/fernet
cryptography.hazmat.primitives.ciphers
Symmetric encryption is a way to encrypt or hide the contents of material where the sender and receiver both use the same secret key. Note that symmetric encryption is not sufficient for most applications because it only provides secrecy but not authenticity. That means an attacker can't see the message but an attacker can create bogus messages and force the application to decrypt them. In many contexts, a lack of authentication on encrypted messages can result in a loss of secrecy as well.
For this reason in nearly all contexts it is necessary to combine encryption with a message authentication code, such as HMAC </hazmat/primitives/mac/hmac>
, in an "encrypt-then-MAC" formulation as described by Colin Percival. cryptography
includes a recipe named /fernet
that does this for you. To minimize the risk of security issues you should evaluate Fernet to see if it fits your needs before implementing anything using this module. If /fernet
is not appropriate for your use-case then you may still benefit from /hazmat/primitives/aead
which combines encryption and authentication securely.
Cipher objects combine an algorithm such as ~cryptography.hazmat.primitives.ciphers.algorithms.AES
with a mode like ~cryptography.hazmat.primitives.ciphers.modes.CBC
or ~cryptography.hazmat.primitives.ciphers.modes.CTR
. A simple example of encrypting and then decrypting content with AES is:
>>> import os >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> key = os.urandom(32) >>> iv = os.urandom(16) >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv)) >>> encryptor = cipher.encryptor() >>> ct = encryptor.update(b"a secret message") + encryptor.finalize() >>> decryptor = cipher.decryptor() >>> decryptor.update(ct) + decryptor.finalize() b'a secret message'
- param algorithm
A
~cryptography.hazmat.primitives.ciphers.CipherAlgorithm
instance such as those describedbelow <symmetric-encryption-algorithms>
.- param mode
A
~cryptography.hazmat.primitives.ciphers.modes.Mode
instance such as those describedbelow <symmetric-encryption-modes>
.- raises cryptography.exceptions.UnsupportedAlgorithm
This is raised if the provided
algorithm
is unsupported.
encryptor()
- return
An encrypting
~cryptography.hazmat.primitives.ciphers.CipherContext
instance.
If the requested combination of algorithm
and mode
is unsupported an ~cryptography.exceptions.UnsupportedAlgorithm
exception will be raised.
decryptor()
- return
A decrypting
~cryptography.hazmat.primitives.ciphers.CipherContext
instance.
If the requested combination of algorithm
and mode
is unsupported an ~cryptography.exceptions.UnsupportedAlgorithm
exception will be raised.
cryptography.hazmat.primitives.ciphers.algorithms
AES (Advanced Encryption Standard) is a block cipher standardized by NIST. AES is both fast, and cryptographically strong. It is a good default choice for encryption.
- param key
The secret key. This must be kept secret. Either
128
,192
, or256
bits
long.- type key
bytes-like
38.0.0
An AES class that only accepts 128 bit keys. This is identical to the standard AES
class except that it will only accept a single key length.
- param key
The secret key. This must be kept secret.
128
bits
long.- type key
bytes-like
38.0.0
An AES class that only accepts 256 bit keys. This is identical to the standard AES
class except that it will only accept a single key length.
- param key
The secret key. This must be kept secret.
256
bits
long.- type key
bytes-like
Camellia is a block cipher approved for use by CRYPTREC and ISO/IEC. It is considered to have comparable security and performance to AES but is not as widely studied or deployed.
- param key
The secret key. This must be kept secret. Either
128
,192
, or256
bits
long.- type key
bytes-like
2.1
Note
In most cases users should use ~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305
instead of this class. ChaCha20 alone does not provide integrity so it must be combined with a MAC to be secure. ~cryptography.hazmat.primitives.ciphers.aead.ChaCha20Poly1305
does this for you.
ChaCha20 is a stream cipher used in several IETF protocols. It is standardized in 7539
.
- param key
The secret key. This must be kept secret.
256
bits
(32 bytes) in length.- type key
bytes-like
- param nonce
Should be unique, a
nonce
. It is critical to never reuse anonce
with a given key. Any reuse of a nonce with the same key compromises the security of every message encrypted with that key. The nonce does not need to be kept secret and may be included with the ciphertext. This must be128
bits
in length. The 128-bit value is a concatenation of 4-byte little-endian counter and the 12-byte nonce (as described in7539
).- type nonce
bytes-like
Note
In
7539
the nonce is defined as a 96-bit value that is later concatenated with a block counter (encoded as a 32-bit little-endian). If you have a separate nonce and block counter you will need to concatenate it yourself before passing it. For example, if you have an initial block counter of 2 and a 96-bit nonce the concatenated nonce would bestruct.pack("<i", 2) + nonce
.
