1. Introduction
This section and its sub-sections are non-normative.
This specification defines an API that enables the use of strong authentication methods in payment flows on the web. It aims to provide the same authentication benefits and user privacy focus as [webauthn-3] with enhancements to meet the needs of payment processing.
Similarly to [webauthn-3], this specification defines two related processes involving a user. The first is § 3 Registration (formerly "enrollment"), where a relationship is created between the user and the Relying Party. The second is § 4 Authentication, where the user responds to a challenge from the Relying Party (possibly via an intermediary payment service provider) to consent to a specific payment.
It is a goal of this specification to reduce authentication friction during checkout, and one aspect of that is to maximize the number of authentications that the user can perform for a given registration. That is, with consent from the Relying Party, ideally the user could "register once" and authenticate on any merchant origin (and via payment service provider), not just the merchant origin where the user first registered.
To that end, an important feature of Secure Payment Confirmation is that the merchant (or another entity) may initiate the authentication ceremony on the Relying Party’s behalf. The Relying Party must opt-in to allowing this behavior during credential creation.
Functionally, this specification defines a new payment method for the PaymentRequest
API, and adds a WebAuthn Extension to extend [webauthn-3] with payment-specific datastructures and to relax assumptions to
allow the API to be called in payment contexts.
1.1. Use Cases
Although [webauthn-3] provides general authentication capabilities for the Web, the following use cases illustrate the value of the payment-specific extension defined in this specification.
We presume that the general use case of cryptographic-based authentication for online transactions is well established.
1.1.1. Cryptographic evidence of transaction confirmation
In many online payment systems, it is common for the entity (e.g., bank) that issues a payment instrument to seek to reduce fraud through authentication. [webauthn-3] and this specification make it possible to use authenticators to cryptographically sign important payment-specific information such as the origin of the merchant and the transaction amount and currency. The bank, as the Relying Party, can then verify the signed payment-specific information as part of the decision to authorize the payment.
If the bank uses plain [webauthn-3], the payment-specific information to be
verified must be stored in the WebAuthn challenge
. This raises several issues:
-
It is a misuse of the
challenge
field (which is intended to defeat replay attacks). -
There is no specification for this, so each bank is likely to have to devise its own format for how payment-specific information should be formatted and encoded in the challenge, complicating deployment and increasing fragmentation.
-
Regulations may require evidence that the user was shown and agreed to the payment-specific information. Plain [webauthn-3] does not provide for this display: there is no specified UX associated with information stored in the
challenge
field.
These limitations motivate the following Secure Payment Confirmation behaviors:
-
The
challenge
field is only used to defeat replay attacks, as with plain [webauthn-3]. -
SPC specifies a format for payment-specific information. This will enable development of generic verification code and test suites.
-
SPC guarantees that the user agent has presented the payment-specific information to the user in a way that a malicious website (or maliciously introduced JavaScript code on a trusted website) cannot bypass.
-
The payment-specific information is included in the
CollectedClientData
dictionary, which cannot be tampered with via JavaScript.
NOTE: Banks and other stakeholders in the payments ecosystem trust payments via browsers sufficiently today using TLS, iframes, and other Web features. The current specification is designed to increase the security and usability of Web payments.
-
1.1.2. Registration in a third-party iframe
If a bank wishes to use [webauthn-3] as the Relying Party, that specification requires the bank to register the user in a first party context. Registration can happen outside of a transaction while the user is visiting the bank’s site. It is also useful to be able to register the user during a transaction, but any registration that interrupts the payment journey creates a risk of transaction abandonment.
This limitation motivates the following Secure Payment Confirmation behavior:
-
SPC supports cross-origin registration from an iframe in a third-party context. For instance, this registration might take place following some other identity and verification (ID&V) flow (e.g., SMS OTP).
1.1.3. Merchant control of authentication
Merchants seek to avoid user drop-off during checkout, in particular by reducing authentication friction. A Relying Party (e.g., a bank) that wishes to use [webauthn-3] to authenticate the user typically does so from an iframe. However, merchants would prefer to manage the user experience of authenticating the user while still enabling the Relying Party to verify the results of authentication.
