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¶
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
PBKDF2HMACorHKDF. 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.
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class
cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC(algorithm, length, salt, iterations, backend)¶ 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
Scryptis generally considered a better solution.This class conforms to the
KeyDerivationFunctioninterface.>>> 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
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is (232 - 1) *
algorithm.digest_size.salt (bytes) – A salt. Secure values 1 are 128-bits (16 bytes) or longer and randomly generated.
iterations (int) – 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.
backend – An instance of
PBKDF2HMACBackend.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementPBKDF2HMACBackendTypeError – This exception is raised if
saltis notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material. For PBKDF2 this should be a password.
- Return bytes
the derived key.
- Raises
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.TypeError – This exception is raised if
key_materialis notbytes.
This generates and returns a new key from the supplied password.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_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.
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class
cryptography.hazmat.primitives.kdf.hkdf.HKDF(algorithm, length, salt, info, backend)¶ 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
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is
255 * (algorithm.digest_size // 8).salt (bytes) – 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 RFC 5869 and the HKDF paper for more details. If
Noneis explicitly passed a default salt ofalgorithm.digest_size // 8null bytes will be used.info (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.backend – An instance of
HMACBackend.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHMACBackendTypeError – This exception is raised if
saltorinfois notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material by performing both the extract and expand operations.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.hkdf.HKDFExpand(algorithm, length, info, backend)¶ 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
HKDFif 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
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is
255 * (algorithm.digest_size // 8).info (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.backend – An instance of
HMACBackend.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHMACBackendTypeError – This exception is raised if
infois notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material by performing both the extract and expand operations.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.TypeError – This is raised if the provided
key_materialis aunicodeobject
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.concatkdf.ConcatKDFHash(algorithm, length, otherinfo, backend)¶ 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=32, ... otherinfo=otherinfo, ... backend=backend ... ) >>> key = ckdf.derive(b"input key") >>> ckdf = ConcatKDFHash( ... algorithm=hashes.SHA256(), ... length=32, ... otherinfo=otherinfo, ... backend=backend ... ) >>> ckdf.verify(b"input key", key)
- Parameters
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is
hashlen * (2^32 -1).otherinfo (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.backend – An instance of
HashBackend.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHashBackendTypeError – This exception is raised if
otherinfois notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.concatkdf.ConcatKDFHMAC(algorithm, length, salt, otherinfo, backend)¶ 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=32, ... salt=salt, ... otherinfo=otherinfo, ... backend=backend ... ) >>> key = ckdf.derive(b"input key") >>> ckdf = ConcatKDFHMAC( ... algorithm=hashes.SHA256(), ... length=32, ... salt=salt, ... otherinfo=otherinfo, ... backend=backend ... ) >>> ckdf.verify(b"input key", key)
- Parameters
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is
hashlen * (2^32 -1).salt (bytes) – 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
Noneis explicitly passed a default salt ofalgorithm.block_sizenull bytes will be used.otherinfo (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.backend – An instance of
HMACBackend.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHMACBackendTypeError – This exception is raised if
saltorotherinfois notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.x963kdf.X963KDF(algorithm, length, otherinfo, backend)¶ 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
ConcatKDFHashbe 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=32, ... sharedinfo=sharedinfo, ... backend=backend ... ) >>> key = xkdf.derive(b"input key") >>> xkdf = X963KDF( ... algorithm=hashes.SHA256(), ... length=32, ... sharedinfo=sharedinfo, ... backend=backend ... ) >>> xkdf.verify(b"input key", key)
- Parameters
algorithm – An instance of
HashAlgorithm.length (int) – The desired length of the derived key in bytes. Maximum is
hashlen * (2^32 -1).sharedinfo (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.backend – A cryptography backend
HashBackendinstance.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHashBackendTypeError – This exception is raised if
sharedinfois notbytes.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.kbkdf.KBKDFHMAC(algorithm, mode, length, rlen, llen, location, label, context, fixed, backend)¶ 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=32, ... 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=32, ... rlen=4, ... llen=4, ... location=CounterLocation.BeforeFixed, ... label=label, ... context=context, ... fixed=None, ... backend=backend ... ) >>> kdf.verify(b"input key", key)
- Parameters
algorithm – An instance of
HashAlgorithm.mode – The desired mode of the PRF. A value from the
Modeenum.length (int) – The desired length of the derived key in bytes.
