Time-based One-Time Password

The OTP method Authelia uses is the Time-Based One-Time Password Algorithm (TOTP) RFC6238 which is an extension of HMAC-Based One-Time Password Algorithm (HOTP) RFC4226.

You have the option to tune the settings of the TOTP generation, and you can see a full example of TOTP configuration below, as well as sections describing them.

Keep in mind the default settings are chosen for compatibility. Many applications do not support digits other than 6, and many only support SHA1.

Configuration#

totp:
  disable: false
  issuer: 'authelia.com'
  algorithm: 'sha1'
  digits: 6
  period: 30
  skew: 1
  secret_size: 32
  allowed_algorithms:
    - 'SHA1'
  allowed_digits:
    - 6
  allowed_periods:
    - 30
  disable_reuse_security_policy: false

Options#

This section describes the individual configuration options.

disable#

This disables One-Time Password (TOTP) if set to true.

issuer#

Applications generating Time-based One-Time Passwords usually display an issuer to differentiate applications registered by the user.

Authelia allows customisation of the issuer to differentiate the entry created by Authelia from others.

algorithm#

The algorithm used for the TOTP key.

Possible Values (case-insensitive):

• sha1
• sha256
• sha512

Changing this value only affects newly registered TOTP keys. See the Registration section for more information.

digits#

The number of digits a user needs to input to perform authentication. It’s generally not recommended for this to be altered as many TOTP applications do not support anything other than 6. What’s worse is some TOTP applications allow you to add the key, but do not use the correct number of digits specified by the key.

The valid values are 6 or 8.

Changing this value only affects newly registered TOTP keys. See the Registration section for more information.

period#

The period of time in seconds between key rotations or the time element of TOTP. Please see the input validation section for how this option and the skew option interact with each other.

It is recommended to keep this value set to 30, the minimum is 15.

Changing this value only affects newly registered TOTP keys. See the Registration section for more information.

skew#

The number of Time-based One-Time Passwords either side of the current valid Time-based One-Time Password that should also be considered valid. The default of 1 results in 3 Time-based One-Time Passwords valid. A setting of 2 would result in 5. With the default period of 30 this would result in 90 and 150 seconds of valid Time-based One-Time Passwords respectively. Please see the input validation section for how this option and the period option interact with each other.

Changing this value affects all TOTP validations, not just newly registered ones.

secret_size#

The length in bytes of generated shared secrets. The minimum is 20 (or 160 bits), and the default is 32 (or 256 bits). In most use cases 32 is sufficient. Though some authenticators may have issues with more than the minimum. Our minimum is the recommended value in RFC4226, though technically according to the specification 16 bytes (or 128 bits) is the minimum.

allowed_algorithms#

Similar to algorithm with the same restrictions except this option allows users to pick from this list. This list will always contain the value configured in the algorithm option.

allowed_digits#

Similar to digits with the same restrictions except this option allows users to pick from this list. This list will always contain the value configured in the digits option.

allowed_periods#

Similar to period with the same restrictions except this option allows users to pick from this list. This list will always contain the value configured in the period option.

disable_reuse_security_policy#

Disables the policy which prevents reuse of a Time-based One-Time Password codes. This is an additional security measure which prevents codes from being replayed. This should only affect codes which are used within the validity period more than once.

Registration#

When users register their TOTP device for the first time, the current issuer, algorithm, and period are used to generate the TOTP link and QR code. These values are saved to the database for future validations.

This means if the configuration options are changed, users will not need to regenerate their keys. This functionality takes effect from 4.33.0 onwards, previously the effect was the keys would just fail to validate. If you’d like to force users to register a new device, you can delete the old device for a particular user by using the authelia storage user totp delete <username> command regardless of if you change the settings or not.

Input Validation#

The period and skew configuration parameters affect each other. The default values are a period of 30 and a skew of 1. It is highly recommended you do not change these unless you wish to set skew to 0.

These options affect security by changing the length of time a Time-based One-Time Password is valid for. The formula to calculate the effective validity period is period + (period * skew * 2). For example period 30 and skew 1 would result in 90 seconds of validity, and period 30 and skew 2 would result in 150 seconds of validity.

System time accuracy#

It’s important to note that if the system time is not accurate enough then clients will seemingly not generate valid passwords for TOTP. Conversely this is the same when the client time is not accurate enough. This is due to the Time-based One-Time Passwords being time-based.

Authelia by default checks the system time against an NTP server on startup. This helps to prevent a time synchronization issue on the server being an issue. There is however no effective and reliable way to check the clients.

Encryption#

The TOTP secret is encrypted in the database in version 4.33.0 and above. This is so a user having access to only the database cannot easily compromise your two-factor authentication method.

This may be inconvenient for some users who wish to export TOTP keys from Authelia to other services. As such there is a command specifically for exporting TOTP configurations from the database. These commands require the configuration or at least a minimal configuration that has the storage backend connection details and the encryption key.

See the CLI Documentation for methods to perform exports.