View Source NimbleTOTP (NimbleTOTP v1.0.0)
NimbleTOTP is a tiny library for Two-factor authentication (2FA) that allows developers to implement Time-Based One-Time Passwords (TOTP) for their applications.
two-factor-authentication-2fa
Two-factor authentication (2FA)
The concept of 2FA is quite simple. It's an extra layer of security that demands a user to provide two pieces of evidence (factors) to the authentication system before access can be granted.
One way to implement 2FA is to generate a random secret for the user and whenever the system needs to perform a critical action it will ask the user to enter a validation code. This validation code is a Time-Based One-Time Password (TOTP) based on the user's secret and can be provided by an authentication app like Google Authenticator or Authy, which should be previously installed and configured on a compatible device, e.g. a smartphone.
Note: A critical action can mean different things depending on the application. For instance, while in a banking system the login itself is already considered a critical action, in other systems a user may be allowed to log in using just the password and only when trying to update critical data (e.g. its profile) 2FA will be required.
using-nimbletotp
Using NimbleTOTP
In order to allow developers to implement 2FA, NimbleTOTP provides functions to:
- Generate secrets composed of random bytes.
- Generate URIs to be encoded in a QR Code.
- Generate Time-Based One-Time Passwords based on a secret.
generating-the-secret
Generating the secret
The first step to set up 2FA for a user is to generate (and later persist) its random
secret. You can achieve that using NimbleTOTP.secret/1
.
Example:
secret = NimbleTOTP.secret()
#=> <<178, 117, 46, 7, 172, 202, 108, 127, 186, 180, ...>>
By default, a binary with 20 random bytes is generated per the HOTP RFC.
generating-uris-for-qr-code
Generating URIs for QR Code
Before persisting the secret, you need to make sure the user has already configured the authentication app in a compatible device. The most common way to do that is to generate a QR Code that can be read by the app.
You can use NimbleTOTP.otpauth_uri/3
along with
eqrcode to generate the QR
code as SVG.
Example:
uri = NimbleTOTP.otpauth_uri("Acme:alice", secret, issuer: "Acme")
#=> "otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
uri |> EQRCode.encode() |> EQRCode.svg()
#=> "<?xml version=\\"1.0\\" standalone=\\"yes\\"?>\\n<svg version=\\"1.1\\" ...
generating-a-time-based-one-time-password
Generating a Time-Based One-Time Password
After successfully reading the QR Code, the app will start generating a
different 6 digit code every 30s
. You can compute the verification code
with:
NimbleTOTP.verification_code(secret)
#=> "569777"
The code can be validated using the valid?/3
function. Example:
NimbleTOTP.valid?(secret, "569777")
#=> true
NimbleTOTP.valid?(secret, "012345")
#=> false
After validating the code, you can finally persist the user's secret so you use it later whenever you need to authorize any critical action using 2FA.
preventing-codes-from-being-reused
Preventing codes from being reused
The TOTP RFC requires that a code can only be used once. This is a security feature that prevents codes from being reused. To ensure codes are only considered valid if they have not been used, you need to keep track of the last time the user entered a TOTP code.
NimbleTOTP.valid?(user.totp_secret, code, since: user.last_totp_at)
Assuming the code itself is valid for the given secret, if since
is nil
,
the code will be considered valid. If since is given, it will not allow
codes in the same time period (30 seconds by default) to be reused. The user
will have to wait for the next code to be generated.
Link to this section Summary
Types
Options for verification_code/2
and valid?/3
.
Unix time in seconds, t:DateTime.t()
or t:NaiveDateTime.t()
.
Options for valid?/3
.
Functions
Generate the uri to be encoded in the QR code.
Generate a binary composed of random bytes.
Checks if the given otp
code matches the secret.
Generate Time-Based One-Time Password.
Link to this section Types
@type option() :: {:time, time()} | {:period, pos_integer()}
Options for verification_code/2
and valid?/3
.
@type time() :: DateTime.t() | NaiveDateTime.t() | integer()
Unix time in seconds, t:DateTime.t()
or t:NaiveDateTime.t()
.
@type validate_option() :: {:since, time() | nil}
Options for valid?/3
.
Link to this section Functions
Generate the uri to be encoded in the QR code.
examples
Examples
iex> NimbleTOTP.otpauth_uri("Acme:alice", "abcd", issuer: "Acme")
"otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
@spec secret(non_neg_integer()) :: binary()
Generate a binary composed of random bytes.
The number of bytes is defined by the size
argument. Default is 20
per the
HOTP RFC.
examples
Examples
NimbleTOTP.secret()
#=> <<178, 117, 46, 7, 172, 202, 108, 127, 186, 180, ...>>
@spec valid?(binary(), String.t(), [option() | validate_option()]) :: boolean()
Checks if the given otp
code matches the secret.
options
Options
- :time - The time (either
%NaiveDateTime{}
,%DateTime{}
or unix format) to be used. Default isSystem.os_time(:second)
- :since - The last time the secret was used, see "Preventing TOTP code reuse" next
- :period - The period (in seconds) in which the code is valid. Default is
30
.
preventing-totp-code-reuse
Preventing TOTP code reuse
The :since
option can be used to prevent TOTP codes from being reused. When set
to the time when the last code was entered, the code generated from within that
period is no longer considered valid. Periods are counted from the Unix epoch.
This means a user may have to wait, in the worst case scenario, for the duration
of :period
before they can enter a valid code again. This implementation meets the
TOTP RFC requirements.
grace-period
Grace period
In some cases it is preferable to allow the user more time to validate the code than
the initial period (mostly 30 seconds), the so-called grace period. Although this library
does not support this out of the box, you can achieve the same functionality by using
the :time
option.
def valid_code?(secret, otp) do
time = System.os_time(:second)
NimbleTOTP.valid?(secret, otp, time: time) or NimbleTOTP.valid?(secret, otp, time: time - 30)
end
In this example by validating first against the current time, but also against 30 seconds ago, we allow the previous code, to be still valid.
Generate Time-Based One-Time Password.
options
Options
- :time - The time (either
%NaiveDateTime{}
,%DateTime{}
or unix format) to be used. Default isSystem.os_time(:second)
- :period - The period (in seconds) in which the code is valid. Default is
30
.
examples
Examples
NimbleTOTP.verification_code(secret)
#=> "569777"