This limitation motivates the following Secure Payment Confirmation behavior:
-
With SPC, other parties than the Relying Party can use authentication credentials on behalf of the Relying Party. The Relying Party can then verify the authentication results.
An additional benefit of this feature to Relying Parties is that they no longer need to build their own front-end experiences for authentication. Instead, payment service providers are likely to build them on behalf of merchants.
NOTE: Relying Parties that wish to provide the authentication user experience may still do so using SPC from an iframe.
1.2. Sample API Usage Scenarios
In this section, we walk through some scenarios for Secure Payment Confirmation and the corresponding sample code for using this API. Note that these are example flows and do not limit the scope of how the API can be used.
1.2.1. Registration during a checkout
This is a first-time flow, in which a new credential is created and stored by an issuing bank during a checkout by the user on some merchant.
-
The user visits
merchant.com
, selects an item to purchase, and proceeds to the checkout flow. They enter their payment instrument details, and indicate that they wish to pay (e.g., by pressing a "Pay" button). -
The merchant communicates out-of-band (e.g., using another protocol) with the bank that issued the payment instrument. The issuing bank requests verification of the user, and provides a bank-controlled URL for the merchant to open in an iframe.
-
The merchant opens an iframe to
bank.com
, with theallow
attribute set to "payment". -
In the iframe, the issuing bank confirms the user’s identity via a traditional means (e.g., SMS OTP). After confirmation, the bank invites the user to register in SPC authentication for future payments.
-
The user consents (e.g., by clicking an "Register" button in the bank UX), and the bank runs code in the iframe (see example below).
-
The user goes through a WebAuthn registration flow. A new credential is created and returned to the issuing bank who stores it in their server-side database associated with the user and payment instrument(s).
-
The verification completes; the bank iframe closes and the merchant finishes the checkout process for the user.
Sample code for registering the user in this way follows:
if ( ! window. PublicKeyCredential) { /* Client not capable. Handle error. */ } const publicKey= { // The challenge should be created by the bank server and sent to the iframe. challenge: new Uint8Array([ 21 , 31 , 105 /* 29 more random bytes generated by the server */ ]), // Relying Party: rp: { name: "Fancy Bank" , }, // User: user: { // Part of WebAuthn. This information is not required by SPC // but may be used by the bank server to identify this user in // future transactions. Inconsistent values for the same user // can result in the creation of multiple credentials for the user // and thus potential UX friction due to credential selection. id: Uint8Array. from ( window. atob( "MIIBkzCCATigAwIBAjCCAZMwggE4oAMCAQIwggGTMII=" ), c=> c. charCodeAt( 0 )), name: "jane.doe@example.com" , displayName: "Jane Doe" , }, // In this example the Relying Party accepts either an ES256 or RS256 // credential, but prefers an ES256 credential. pubKeyCredParams: [ { type: "public-key" , alg: - 7 // "ES256" }, { type: "public-key" , alg: - 257 // "RS256" } ], authenticatorSelection: { userVerification: "required" , residentKey: "required" , authenticatorAttachment: "platform" , }, timeout: 360000 , // 6 minutes // Indicate that this is an SPC credential. This is currently required so // that the browser knows this credential relates to SPC. It also enables // credential creation in a cross-origin iframe, which is required for this // example. // // A future version of the spec may remove the need for this extension. extensions: { "payment" : { isPayment: true , } } }; // Note: The following call will cause the authenticator to display UI. navigator. credentials. create({ publicKey}) . then( function ( newCredentialInfo) { // Send new credential info to server for verification and registration. }). catch ( function ( err) { // No acceptable authenticator or user refused consent. Handle appropriately. });
1.2.2. Authentication on merchant site
This is the flow when a user with an already registered credential is performing a transaction and the issuing bank and merchant wish to use Secure Payment Confirmation.
-
The user visits
merchant.com
, selects an item to purchase, and proceeds to the checkout flow. They enter their payment instrument details, and indicate that they wish to pay (e.g., by pressing a "Pay" button). -
The merchant communicates out-of-band with the issuing bank of the payment instrument (e.g., using another protocol). The issuing bank requests verification of the user, and at the same time informs the merchant that it accepts SPC by providing the information necessary to use the API. This information includes a challenge and any credential IDs associated with this user and payment instrument(s).