>>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> nonce = os.urandom(16) >>> algorithm = algorithms.ChaCha20(key, nonce) >>> cipher = Cipher(algorithm, mode=None) >>> encryptor = cipher.encryptor() >>> ct = encryptor.update(b"a secret message") >>> decryptor = cipher.decryptor() >>> decryptor.update(ct) b'a secret message'
Triple DES (Data Encryption Standard), sometimes referred to as 3DES, is a block cipher standardized by NIST. Triple DES has known crypto-analytic flaws, however none of them currently enable a practical attack. Nonetheless, Triple DES is not recommended for new applications because it is incredibly slow; old applications should consider moving away from it.
- param key
The secret key. This must be kept secret. Either
64
,128
, or192
bits
long. DES only uses56
,112
, or168
bits of the key as there is a parity byte in each component of the key. Some writing refers to there being up to three separate keys that are each56
bits long, they can simply be concatenated to produce the full key.- type key
bytes-like
0.2
CAST5 (also known as CAST-128) is a block cipher approved for use in the Canadian government by the Communications Security Establishment. It is a variable key length cipher and supports keys from 40-128 bits
in length.
- param key
The secret key, This must be kept secret. 40 to 128
bits
in length in increments of 8 bits.- type key
bytes-like
0.4
SEED is a block cipher developed by the Korea Information Security Agency (KISA). It is defined in 4269
and is used broadly throughout South Korean industry, but rarely found elsewhere.
- param key
The secret key. This must be kept secret.
128
bits
in length.- type key
bytes-like
35.0.0
SM4 is a block cipher developed by the Chinese Government and standardized in the GB/T 32907-2016. It is used in the Chinese WAPI (Wired Authentication and Privacy Infrastructure) standard. (An English description is available at draft-ribose-cfrg-sm4-10.) This block cipher should be used for compatibility purposes where required and is not otherwise recommended for use.
- param key
The secret key. This must be kept secret.
128
bits
in length.- type key
bytes-like
Warning
These ciphers are considered weak for a variety of reasons. New applications should avoid their use and existing applications should strongly consider migrating away.
Blowfish is a block cipher developed by Bruce Schneier. It is known to be susceptible to attacks when using weak keys. The author has recommended that users of Blowfish move to newer algorithms such as AES
.
- param key
The secret key. This must be kept secret. 32 to 448
bits
in length in increments of 8 bits.- type key
bytes-like
ARC4 (Alleged RC4) is a stream cipher with serious weaknesses in its initial stream output. Its use is strongly discouraged. ARC4 does not use mode constructions.
- param key
The secret key. This must be kept secret. Either
40
,56
,64
,80
,128
,192
, or256
bits
in length.- type key
bytes-like
>>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> algorithm = algorithms.ARC4(key) >>> cipher = Cipher(algorithm, mode=None) >>> encryptor = cipher.encryptor() >>> ct = encryptor.update(b"a secret message") >>> decryptor = cipher.decryptor() >>> decryptor.update(ct) b'a secret message'
IDEA (International Data Encryption Algorithm) is a block cipher created in 1991. It is an optional component of the OpenPGP standard. This cipher is susceptible to attacks when using weak keys. It is recommended that you do not use this cipher for new applications.
- param key
The secret key. This must be kept secret.
128
bits
in length.- type key
bytes-like
cryptography.hazmat.primitives.ciphers.modes
CBC (Cipher Block Chaining) is a mode of operation for block ciphers. It is considered cryptographically strong.
Padding is required when using this mode.
- param initialization_vector
Must be
random bytes </random-numbers>
. They do not need to be kept secret and they can be included in a transmitted message. Must be the same number of bytes as theblock_size
of the cipher. Each time something is encrypted a newinitialization_vector
should be generated. Do not reuse aninitialization_vector
with a givenkey
, and particularly do not use a constantinitialization_vector
.- type initialization_vector
bytes-like
A good construction looks like:
>>> import os >>> from cryptography.hazmat.primitives.ciphers.modes import CBC >>> iv = os.urandom(16) >>> mode = CBC(iv)
While the following is bad and will leak information:
>>> from cryptography.hazmat.primitives.ciphers.modes import CBC >>> iv = b"a" * 16 >>> mode = CBC(iv)
Warning
Counter mode is not recommended for use with block ciphers that have a block size of less than 128-bits
.
