This content originally appeared on web.dev and was authored by Eiji Kitamura
If you want to learn more general SMS OTP form best practices including Web OTP API, checkout SMS OTP form best practices.
What is the Web OTP API?
These days, most people in the world own a mobile device and developers are commonly using phone numbers as an identifier for users of their services.
There are a variety of ways to verify phone numbers, but a randomly generated one-time password (OTP) sent by SMS is one of the most common. Sending this code back to the developer's server demonstrates control of the phone number.
The Web OTP API was originally called the SMS Receiver API. You may still see it named that way in some places. If you used that API, you should still read this article. There are significant differences between the current and earlier versions of the API.
This idea is already deployed in many scenarios to achieve:
- Phone number as an identifier for the user. When signing up for a new service, some websites ask for a phone number instead of an email address and use it as an account identifier.
- Two step verification. When signing in, a website asks for a one-time code sent via SMS on top of a password or other knowledge factor for extra security.
- Payment confirmation. When a user is making a payment, asking for a one-time code sent via SMS can help verify the person's intent.
The current process creates friction for users. Finding an OTP within an SMS message, then copying and pasting it to the form is cumbersome, lowering conversion rates in critical user journeys. Easing this has been a long standing request for the web from many of the largest global developers. Android has an API that does exactly this. So does iOS and Safari.
The Web OTP API lets your app receive specially-formatted messages bound to your app's domain. From this, you can programmatically obtain an OTP from an SMS message and verify a phone number for the user more easily.
Warning: Attackers can spoof SMS and hijack a person's phone number. Carriers can also recycle phone numbers to new users after an account is closed. While SMS OTP is useful to verify a phone number for the use cases above, we recommend using additional and stronger forms of authentication (such as multiple factors and the Web Authentication API to establish new sessions for these users.
Current status
The table below explains the current status of the Web OTP API.
| Step | Status |
|---|---|
| 1. Create explainer | Complete |
| 2. Create initial draft of specification | Complete |
| 3. Gather feedback and iterate on design | Complete |
| 4. Origin trial | Complete |
| 5. Launch | Chrome 84 |
Changes from earlier versions
Early versions of this API were called SMS Receiver. If you are famillar with that version of the API be aware of the changes made to it. Improvements from SMS Receiver API include:
- The SMS message format is now aligned with WebKit's.
- The web page only receives an OTP code regardless of whatever else is in the message.
- The browser's application hash code is no longer required in the message.
See it in action
Let's say a user wants to verify their phone number with a website. The website sends a text message to the user over SMS and the user enters the OTP from the message to verify the ownership of the phone number.
With the Web OTP API, these steps are as easy as one tap for the user, as demonstrated in the video. When the text message arrives, a bottom sheet pops up and prompts the user to verify their phone number. After clicking the Verify button on the bottom sheet, the browser pastes the OTP into the form and the form is submitted without the user needing to press Continue.
The whole process is diagrammed in the image below.
Try the demo yourself. It doesn't ask for your phone number or send an SMS to your device, but you can send one from another device by copying the text displayed in the demo. This works because it doesn't matter who the sender is when using the Web OTP API.
- Go to https://web-otp.glitch.me in Chrome 84 or later on an Android device.
- Send your phone the following SMS text message from the another phone.
Your OTP is: 123456.
@web-otp.glitch.me #12345Did you receive the SMS and see the prompt to enter the code to the input area? That is how the Web OTP API works for users.
If the dialog doesn't appear for you, check out the FAQ.
Using the Web OTP API consists of three parts:
- A properly annotated
<input>tag - JavaScript in your web app
- Formatted message text sent via SMS.
I'll cover the <input> tag first.
Annotate an <input> tag
Web OTP itself works without any HTML annotation, but for cross-browser
compatibility, I highly recommend that you add autocomplete="one-time-code" to
the <input> tag where you expect the user entering an OTP.
This allows Safari 14 or later to suggest that the user to autofill the <input>
field with an OTP when they receive an SMS with the format described in Format
the SMS message even though it doesn't support Web OTP.
HTML
<form>
<input autocomplete="one-time-code" required/>
<input type="submit">
</form>Use the Web OTP API
Because Web OTP is simple, just copying and pasting the following code will do the job. I'll walk you through what's happening anyway.
JavaScript
if ('OTPCredential' in window) {
window.addEventListener('DOMContentLoaded', e => {
const input = document.querySelector('input[autocomplete="one-time-code"]');
if (!input) return;
const ac = new AbortController();
const form = input.closest('form');
if (form) {
form.addEventListener('submit', e => {
ac.abort();
});
}
navigator.credentials.get({
otp: { transport:['sms'] },
signal: ac.signal
}).then(otp => {
input.value = otp.code;
if (form) form.submit();
}).catch(err => {
console.log(err);
});
});
}Feature detection
Feature detection is the same as for many other APIs. Listening to
DOMContentLoaded event will wait for the DOM tree to be ready to query.