-
The merchant runs the example code shown below.
-
The user agrees to the payment-specific information displayed in the SPC UX, and performs a subsequent WebAuthn authentication ceremony. The signed cryptogram is returned to the merchant.
-
The merchant communicates the signed cryptogram to the issuing bank out-of-band. The issuing bank verifies the cryptogram, and knows that the user is valid, what payment-specific information has been displayed, and that the user has consented to the transaction. The issuing bank authorizes the transaction and the merchant finishes the checkout process for the user.
The sample code for authenticating the user follows. Note that the example code presumes access to await/async, for easier to read promise handling.
/* isSecurePaymentConfirmationAvailable indicates whether the browser */ /* supports SPC. It does not indicate whether the user has a credential */ /* ready to go on this device. */ const spcAvailable= PaymentRequest&& PaymentRequest. isSecurePaymentConfirmationAvailable&& await PaymentRequest. isSecurePaymentConfirmationAvailable(); if ( ! spcAvailable) { /* Browser does not support SPC; merchant should fallback to traditional flows. */ } const request= new PaymentRequest([{ supportedMethods: "secure-payment-confirmation" , data: { // List of credential IDs obtained from the bank. credentialIds, rpId: "fancybank.com" , // The challenge is also obtained from the bank. challenge: new Uint8Array([ 21 , 31 , 105 /* 29 more random bytes generated by the bank */ ]), instrument: { displayName: "Fancy Card ****1234" , icon: "https://fancybank.com/card-art.png" , }, payeeName: "Merchant Shop" , payeeOrigin: "https://merchant.com" , // Caller’s requested localized experience locale: [ "en" ], timeout: 360000 , // 6 minutes }], { total: { label: "Total" , amount: { currency: "USD" , value: "5.00" , }, }, }); try { const response= await request. show(); await response. complete( 'success' ); // response.data is a PublicKeyCredential, with a clientDataJSON that // contains the transaction data for verification by the issuing bank. /* send response.data to the issuing bank for verification */ } catch ( err) { /* SPC cannot be used; merchant should fallback to traditional flows */ }
2. Terminology
- SPC Credential
-
A WebAuthn credential that can be used for the behaviors defined in this specification. When an SPC Credential is to be used by a party other than the Relying Party, the Relying Party must explicitly opt in by declaring the credential for SPC at creation time.
This specification does not intend to limit how SPC credentials may (or may not) be used by a Relying Party for other authentication flows (e.g., login).
Note: The current version of this specification requires the Relying Party to explicitly opt in for a credential to be used in either a first-party or third-party context. Longer-term, our intention is that all WebAuthn credentials will be usable for SPC in a first-party context (e.g., on the Relying Party’s domain) and opt-in will only be required to allow a credential to be used by a third-party.
- Steps to silently determine if a credential is SPC-enabled
-
An as-yet undefined process by which a user agent can, given a Relying Party Identifier and a credential ID, silently (i.e., without user interaction) determine if the credential represented by that ID is an SPC Credential.
NOTE: See WebAuthn issue 1667.
- Steps to silently determine if a credential is available for the current device
-
An as-yet undefined process by which a user agent can, given a Relying Party Identifier and a credential ID, silently (i.e., without user interaction) determine if the credential represented by that credential ID is available for the current device (i.e., could be successfully used as part of a WebAuthn Get call).
This allows the user agent to only conditionally display the transaction UX to the user if there is some chance that they can successfully complete the transaction.
NOTE: This property will likely require that SPC Credentials be discoverable; as such this specification currently encodes that as a requirement.
NOTE: This property is very similar to that which is required for the WebAuthn Conditional UI Proposal. It is likely that both it and SPC could be supported by the same underlying API.
3. Registration
To register a user for Secure Payment Confirmation, relying parties should call navigator.credentials.create()
, with the payment
WebAuthn Extension specified.
Tests
Note: In this specification we define an extension in order to allow (1) credential creation in a cross-origin iframe (which WebAuthn Levels 1 and 2 do not allow, but Level 3 is expected to allow) and (2) the browser to cache SPC credential IDs in the absence of Conditional UI. If these capabilities are available in future versions of WebAuthn, we may remove the requirement for the extension from SPC. Note that SPC credentials (with the extension) are otherwise full-fledged WebAuthn credentials.