CTR (Counter) is a mode of operation for block ciphers. It is considered cryptographically strong. It transforms a block cipher into a stream cipher.
This mode does not require padding.
- param nonce
Should be unique, a
nonce
. It is critical to never reuse anonce
with a given key. Any reuse of a nonce with the same key compromises the security of every message encrypted with that key. Must be the same number of bytes as theblock_size
of the cipher with a given key. The nonce does not need to be kept secret and may be included with the ciphertext.- type nonce
bytes-like
OFB (Output Feedback) is a mode of operation for block ciphers. It transforms a block cipher into a stream cipher.
This mode does not require padding.
- param initialization_vector
Must be
random bytes </random-numbers>
. They do not need to be kept secret and they can be included in a transmitted message. Must be the same number of bytes as theblock_size
of the cipher. Do not reuse aninitialization_vector
with a givenkey
.- type initialization_vector
bytes-like
CFB (Cipher Feedback) is a mode of operation for block ciphers. It transforms a block cipher into a stream cipher.
This mode does not require padding.
- param initialization_vector
Must be
random bytes </random-numbers>
. They do not need to be kept secret and they can be included in a transmitted message. Must be the same number of bytes as theblock_size
of the cipher. Do not reuse aninitialization_vector
with a givenkey
.- type initialization_vector
bytes-like
CFB (Cipher Feedback) is a mode of operation for block ciphers. It transforms a block cipher into a stream cipher. The CFB8 variant uses an 8-bit shift register.
This mode does not require padding.
- param initialization_vector
Must be
random bytes </random-numbers>
. They do not need to be kept secret and they can be included in a transmitted message. Must be the same number of bytes as theblock_size
of the cipher. Do not reuse aninitialization_vector
with a givenkey
.- type initialization_vector
bytes-like
Danger
If you are encrypting data that can fit into memory you should strongly consider using ~cryptography.hazmat.primitives.ciphers.aead.AESGCM
instead of this.
When using this mode you must not use the decrypted data until the appropriate finalization method (~cryptography.hazmat.primitives.ciphers.CipherContext.finalize
or ~cryptography.hazmat.primitives.ciphers.AEADDecryptionContext.finalize_with_tag
) has been called. GCM provides no guarantees of ciphertext integrity until decryption is complete.
GCM (Galois Counter Mode) is a mode of operation for block ciphers. An AEAD (authenticated encryption with additional data) mode is a type of block cipher mode that simultaneously encrypts the message as well as authenticating it. Additional unencrypted data may also be authenticated. Additional means of verifying integrity such as HMAC </hazmat/primitives/mac/hmac>
are not necessary.
This mode does not require padding.
- param initialization_vector
Must be unique, a
nonce
. They do not need to be kept secret and they can be included in a transmitted message. NIST recommends a 96-bit IV length for performance critical situations but it can be up to 264 - 1bits
. Do not reuse aninitialization_vector
with a givenkey
.- type initialization_vector
bytes-like
Note
Cryptography will generate a 128-bit tag when finalizing encryption. You can shorten a tag by truncating it to the desired length but this is not recommended as it makes it easier to forge messages, and also potentially leaks the key (NIST SP-800-38D recommends 96-bits
or greater). Applications wishing to allow truncation can pass the min_tag_length
parameter.
0.5
The min_tag_length
parameter was added in 0.5
, previously truncation down to 4
bytes was always allowed.
- param bytes tag
The tag bytes to verify during decryption. When encrypting this must be
None
. When decrypting, it may beNone
if the tag is supplied on finalization using~cryptography.hazmat.primitives.ciphers.AEADDecryptionContext.finalize_with_tag
. Otherwise, the tag is mandatory.- param int min_tag_length
The minimum length
tag
must be. By default this is16
, meaning tag truncation is not allowed. Allowing tag truncation is strongly discouraged for most applications.- raises ValueError
This is raised if
len(tag) < min_tag_length
or theinitialization_vector
is too short.
