n=2*14, ... r=8, ... p=1, ... ) >>> kdf.verify(b"my great password", key)

param bytes salt

A salt.

param int length

The desired length of the derived key in bytes.

param int n

CPU/Memory cost parameter. It must be larger than 1 and be a power of 2.

param int r

Block size parameter.

param int p

Parallelization parameter.

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.

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).

raises cryptography.exceptions.UnsupportedAlgorithm

If Scrypt is not supported by the OpenSSL version cryptography is using.

raises TypeError

This exception is raised if salt is not bytes.

raises ValueError

This exception is raised if n is less than 2, if n is not a power of 2, if r is less than 1 or if p is less than 1.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

the derived key.

raises TypeError

This exception is raised if key_material is not bytes.

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 password.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

Fixed cost algorithms

ConcatKDF

cryptography.hazmat.primitives.kdf.concatkdf

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 >>> otherinfo = b"concatkdf-example" >>> ckdf = ConcatKDFHash( ... algorithm=hashes.SHA256(), ... length=32, ... otherinfo=otherinfo, ... ) >>> key = ckdf.derive(b"input key") >>> ckdf = ConcatKDFHash( ... algorithm=hashes.SHA256(), ... length=32, ... otherinfo=otherinfo, ... ) >>> ckdf.verify(b"input key", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is hashlen * (2^32 -1).

param bytes otherinfo

Application specific context information. If None is explicitly passed an empty byte string will be used.

raises TypeError

This exception is raised if otherinfo is not bytes.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

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 >>> salt = os.urandom(16) >>> otherinfo = b"concatkdf-example" >>> ckdf = ConcatKDFHMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... otherinfo=otherinfo, ... ) >>> key = ckdf.derive(b"input key") >>> ckdf = ConcatKDFHMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... otherinfo=otherinfo, ... ) >>> ckdf.verify(b"input key", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is hashlen * (2^32 -1).

param bytes salt

A salt. Randomizes the KDF's output. Optional, but highly recommended. Ideally as many bits of entropy as the security level of the hash: often that means cryptographically random and as long as the hash output. Does not have to be secret, but may cause stronger security guarantees if secret; If None is explicitly passed a default salt of algorithm.block_size null bytes will be used.

param bytes otherinfo

Application specific context information. If None is explicitly passed an empty byte string will be used.

raises TypeError

This exception is raised if salt or otherinfo is not bytes.

derive(key_material)

param bytes key_material

The input key material.

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

HKDF

cryptography.hazmat.primitives.kdf.hkdf

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 >>> salt = os.urandom(16) >>> info = b"hkdf-example" >>> hkdf = HKDF( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... info=info, ... ) >>> key = hkdf.derive(b"input key") >>> hkdf = HKDF( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... info=info, ... ) >>> hkdf.verify(b"input key", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is 255 * (algorithm.digest_size // 8).

param bytes salt

A salt. Randomizes the KDF's output. Optional, but highly recommended. Ideally as many bits of entropy as the security level of the hash: often that means cryptographically random and as long as the hash output. Worse (shorter, less entropy) salt values can still meaningfully contribute to security. May be reused. Does not have to be secret, but may cause stronger security guarantees if secret; see 5869 and the HKDF paper for more details. If None is explicitly passed a default salt of algorithm.digest_size // 8 null bytes will be used.

param bytes info

Application specific context information. If None is explicitly passed an empty byte string will be used.

raises TypeError

This exception is raised if salt or info is not bytes.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material by performing both the extract and expand operations.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

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 ~cryptography.hazmat.primitives.kdf.hkdf.HKDF if you are unsure.

>>> import os >>> from cryptography.hazmat.primitives import hashes >>> from cryptography.hazmat.primitives.kdf.hkdf import HKDFExpand >>> info = b"hkdf-example" >>> key_material = os.urandom(16) >>> hkdf = HKDFExpand( ... algorithm=hashes.SHA256(), ... length=32, ... info=info, ... ) >>> key = hkdf.derive(key_material) >>> hkdf = HKDFExpand( ... algorithm=hashes.SHA256(), ... length=32, ... info=info, ... ) >>> hkdf.verify(key_material, key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is 255 * (algorithm.digest_size // 8).

param bytes info

Application specific context information. If None is explicitly passed an empty byte string will be used.

raises TypeError

This exception is raised if info is not bytes.

derive(key_material)

param bytes key_material

The input key material.

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material by performing both the extract and expand operations.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

raises TypeError

This is raised if the provided key_material is a unicode object

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.

KBKDF

cryptography.hazmat.primitives.kdf.kbkdf

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 ... ) >>> label = b"KBKDF HMAC Label" >>> context = b"KBKDF HMAC Context" >>> kdf = KBKDFHMAC( ... algorithm=hashes.SHA256(), ... mode=Mode.CounterMode, ... length=32, ... rlen=4, ... llen=4, ... location=CounterLocation.BeforeFixed, ... label=label, ... context=context, ... fixed=None, ... ) >>> key = kdf.derive(b"input key") >>> kdf = KBKDFHMAC( ... algorithm=hashes.SHA256(), ... mode=Mode.CounterMode, ... length=32, ... rlen=4, ... llen=4, ... location=CounterLocation.BeforeFixed, ... label=label, ... context=context, ... fixed=None, ... ) >>> kdf.verify(b"input key", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param mode

The desired mode of the PRF. A value from the ~cryptography.hazmat.primitives.kdf.kbkdf.Mode enum.

param int length

The desired length of the derived key in bytes.

param int rlen

An integer that indicates the length of the binary representation of the counter in bytes.

param int llen

An integer that indicates the binary representation of the length in bytes.

param location

The desired location of the counter. A value from the ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation enum.

param bytes label

Application specific label information. If None is explicitly passed an empty byte string will be used.

param bytes context

Application specific context information. If None is explicitly passed an empty byte string will be used.

param bytes fixed

Instead of specifying label and context you may supply your own fixed data. If fixed is specified, label and context is ignored.

param int break_location

A keyword-only argument. An integer that indicates the bytes offset where counter bytes are to be located. Required when location is ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation.MiddleFixed.

raises TypeError

This exception is raised if label or context is not bytes. Also raised if rlen, llen, or break_location is not int.

raises ValueError

This exception is raised if rlen or llen is greater than 4 or less than 1. This exception is also raised if you specify a label or context and fixed. This exception is also raised if you specify break_location and location is not ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation.MiddleFixed.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

35.0

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

KBKDFCMAC should not be used for password storage.