4. Authentication
To authenticate a payment via Secure Payment Confirmation, this specification defines a new payment method, "secure-payment-confirmation". This payment method confirms the transaction with the user and then performs an authentication ceremony to authenticate the user and create a signed blob representing the authentication ceremony.
At a high level, authentication for Secure Payment Confirmation is similar to [webauthn-3], with one major conceptual shift. Secure Payment Confirmation allows a third-party (e.g., the merchant) to trigger an authentication ceremony on behalf of the Relying Party, passing in credentials that it has obtained from the Relying Party on some other unspecified channel. See § 1.1.3 Merchant control of authentication.
4.1. Payment Method: Secure Payment Confirmation
This specification defines a new payment handler, the Secure Payment Confirmation payment handler, which handles requests to authenticate a given payment.
NOTE: To quickly support an initial SPC experiment, this API was designed atop existing implementations of the Payment Request and Payment Handler APIs. There is now general agreement to explore a design of SPC independent of Payment Request. We therefore expect (without a concrete timeline) that SPC will move away from its Payment Request origins. For developers, this should improve feature detection, invocation, and other aspects of the API.
4.1.1. Payment Method Identifier
The standardized payment method identifier for the Secure Payment Confirmation payment handler is "secure-payment-confirmation".
4.1.2. Registration in [payment-method-id]
Add the following to the registry of standardized payment methods in [payment-method-id]:
- "secure-payment-confirmation"
-
The Secure Payment Confirmation specification.
4.1.3. Modification of Payment Request constructor
In the steps for the PaymentRequest object’s constructor
,
add a new step after step 4.3:
-
Process payment methods: [substeps 1-3 elided]
-
If seenPMIs contains "secure-payment-confirmation" and the size of seenPMIs is greater than 1, throw a
RangeError
.
-
4.1.4. Modification of user activation requirement
In the steps for the PaymentRequest.show()
method,
modify steps 2 and 3:
-
If the relevant global object of request does not have transient activation, the user agent MAY:
-
Return a promise rejected with with a
"SecurityError"
DOMException
.
-
-
Otherwise, consume user activation of the relevant global object.
NOTE: This allows the user agent to not require user activation, for example to support redirect authentication flows where a user activation may not be present upon redirect. See § 10.3 Lack of user activation requirement for security considerations.
4.1.5. SecurePaymentConfirmationRequest
Dictionary
dictionary SecurePaymentConfirmationRequest {required BufferSource challenge ;required USVString rpId ;required sequence <BufferSource >credentialIds ;required PaymentCredentialInstrument instrument ;unsigned long timeout ;USVString payeeName ;USVString payeeOrigin ;AuthenticationExtensionsClientInputs extensions ;sequence <USVString >locale ;boolean showOptOut ; };
The SecurePaymentConfirmationRequest
dictionary contains the following
members:
challenge
member, of type BufferSource-
A random challenge that the relying party generates on the server side to prevent replay attacks.
rpId
member, of type USVString-
The Relying Party Identifier of the credentials.
credentialIds
member, of type sequence<BufferSource>-
The list of credential identifiers for the given instrument.
instrument
member, of type PaymentCredentialInstrument-
The description of the instrument name and icon to display during registration and to be signed along with the transaction details.
timeout
member, of type unsigned long-
The number of milliseconds before the request to sign the transaction details times out. At most 1 hour.
payeeName
member, of type USVString-
The display name of the payee that this SPC call is for (e.g., the merchant). Optional, may be provided alongside or instead of
payeeOrigin
. payeeOrigin
member, of type USVString-
The origin of the payee that this SPC call is for (e.g., the merchant). Optional, may be provided alongside or instead of
payeeName
. extensions
member, of type AuthenticationExtensionsClientInputs-
Any WebAuthn extensions that should be used for the passed credential(s). The caller does not need to specify the payment extension; it is added automatically.
locale
member, of type sequence<USVString>-
An optional list of well-formed [BCP47] language tags, in descending order of priority, that identify the locale preferences of the website, i.e. a language priority list [RFC4647], which the user agent can use to perform language negotiation and locale-affected formatting with the caller.