An example of securely encrypting and decrypting data with AES
in the GCM
mode looks like:
import os
- from cryptography.hazmat.primitives.ciphers import (
Cipher, algorithms, modes
)
- def encrypt(key, plaintext, associated_data):
# Generate a random 96-bit IV. iv = os.urandom(12)
# Construct an AES-GCM Cipher object with the given key and a # randomly generated IV. encryptor = Cipher( algorithms.AES(key), modes.GCM(iv), ).encryptor()
# associated_data will be authenticated but not encrypted, # it must also be passed in on decryption. encryptor.authenticate_additional_data(associated_data)
# Encrypt the plaintext and get the associated ciphertext. # GCM does not require padding. ciphertext = encryptor.update(plaintext) + encryptor.finalize()
return (iv, ciphertext, encryptor.tag)
- def decrypt(key, associated_data, iv, ciphertext, tag):
# Construct a Cipher object, with the key, iv, and additionally the # GCM tag used for authenticating the message. decryptor = Cipher( algorithms.AES(key), modes.GCM(iv, tag), ).decryptor()
# We put associated_data back in or the tag will fail to verify # when we finalize the decryptor. decryptor.authenticate_additional_data(associated_data)
# Decryption gets us the authenticated plaintext. # If the tag does not match an InvalidTag exception will be raised. return decryptor.update(ciphertext) + decryptor.finalize()
- iv, ciphertext, tag = encrypt(
key, b"a secret message!", b"authenticated but not encrypted payload"
)
- print(decrypt(
key, b"authenticated but not encrypted payload", iv, ciphertext, tag
))
b'a secret message!'
2.1
Warning
XTS mode is meant for disk encryption and should not be used in other contexts. cryptography
only supports XTS mode with ~cryptography.hazmat.primitives.ciphers.algorithms.AES
.
Note
AES XTS keys are double length. This means that to do AES-128 encryption in XTS mode you need a 256-bit key. Similarly, AES-256 requires passing a 512-bit key. AES 192 is not supported in XTS mode.
XTS (XEX-based tweaked-codebook mode with ciphertext stealing) is a mode of operation for the AES block cipher that is used for disk encryption.
This mode does not require padding.
- param tweak
The tweak is a 16 byte value typically derived from something like the disk sector number. A given
(tweak, key)
pair should not be reused, although doing so is less catastrophic than in CTR mode.- type tweak
bytes-like
Warning
These modes are insecure. New applications should never make use of them, and existing applications should strongly consider migrating away.
ECB (Electronic Code Book) is the simplest mode of operation for block ciphers. Each block of data is encrypted in the same way. This means identical plaintext blocks will always result in identical ciphertext blocks, which can leave significant patterns in the output.
Padding is required when using this mode.
cryptography.hazmat.primitives.ciphers
When calling encryptor()
or decryptor()
on a Cipher
object the result will conform to the CipherContext
interface. You can then call update(data)
with data until you have fed everything into the context. Once that is done call finalize()
to finish the operation and obtain the remainder of the data.
Block ciphers require that the plaintext or ciphertext always be a multiple of their block size. Because of that padding is sometimes required to make a message the correct size. CipherContext
will not automatically apply any padding; you'll need to add your own. For block ciphers the recommended padding is ~cryptography.hazmat.primitives.padding.PKCS7
. If you are using a stream cipher mode (such as ~cryptography.hazmat.primitives.ciphers.modes.CTR
) you don't have to worry about this.
update(data)
- param data
The data you wish to pass into the context.
- type data
bytes-like
- return bytes
Returns the data that was encrypted or decrypted.
- raises cryptography.exceptions.AlreadyFinalized
See
finalize
When the Cipher
was constructed in a mode that turns it into a stream cipher (e.g. ~cryptography.hazmat.primitives.ciphers.modes.CTR
), this will return bytes immediately, however in other modes it will return chunks whose size is determined by the cipher's block size.
update_into(data, buf)
1.8
Warning
This method allows you to avoid a memory copy by passing a writable buffer and reading the resulting data. You are responsible for correctly sizing the buffer and properly handling the data. This method should only be used when extremely high performance is a requirement and you will be making many small calls to update_into
.
- param data
The data you wish to pass into the context.
- type data
bytes-like
- param buf
A writable Python buffer that the data will be written into. This buffer should be
len(data) + n - 1
bytes wheren
is the block size (in bytes) of the cipher and mode combination.- return int
Number of bytes written.
- raises ValueError
This is raised if the supplied buffer is too small.
>>> import os >>> from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes >>> key = os.urandom(32) >>> iv = os.urandom(16) >>> cipher = Cipher(algorithms.AES(key), modes.CBC(iv)) >>> encryptor = cipher.encryptor() >>> # the buffer needs to be at least len(data) + n - 1 where n is cipher/mode block size in bytes >>> buf = bytearray(31) >>> len_encrypted = encryptor.update_into(b"a secret message", buf) >>> # get the ciphertext from the buffer reading only the bytes written to it (len_encrypted) >>> ct = bytes(buf[:len_encrypted]) + encryptor.finalize() >>> decryptor = cipher.decryptor() >>> len_decrypted = decryptor.update_into(ct, buf) >>> # get the plaintext from the buffer reading only the bytes written (len_decrypted) >>> bytes(buf[:len_decrypted]) + decryptor.finalize() b'a secret message'
finalize()
- return bytes
Returns the remainder of the data.