JavaScript
if ('OTPCredential' in window) {
window.addEventListener('DOMContentLoaded', e => {
const input = document.querySelector('input[autocomplete="one-time-code"]');
if (!input) return;
…
const form = input.closest('form');
…
});
}Caution: The Web OTP API requires a secure origin (HTTPS). The feature detection on an HTTP website will fail.
Process the OTP
The Web OTP API itself is simple enough. Use
navigator.credentials.get()
to obtain the OTP. Web OTP adds a new otp option to that method. It only has
one property: transport, whose value must be an array with the string 'sms'.
JavaScript
…
navigator.credentials.get({
otp: { transport:['sms'] }
…
}).then(otp => {
…This triggers the browser's permission flow when an SMS message arrives. If permission is
granted, the returned promise resolves with an OTPCredential object.
Content of obtained OTPCredential object
{
code: "123456" // Obtained OTP
type: "otp" // `type` is always "otp"
}Next, pass the OTP value to the <input> field. Submitting the form directly
will eliminate the step requiring the user to tap a button.
JavaScript
…
navigator.credentials.get({
otp: { transport:['sms'] }
…
}).then(otp => {
input.value = otp.code;
if (form) form.submit();
}).catch(err => {
console.error(err);
});
…Aborting the message
In case the user manually enters an OTP and submits the form, you can cancel the
get() call by using an AbortController instance in the options
object.
JavaScript
…
const ac = new AbortController();
…
if (form) {
form.addEventListener('submit', e => {
ac.abort();
});
}
…
navigator.credentials.get({
otp: { transport:['sms'] },
signal: ac.signal
}).then(otp => {
…Format the SMS message
The API itself should look simple enough, but there are a few things you should
know before using it. The message must be sent after
navigator.credentials.get() is called and it must be received on the device
where get() was called. Finally, the message must adhere to the following
formatting:
- The message begins with (optional) human-readable text that contains a four to ten character alphanumeric string with at least one number leaving the last line for the URL and the OTP.
- The domain part of the URL of the website that invoked the API must be preceded
by
@. - The URL must contain a pound sign ('
#') followed by the OTP.
For example:
Your OTP is: 123456.
@www.example.com #123456Demos
Try various messages with the demo: https://web-otp.glitch.me
You may also fork it and create your version: https://glitch.com/edit/#!/web-otp.
FAQ
The dialog doesn't appear though I'm sending a properly formatted message. What's going wrong?
There are a couple of caveats when testing the API:
- If the sender's phone number is included in the receiver's contact list, this API will not be triggered due to the design of the underlying SMS User Consent API.
- If you are using a work profile on your Android device and the Web OTP does not work, try installing and using Chrome on your personal profile instead (i.e. the same profile in which you receive SMS messages).
Where do I report bugs in Chrome's implementation?
Did you find a bug with Chrome's implementation?
- File a bug at
https://new.crbug.com.
Include as much detail as you can, simple instructions for reproducing, and
set Components to
Blink>WebOTP.
How can I help this feature?
Are you planning to use the Web OTP API? Your public support helps us prioritize
features, and shows other browser vendors how critical it is to support them.
Send a Tweet to @ChromiumDev with
#webotp and let us know where and how you're using it.
What are the differences with the SMS Receiver API?
Consider Web OTP API an evolved version of the SMS Receiver API. Web OTP API has a few significant differences compared to the SMS Receiver API.
- The expected text format for the SMS message has changed.
- It no longer requires an app hash string to be included in the SMS message.
- The method called is now
navigator.credentials.get()rather thannavigator.sms.receive(). - The
get()receives only the OTP rather than the entire SMS message asreceive()did before. - It's now possible to abort the call to
get().
Is this API compatible between different browsers?
Chromium and WebKit agreed on the SMS text message
format and Apple announced Safari's
support for it starting in iOS 14
and macOS Big Sur. Though Safari doesn't support the Web OTP JavaScript API, by
annotating input element with autocomplete=["one-time-code"], the default
keyboard automatically suggests that you enter the OTP if the SMS message complies
with the format.
Is it safe to use SMS as a way to authenticate?
While SMS OTP is useful to verify a phone number when the number is first provided, phone number verification via SMS must be used carefully for re-authentication since phone numbers can be hijacked and recycled by carriers. Web OTP is a convenient re-auth and recovery mechanism, but services should combine it with additional factors, such as a knowledge challenge, or use the Web Authentication API for strong authentication.
Find more questions at the FAQ section in the explainer.
This content originally appeared on web.dev and was authored by Eiji Kitamura
Eiji Kitamura | Sciencx (2019-10-07T00:00:00+00:00) Verify phone numbers on the web with the Web OTP API. Retrieved from https://www.scien.cx/2019/10/07/verify-phone-numbers-on-the-web-with-the-web-otp-api/
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