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.
Two-factor authentication
This module contains algorithms related to two-factor authentication.
Currently, it contains an algorithm for generating and verifying one time password values based on Hash-based message authentication codes (HMAC).
- class cryptography.hazmat.primitives.twofactor.InvalidToken[source]
This is raised when the verify method of a one time password function’s computed token does not match the expected token.
- cryptography.hazmat.primitives.twofactor.hotp.HOTPHashTypes
Added in version 40.0.0.
Type alias: A union of supported hash algorithm types:
SHA1
,SHA256
orSHA512
.
- class cryptography.hazmat.primitives.twofactor.hotp.HOTP(key, length, algorithm, *, enforce_key_length=True)[source]
Added in version 0.3.
HOTP objects take a
key
,length
andalgorithm
parameter. Thekey
should be randomly generated bytes and is recommended to be 160 bits in length. Thelength
parameter controls the length of the generated one time password and must be >= 6 and <= 8.This is an implementation of RFC 4226.
>>> import os >>> from cryptography.hazmat.primitives.twofactor.hotp import HOTP >>> from cryptography.hazmat.primitives.hashes import SHA1 >>> key = os.urandom(20) >>> hotp = HOTP(key, 6, SHA1()) >>> hotp_value = hotp.generate(0) >>> hotp.verify(hotp_value, 0)
- Parameters:
key (bytes-like) – Per-user secret key. This value must be kept secret and be at least 128 bits. It is recommended that the key be 160 bits.
length (int) – Length of generated one time password as
int
.algorithm (cryptography.hazmat.primitives.hashes.HashAlgorithm) – A
hashes
instance (must matchHOTPHashTypes
).enforce_key_length –
A boolean flag defaulting to True that toggles whether a minimum key length of 128 bits is enforced. This exists to work around the fact that as documented in Issue #2915, the Google Authenticator PAM module by default generates 80 bit keys. If this flag is set to False, the application developer should implement additional checks of the key length before passing it into
HOTP
.Added in version 1.5.
- Raises:
- generate(counter)[source]
- Parameters:
counter (int) – The counter value used to generate the one time password.
- Return bytes:
A one time password value.
- verify(hotp, counter)[source]
- Parameters:
- Raises:
cryptography.hazmat.primitives.twofactor.InvalidToken – This is raised when the supplied HOTP does not match the expected HOTP.
Throttling
Due to the fact that the HOTP algorithm generates rather short tokens that are 6 - 8 digits long, brute force attacks are possible. It is highly recommended that the server that validates the token implement a throttling scheme that locks out the account for a period of time after a number of failed attempts. The number of allowed attempts should be as low as possible while still ensuring that usability is not significantly impacted.
Re-synchronization of the counter
The server’s counter value should only be incremented on a successful HOTP authentication. However, the counter on the client is incremented every time a new HOTP value is requested. This can lead to the counter value being out of synchronization between the client and server.
Due to this, it is highly recommended that the server sets a look-ahead window
that allows the server to calculate the next x
HOTP values and check them
against the supplied HOTP value. This can be accomplished with something
similar to the following code.
def verify(hotp, counter, look_ahead):
assert look_ahead >= 0
correct_counter = None
otp = HOTP(key, 6)
for count in range(counter, counter + look_ahead):
try:
otp.verify(hotp, count)
correct_counter = count
except InvalidToken:
pass
return correct_counter
- class cryptography.hazmat.primitives.twofactor.totp.TOTP(key, length, algorithm, time_step, *, enforce_key_length=True)[source]
TOTP objects take a
key
,length
,algorithm
andtime_step
parameter. Thekey
should be randomly generated bytes and is recommended to be as long as your hash function’s output (e.g 256-bit for SHA256). Thelength
parameter controls the length of the generated one time password and must be >= 6 and <= 8.This is an implementation of RFC 6238.
>>> import os >>> import time >>> from cryptography.hazmat.primitives.twofactor.totp import TOTP >>> from cryptography.hazmat.primitives.hashes import SHA1 >>> key = os.urandom(20) >>> totp = TOTP(key, 8, SHA1(), 30) >>> time_value = time.time() >>> totp_value = totp.generate(time_value) >>> totp.verify(totp_value, time_value)
- Parameters:
key (bytes-like) – Per-user secret key. This value must be kept secret and be at least 128 bits. It is recommended that the key be 160 bits.
length (int) – Length of generated one time password as
int
.algorithm (cryptography.hazmat.primitives.hashes.HashAlgorithm) – A
hashes
instance.time_step (int) – The time step size. The recommended size is 30.
enforce_key_length –
A boolean flag defaulting to True that toggles whether a minimum key length of 128 bits is enforced. This exists to work around the fact that as documented in Issue #2915, the Google Authenticator PAM module by default generates 80 bit keys. If this flag is set to False, the application develop should implement additional checks of the key length before passing it into
TOTP
.Added in version 1.5.
- Raises:
- generate(time)[source]
- Parameters:
time (int) – The time value used to generate the one time password.
- Return bytes:
A one time password value.
- verify(totp, time)[source]
- Parameters:
- Raises:
cryptography.hazmat.primitives.twofactor.InvalidToken – This is raised when the supplied TOTP does not match the expected TOTP.
- get_provisioning_uri(account_name, issuer)[source]
Added in version 1.0.
- Parameters:
account_name (str) – The display name of account, such as
'Alice Smith'
or'alice@example.com'
.issuer (
str
orNone
) – The optional display name of issuer. This is typically the provider or service the user wants to access using the OTP token.
- Returns:
A URI string.
Provisioning URI
The provisioning URI of HOTP and TOTP is a feature of Google Authenticator and not actually part of the HOTP or TOTP RFCs. However, it is widely supported by web sites and mobile applications which are using Two-Factor authentication.
For generating a provisioning URI you can use the get_provisioning_uri
method of HOTP/TOTP instances.
counter = 5
account_name = 'alice@example.com'
issuer_name = 'Example Inc'
hotp_uri = hotp.get_provisioning_uri(account_name, counter, issuer_name)
totp_uri = totp.get_provisioning_uri(account_name, issuer_name)
A common usage is encoding the provisioning URI into QR code and guiding users to scan it with Two-Factor authentication applications in their mobile devices.