rlen (int) – An integer that indicates the length of the binary representation of the counter in bytes.
llen (int) – An integer that indicates the binary representation of the
lengthin bytes.location – The desired location of the counter. A value from the
CounterLocationenum.label (bytes) – Application specific label information. If
Noneis explicitly passed an empty byte string will be used.context (bytes) – Application specific context information. If
Noneis explicitly passed an empty byte string will be used.fixed (bytes) – Instead of specifying
labelandcontextyou may supply your own fixed data. Iffixedis specified,labelandcontextis ignored.backend – A cryptography backend
HMACBackendinstance.
- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementHMACBackendTypeError – This exception is raised if
labelorcontextis notbytes. Also raised ifrlenorllenis notint.ValueError – This exception is raised if
rlenorllenis greater than 4 or less than 1. This exception is also raised if you specify alabelorcontextandfixed.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material.
- Return bytes
The derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.
Derives a new key from the input key material.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_key, and raises an exception if they do not match.
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class
cryptography.hazmat.primitives.kdf.kbkdf.Mode¶ An enumeration for the key based key derivative modes.
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CounterMode¶ The output of the PRF is computed with a counter as the iteration variable.
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class
cryptography.hazmat.primitives.kdf.kbkdf.CounterLocation¶ An enumeration for the key based key derivative counter location.
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BeforeFixed¶ The counter iteration variable will be concatenated before the fixed input data.
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AfterFixed¶ The counter iteration variable will be concatenated after the fixed input data.
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class
cryptography.hazmat.primitives.kdf.scrypt.Scrypt(salt, length, n, r, p, backend)¶ 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
KeyDerivationFunctioninterface.>>> 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=32, ... n=2**14, ... r=8, ... p=1, ... backend=backend ... ) >>> key = kdf.derive(b"my great password") >>> # verify >>> kdf = Scrypt( ... salt=salt, ... length=32, ... n=2**14, ... r=8, ... p=1, ... backend=backend ... ) >>> kdf.verify(b"my great password", key)
- Parameters
The computational and memory cost of Scrypt can be adjusted by manipulating the 3 parameters:
n,r, andp. In general, the memory cost of Scrypt is affected by the values of bothnandr, whilenalso determines the number of iterations performed.pincreases 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=8andp=1while scalingnto a number appropriate for your system. The scrypt paper suggests a minimum value ofn=2**14for interactive logins (t < 100ms), orn=2**20for more sensitive files (t < 5s).- Raises
cryptography.exceptions.UnsupportedAlgorithm – This is raised if the provided
backenddoes not implementScryptBackendTypeError – This exception is raised if
saltis notbytes.ValueError – This exception is raised if
nis less than 2, ifnis not a power of 2, ifris less than 1 or ifpis less than 1.
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derive(key_material)¶ - Parameters
key_material (bytes-like) – The input key material.
- Return bytes
the derived key.
- Raises
TypeError – This exception is raised if
key_materialis notbytes.cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This generates and returns a new key from the supplied password.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_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.
Interface¶
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class
cryptography.hazmat.primitives.kdf.KeyDerivationFunction¶ New in version 0.2.
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derive(key_material)¶ - 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.
- Returns
The new key.
- Raises
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This generates and returns a new key from the supplied key material.
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verify(key_material, expected_key)¶ - Parameters
- Raises
cryptography.exceptions.InvalidKey – This is raised when the derived key does not match the expected key.
cryptography.exceptions.AlreadyFinalized – This is raised when
derive()orverify()is called more than once.
This checks whether deriving a new key from the supplied
key_materialgenerates the same key as theexpected_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.