NOTE: The
locale
is distinct from language or direction metadata associated with specific input members, in that it represents the caller’s requested localized experience rather than assertion about a specific string value. See § 13 Internationalization Considerations for more discussion. showOptOut
member, of type boolean-
Whether the user should be given a chance to opt-out during the transaction confirmation UX. Optional, default false.
4.1.6. Payment Method additional data type
The payment method additional data type for this payment method is SecurePaymentConfirmationRequest
.
4.1.7. Checking if Secure Payment Confirmation is available
A static API is added to PaymentRequest
in order to provide developers a
simplified method of checking whether Secure Payment Confirmation is available.
partial interface PaymentRequest {static Promise <boolean >(); };
isSecurePaymentConfirmationAvailable
isSecurePaymentConfirmationAvailable()
-
Upon invocation, a promise is returned that resolves with a value of
true
if the Secure Payment Confirmation feature is available, orfalse
otherwise.
This allows a developer to perform the following check when deciding whether to initiate a SPC flow:
const spcAvailable= PaymentRequest&& PaymentRequest. isSecurePaymentConfirmationAvailable&& await PaymentRequest. isSecurePaymentConfirmationAvailable();
NOTE: The use of the static isSecurePaymentConfirmationAvailable
method is recommended for
SPC feature detection, instead of calling canMakePayment
on an already-constructed
PaymentRequest object.
4.1.8. Steps to validate payment method data
The steps to validate payment method data for this payment method, for an
input SecurePaymentConfirmationRequest
data, are:
Tests
-
If data["
credentialIds
"] is empty, throw aRangeError
. -
For each id in data["
credentialIds
"]:-
If id is empty, throw a
RangeError
.
-
-
If data["
instrument
"]["displayName
"] is empty, throw aTypeError
. -
If data["
instrument
"]["icon
"] is empty, throw aTypeError
. -
Run the URL parser on data["
instrument
"] ["icon
"]. If this returns failure, throw aTypeError
. -
If data["
rpId
"] is not a valid domain, throw aTypeError
. -
If both data["
payeeName
"] and data["payeeOrigin
"] are omitted, throw aTypeError
. -
If either of data["
payeeName
"] or data["payeeOrigin
"] is present and empty, throw aTypeError
. -
If data["
payeeOrigin
"] is present:-
Let parsedURL be the result of running the URL parser on data["
payeeOrigin
"]. -
If parsedURL is failure, then throw a
TypeError
. -
If parsedURL’s scheme is not "
https
", then throw aTypeError
.
-
4.1.9. Steps to check if a payment can be made
The steps to check if a payment can be made for this payment method, for an
input SecurePaymentConfirmationRequest
data, are:
Tests
-
If data["
payeeOrigin
"] is present:-
Let parsedURL be the result of running the URL parser on data["
payeeOrigin
"]. -
Assert that parsedURL is not failure.
-
Assert that parsedURL’s scheme is "
https
".
NOTE: These pre-conditions were previously checked in the steps to validate payment method data.
-
Set data["
payeeOrigin
"] to the serialization of parsedURL’s origin.
-
-
Fetch the image resource for the icon, passing «["
src
" → data["instrument
"]["icon
"]]» for image. If this fails:-
If data["
instrument
"]["iconMustBeShown
"] istrue
, then returnfalse
. -
Otherwise, set data["
instrument
"]["icon
"] to an empty string.Note: This lets the RP know that the specified icon was not shown, as the output
instrument
will have an empty icon string.
Note: The image resource must be fetched whether or not any credential matches, to defeat attempts to probe for credential existence.
-
-
For each id in data["
credentialIds
"]:-
Run the steps to silently determine if a credential is available for the current device and the steps to silently determine if a credential is SPC-enabled, passing in data["
rpId
"] and id. If the result of either of these isfalse
, remove id from data["credentialIds
"].
-
-
If data["
credentialIds
"] is now empty, returnfalse
. The user agent must maintain authentication ceremony privacy and not leak this lack of matching credentials to the caller, by:-
Not allowing the caller to perform a timing attack on this outcome versus the user declining to authenticate on the transaction confirmation UX, e.g., by presenting an alternative interstitial that the user must interact with.