- raises ValueError
This is raised when the data provided isn't a multiple of the algorithm's block size.
Once finalize
is called this object can no longer be used and update
and finalize
will raise an ~cryptography.exceptions.AlreadyFinalized
exception.
When calling encryptor
or decryptor
on a Cipher
object with an AEAD mode (e.g. ~cryptography.hazmat.primitives.ciphers.modes.GCM
) the result will conform to the AEADCipherContext
and CipherContext
interfaces. If it is an encryption or decryption context it will additionally be an AEADEncryptionContext
or AEADDecryptionContext
instance, respectively. AEADCipherContext
contains an additional method authenticate_additional_data
for adding additional authenticated but unencrypted data (see note below). You should call this before calls to update
. When you are done call finalize
to finish the operation.
Note
In AEAD modes all data passed to update()
will be both encrypted and authenticated. Do not pass encrypted data to the authenticate_additional_data()
method. It is meant solely for additional data you may want to authenticate but leave unencrypted.
authenticate_additional_data(data)
- param data
Any data you wish to authenticate but not encrypt.
- type data
bytes-like
- raises
~cryptography.exceptions.AlreadyFinalized
When creating an encryption context using encryptor
on a Cipher
object with an AEAD mode such as ~cryptography.hazmat.primitives.ciphers.modes.GCM
an object conforming to both the AEADEncryptionContext
and AEADCipherContext
interfaces will be returned. This interface provides one additional attribute tag
. tag
can only be obtained after finalize
has been called.
tag
- return bytes
Returns the tag value as bytes.
- raises
~cryptography.exceptions.NotYetFinalized
if called before the context is finalized.
1.9
When creating an encryption context using decryptor
on a Cipher
object with an AEAD mode such as ~cryptography.hazmat.primitives.ciphers.modes.GCM
an object conforming to both the AEADDecryptionContext
and AEADCipherContext
interfaces will be returned. This interface provides one additional method finalize_with_tag
that allows passing the authentication tag for validation after the ciphertext has been decrypted.
finalize_with_tag(tag)
- param bytes tag
The tag bytes to verify after decryption.
- return bytes
Returns the remainder of the data.
- raises ValueError
This is raised when the data provided isn't a multiple of the algorithm's block size, if
min_tag_length
is less than 4, or iflen(tag) < min_tag_length
.min_tag_length
is an argument to theGCM
constructor.
If the authentication tag was not already supplied to the constructor of the ~cryptography.hazmat.primitives.ciphers.modes.GCM
mode object, this method must be used instead of ~cryptography.hazmat.primitives.ciphers.CipherContext.finalize
.
A named symmetric encryption algorithm.
name
- type
str
The standard name for the mode, for example, "AES", "Camellia", or "Blowfish".
key_size
- type
int
The number of bits
in the key being used.
A block cipher algorithm.
block_size
- type
int
The number of bits
in a block.
Interfaces used by the symmetric cipher modes described in Symmetric Encryption Modes <symmetric-encryption-modes>
.
cryptography.hazmat.primitives.ciphers.modes
A named cipher mode.
name
- type
str
This should be the standard shorthand name for the mode, for example Cipher-Block Chaining mode is "CBC".
validate_for_algorithm(algorithm)
- param cryptography.hazmat.primitives.ciphers.CipherAlgorithm algorithm
Checks that the combination of this mode with the provided algorithm meets any necessary invariants. This should raise an exception if they are not met.
For example, the ~cryptography.hazmat.primitives.ciphers.modes.CBC
mode uses this method to check that the provided initialization vector's length matches the block size of the algorithm.
A cipher mode with an initialization vector.
initialization_vector
- type
bytes-like
Exact requirements of the initialization are described by the documentation of individual modes.
A cipher mode with a nonce.
nonce
- type
bytes-like
Exact requirements of the nonce are described by the documentation of individual modes.
A cipher mode with an authentication tag.
tag
- type
bytes-like
Exact requirements of the tag are described by the documentation of individual modes.
2.1
A cipher mode with a tweak.
tweak
- type
bytes-like
Exact requirements of the tweak are described by the documentation of individual modes.
cryptography.exceptions
This is raised if an authenticated encryption tag fails to verify during decryption.