Learn how to optimize your SMS OTP form and improve user experience.
Asking a user to provide the OTP (one time password) delivered via SMS is a common way to confirm a user's phone number. There are a few use cases for SMS OTP:
- Two-factor authentication. In addition to username and password, SMS OTP can be used as a strong signal that the account is owned by the person who received the SMS OTP.
- Phone number verification. Some services use a phone number as the user's primary identifier. In such services, users can enter their phone number and the OTP received via SMS to prove their identity. Sometimes it's combined with a PIN to constitute a two-factor authentication.
- Account recovery. When a user loses access to their account, there needs to be a way to recover it. Sending an email to their registered email address or an SMS OTP to their phone number are common account recovery methods.
- Payment confirmation In payment systems, some banks or credit card issuers request additional authentication from the payer for security reasons. SMS OTP is commonly used for that purpose.
This post explains best practices to build an SMS OTP form for the above use cases.
Checklist
To provide the best user experience with the SMS OTP, follow these steps:
- Use the
<input>
element with:type="text"
inputmode="numeric"
autocomplete="one-time-code"
- Use
@BOUND_DOMAIN #OTP_CODE
as the last line of the OTP SMS message. - Use the WebOTP API.
Use the <input>
element
Using a form with an <input>
element is the most important best practice you
can follow because it works in all browsers. Even if other suggestions from
this post don't work in some browser, the user will still be able to enter and submit the OTP
manually.
<form action="/verify-otp" method="POST">
<input type="text"
inputmode="numeric"
autocomplete="one-time-code"
pattern="\d{6}"
required>
</form>
The following are a few ideas to ensure an input field gets the best out of browser functionality.
type="text"
Since OTPs are usually five or six digit numbers, using
type="number"
for an input field might seem intuitive because it changes the mobile
keyboard to numbers only. This is not recommended because the browser expects an
input field to be a countable number rather than a sequence of multiple numbers,
which can cause unexpected behavior. Using type="number"
causes up and down
buttons to be displayed beside the input field; pressing these buttons
increments or decrements the number and may remove preceding zeros.
Use type="text"
instead. This won't turn the mobile keyboard into numbers
only, but that is fine because the next tip for using inputmode="numeric"
does
that job.
inputmode="numeric"
Use inputmode="numeric"
to change the mobile keyboard to numbers only.
Some websites use type="tel"
for OTP input fields since it also
turns the mobile keyboard to numbers only (including *
and #
) when
focused. This hack was used in the past when inputmode="numeric"
wasn't widely supported. Since Firefox started supporting
inputmode="numeric"
,
there's no need to use the semantically incorrect type="tel"
hack.
autocomplete="one-time-code"
autocomplete
attribute lets developers specify what permission the browser
has to provide autocomplete assistance and informs the browser about the
type of information expected in the field.
With autocomplete="one-time-code"
whenever a user receives an SMS message while a
form is open, the operating system will parse the OTP in the SMS heuristically and
the keyboard will suggest the OTP for the user to enter. It works only on Safari 12 and
later on iOS, iPadOS, and macOS, but we strongly recommend using it, because it is an
easy way to improve the SMS OTP experience on those platforms.
autocomplete="one-time-code"
improves the user experience, but there's more you
can do by ensuring that the SMS message complies with the origin-bound message
format.
Format the SMS text
Enhance the user experience of entering an OTP by aligning with the origin-bound one-time codes delivered via SMS specification.
The format rule is simple: Finish the SMS message with the receiver domain
preceded with @
and the OTP preceded with #
.
For example:
Your OTP is 123456
@web-otp.glitch.me #123456
Using a standard format for OTP messages makes extraction of codes from them easier and more reliable. Associating OTP codes with websites makes it harder to trick users into providing a code to malicious sites.
Using this format provides a couple of benefits:
- The OTP will be bound to the domain. If the user is on domains other than the one specified in the SMS message, the OTP suggestion won't appear. This also mitigates the risk of phishing attacks and potential account hijacks.
- Browser will now be able to reliably extract the OTP without depending on mysterious and flaky heuristics.
When a website uses autocomplete="one-time-code"
, Safari with iOS 14 or later
will suggest the OTP following the above rules.
This SMS message format also benefits browsers other than Safari. Chrome, Opera,
and Vivaldi on Android also support the origin-bound one-time codes rule with
the WebOTP API, though not through autocomplete="one-time-code"
.
Use the WebOTP API
The WebOTP API provides access to the OTP
received in an SMS message. By calling
navigator.credentials.get()
with otp
type (OTPCredential
) where transport
includes sms
, the website
will wait for an SMS that complies with the origin-bound one-time codes to be
delivered and granted access by the user. Once the OTP is passed to JavaScript,
the website can use it in a form or POST it directly to the server.
navigator.credentials.get({
otp: {transport:['sms']}
})
.then(otp => input.value = otp.code);
Learn how to use the WebOTP API in detail in Verify phone numbers on the web
with the WebOTP API or copy and paste the following snippet. (Make
sure the <form>
element has an action
and method
attribute properly set.)
// Feature detection
if ('OTPCredential' in window) {
window.addEventListener('DOMContentLoaded', e => {
const input = document.querySelector('input[autocomplete="one-time-code"]');
if (!input) return;
// Cancel the WebOTP API if the form is submitted manually.
const ac = new AbortController();
const form = input.closest('form');
if (form) {
form.addEventListener('submit', e => {
// Cancel the WebOTP API.
ac.abort();
});
}
// Invoke the WebOTP API
navigator.credentials.get({
otp: { transport:['sms'] },
signal: ac.signal
}).then(otp => {
input.value = otp.code;
// Automatically submit the form when an OTP is obtained.
if (form) form.submit();
}).catch(err => {
console.log(err);
});
});
}
Photo by Jason Leung on Unsplash.