-
Rejecting the
show()
promise with a "NotAllowedError
"DOMException
.
-
-
Return
true
.
4.1.10. Displaying a transaction confirmation UX
To avoid restricting User Agent implementation choice, this specification does
not require a User Agent to display a particular user interface when PaymentRequest.show()
is called and the Secure
Payment Confirmation payment handler is selected. However, so that a Relying Party can trust the information included in CollectedClientPaymentData
, the User Agent MUST ensure that the following
is communicated to the user and that the user’s consent is collected for the
authentication:
-
The
payeeName
if it is present. -
The
payeeOrigin
if it is present. -
The
total
, that is thecurrency
andvalue
of the transaction. -
The
instrument
details, that is the payment instrumentdisplayName
andicon
. If an image resource could not be fetched or decoded from the inputicon
, then the User Agent may show no icon or a generic payment instrument icon in its place.NOTE: If the specified icon could not be fetched or decoded, then
iconMustBeShown
must befalse
here as otherwise the the steps to check if a payment can be made would have failed previously.
The user agent MAY utilize the information in locale
, if any, to display a UX localized
into a language and using locale-based formatting consistent with that of the
website.
If showOptOut
is true
, the user agent
MUST give the user the opportunity to indicate that they want to opt out of the
process for the given relying party
.
If the user indicates that they wish to opt-out, then the user agent must reject
the show()
promise with an
"OptOutError
" DOMException
. See § 11.5 User opt out.
If the current transaction automation mode is not "none
", the user agent
should first verify that it is in an automation context (see WebDriver’s Security considerations). The user agent
should then bypass the above communication of information and gathering of user
consent, and instead do the following based on the value of the current
transaction automation mode:
- "
autoAccept
" -
Act as if the user has seen the transaction details and accepted the authentication.
- "
autoReject
" -
Act as if the user has seen the transaction details and rejected the authentication.
- "
autoOptOut
" -
Act as if the user has seen the transaction details and indicated they want to opt out.
4.1.11. Steps to respond to a payment request
The steps to respond to a payment request for this payment method, for a given PaymentRequest
request and SecurePaymentConfirmationRequest
data, are:
-
Let topOrigin be the top-level origin of the relevant settings object of request.
-
Let payment be a new a
AuthenticationExtensionsPaymentInputs
dictionary, whose fields are:isPayment
-
The boolean value
true
. rpId
-
data["
rpId
"] topOrigin
-
topOrigin
payeeName
-
data["
payeeName
"] if it is present, otherwise omitted. payeeOrigin
-
data["
payeeOrigin
"] if it is present, otherwise omitted. total
-
request.[[details]]["
total
"] instrument
-
data["
instrument
"]
-
Let extensions be a new
AuthenticationExtensionsClientInputs
dictionary whosepayment
member is set to payment, and whose other members are set from data["extensions
"]. -
Let publicKeyOpts be a new
PublicKeyCredentialRequestOptions
dictionary, whose fields are:challenge
-
data["
challenge
"] timeout
-
data["
timeout
"] rpId
-
data["
rpId
"] userVerification
extensions
-
extensions
Note: This algorithm hard-codes "required" as the value for
userVerification
, because that is what Chrome’s initial implementation supports. The current limitations may change. The Working Group invites implementers to share use cases that would benefit from support for other values (e.g., "preferred" or "discouraged"). -
For each id in data["
credentialIds
"]:-
Let descriptor be a new
PublicKeyCredentialDescriptor
dictionary, whose fields are:type
id
-
id
transports
-
A sequence of length 1 whose only member is
internal
.
-
Append descriptor to publicKeyOpts["
allowCredentials
"].
-
-
Let outputCredential be the result of running the algorithm to Request a Credential, passing «["
publicKey
" → publicKeyOpts]».Note: Chrome’s initial implementation does not pass the full
data.credentialIds
list to Request a Credential. Instead, it chooses one credential in the list that matches the current device and passes only that in.Note: This triggers [webauthn-3]'s Get behavior
-
Return outputCredential.
5. WebAuthn Extension - "payment
"
This client registration extension and authentication extension indicates that a credential is either being created for or used for Secure Payment Confirmation, respectively.
