Authelia uses time based one-time passwords as the OTP method. 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.
totp: issuer: authelia.com period: 30 skew: 1
Applications generating 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.
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.
The way you configure these affects security by changing the length of time a 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.
Configures the period of time in seconds a one-time password is current for. It is important to note that changing this value will require your users to register their application again.
It is recommended to keep this value set to 30, the minimum is 1.
Configures the number of one-time passwords either side of the current one that are considered valid, each time you increase this it makes two more one-time passwords valid. For example the default of 1 has a total of 3 keys valid. A value of 2 has 5 one-time passwords valid.
It is recommended to keep this value set to 0 or 1, the minimum is 0.
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.