>>> from cryptography.hazmat.primitives.ciphers import algorithms >>> from cryptography.hazmat.primitives.kdf.kbkdf import ( ... CounterLocation, KBKDFCMAC, Mode ... ) >>> label = b"KBKDF CMAC Label" >>> context = b"KBKDF CMAC Context" >>> kdf = KBKDFCMAC( ... algorithm=algorithms.AES, ... mode=Mode.CounterMode, ... length=32, ... rlen=4, ... llen=4, ... location=CounterLocation.BeforeFixed, ... label=label, ... context=context, ... fixed=None, ... ) >>> key = kdf.derive(b"32 bytes long input key material") >>> kdf = KBKDFCMAC( ... algorithm=algorithms.AES, ... mode=Mode.CounterMode, ... length=32, ... rlen=4, ... llen=4, ... location=CounterLocation.BeforeFixed, ... label=label, ... context=context, ... fixed=None, ... ) >>> kdf.verify(b"32 bytes long input key material", key)

param algorithm

A class implementing a block cipher algorithm being a subclass of ~cryptography.hazmat.primitives.ciphers.CipherAlgorithm and ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm.

param mode

The desired mode of the PRF. A value from the ~cryptography.hazmat.primitives.kdf.kbkdf.Mode enum.

param int length

The desired length of the derived key in bytes.

param int rlen

An integer that indicates the length of the binary representation of the counter in bytes.

param int llen

An integer that indicates the binary representation of the length in bytes.

param location

The desired location of the counter. A value from the ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation enum.

param bytes label

Application specific label information. If None is explicitly passed an empty byte string will be used.

param bytes context

Application specific context information. If None is explicitly passed an empty byte string will be used.

param bytes fixed

Instead of specifying label and context you may supply your own fixed data. If fixed is specified, label and context is ignored.

param int break_location

A keyword-only argument. An integer that indicates the bytes offset where counter bytes are to be located. Required when location is ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation.MiddleFixed.

raises cryptography.exceptions.UnsupportedAlgorithm

This is raised if algorithm is not a subclass of ~cryptography.hazmat.primitives.ciphers.CipherAlgorithm and ~cryptography.hazmat.primitives.ciphers.BlockCipherAlgorithm.

raises TypeError

This exception is raised if label or context is not bytes, rlen, llen, or break_location is not int, mode is not ~cryptography.hazmat.primitives.kdf.kbkdf.Mode or location is not ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation.

raises ValueError

This exception is raised if rlen or llen is greater than 4 or less than 1. This exception is also raised if you specify a label or context and fixed. This exception is also raised if you specify break_location and location is not ~cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation.MiddleFixed.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises ValueError

This exception is raised if key_material is not a valid key for algorithm passed to ~cryptography.hazmat.primitives.kdf.kbkdf.KBKDFCMAC constructor.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises

Exceptions raised by derive.

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.

CounterMode

The output of the PRF is computed with a counter as the iteration variable.

An enumeration for the key based key derivative counter location.

BeforeFixed

The counter iteration variable will be concatenated before the fixed input data.

AfterFixed

The counter iteration variable will be concatenated after the fixed input data.

MiddleFixed

38.0

The counter iteration variable will be concatenated in the middle of the fixed input data.

X963KDF

cryptography.hazmat.primitives.kdf.x963kdf

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 ~cryptography.hazmat.primitives.kdf.concatkdf.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 >>> sharedinfo = b"ANSI X9.63 Example" >>> xkdf = X963KDF( ... algorithm=hashes.SHA256(), ... length=32, ... sharedinfo=sharedinfo, ... ) >>> key = xkdf.derive(b"input key") >>> xkdf = X963KDF( ... algorithm=hashes.SHA256(), ... length=32, ... sharedinfo=sharedinfo, ... ) >>> xkdf.verify(b"input key", key)

param algorithm

An instance of ~cryptography.hazmat.primitives.hashes.HashAlgorithm.

param int length

The desired length of the derived key in bytes. Maximum is hashlen * (2^32 -1).

param bytes sharedinfo

Application specific context information. If None is explicitly passed an empty byte string will be used.

raises TypeError

This exception is raised if sharedinfo is not bytes.

derive(key_material)

param key_material

The input key material.

type key_material

bytes-like

return bytes

The derived key.

raises TypeError

This exception is raised if key_material is not bytes.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

Derives a new key from the input key material.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.

Interface

cryptography.hazmat.primitives.kdf

0.2

derive(key_material)

param bytes key_material

The input key material. Depending on what key derivation function you are using this could be either random bytes, or a user supplied password.

return

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.

verify(key_material, expected_key)

param bytes key_material

The input key material. This is the same as key_material in derive.

param bytes expected_key

The expected result of deriving a new key, this is the same as the return value of derive.

raises cryptography.exceptions.InvalidKey

This is raised when the derived key does not match the expected key.

raises cryptography.exceptions.AlreadyFinalized

This is raised when derive or verify is called more than once.

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.↩