For registration, this extension relaxes the WebAuthn requirements to allow credential creation in a cross-origin iframe, and also allows the browser to identify and cache Secure Payment Confirmation credential IDs. For authentication, this extension allows a third-party to perform an authentication ceremony on behalf of the Relying Party, and also adds transaction information to the signed cryptogram.
Notably, a website should not call navigator.credentials.get()
with this extension
directly; for authentication the extension can only be accessed via PaymentRequest
with a "secure-payment-confirmation" payment method.
Tests
This test does not directly correspond to a spec line, but instead tests that authentication can be triggered from inside a cross-origin iframe. That behavior is specified by the lack of any line forbidding it.
- Extension identifier
-
payment
- Operation applicability
- Client extension input
-
partial dictionary AuthenticationExtensionsClientInputs {AuthenticationExtensionsPaymentInputs
; };payment dictionary
{AuthenticationExtensionsPaymentInputs boolean isPayment ; // Only used for authentication.USVString rpId ;USVString topOrigin ;USVString payeeName ;USVString payeeOrigin ;PaymentCurrencyAmount total ;PaymentCredentialInstrument instrument ; };isPayment
member, of type boolean-
Indicates that the extension is active.
rpId
member, of type USVString-
The Relying Party id of the credential(s) being used. Only used at authentication time; not registration.
topOrigin
member, of type USVString-
The origin of the top-level frame. Only used at authentication time; not registration.
payeeName
member, of type USVString-
The payee name, if present, that was displayed to the user. Only used at authentication time; not registration.
payeeOrigin
member, of type USVString-
The payee origin, if present, that was displayed to the user. Only used at authentication time; not registration.
total
member, of type PaymentCurrencyAmount-
The transaction amount that was displayed to the user. Only used at authentication time; not registration.
instrument
member, of type PaymentCredentialInstrument-
The instrument details that were displayed to the user. Only used at authentication time; not registration.
- Client extension processing (registration)
-
When creating a new credential:
-
Modify step 2 (the check for sameOriginWithAncestors) as follows:
-
If sameOriginWithAncestors is
false
:-
If the relevant global object, as determined by the calling
create()
implementation, does not have transient activation:-
Return a
DOMException
whose name is "SecurityError
", and terminate this algorithm.
-
-
Note: This allows for creating SPC credentials in a cross-origin iframe, as long as the correct permission policy is set (see § 7 Permissions Policy integration). A transient activation is also required in this case to mitigate privacy risks; see § 11.1 Registration in a Cross-Origin iframe.
-
-
After step 3, insert the following step:
-
If any of the following are true:
-
options["
authenticatorSelection
"]["authenticatorAttachment
"] is not "platform
". -
options["
authenticatorSelection
"]["residentKey
"] is not "required
" or "preferred
". -
options["
authenticatorSelection
"]["userVerification
"] is not "required
".
then return a
TypeError
.Note: These values are hard-coded as that is what Chrome’s initial implementation supports. The current limitations may change. The Working Group invites implementers to share use cases that would benefit from support for other values.
-
-
-
- Client extension processing (authentication)
-
When making an assertion with a
AuthenticationExtensionsPaymentInputs
extension_inputs:-
If not in a "secure-payment-confirmation" payment handler, return a "
NotAllowedError
"DOMException
.Note: This guards against websites trying to access the extended powers of SPC without going through the transaction UX.
-
During
[[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors)
:-
Skip step 6.1, which compares options.rpId to effectiveDomain
Note: This enables cross-domain authentication ceremonies; see § 1.1.3 Merchant control of authentication.
-
In step 9, instead of creating a
CollectedClientData
, instead create aCollectedClientPaymentData
with:-
type
set to "payment.get
" -
payment
set to a newCollectedClientAdditionalPaymentData
whose fields are:rpId
-
extension_inputs["
rpId
"] topOrigin
-
extension_inputs["
topOrigin
"] payeeName
-
extension_inputs["
payeeName
"] if it is present, otherwise omitted. payeeOrigin
-
extension_inputs["
payeeOrigin
"] if it is present, otherwise omitted. total
-
extension_inputs["
total
"] instrument
-
extension_inputs["
instrument
"]
-
All other fields set as per the original step 9.
-
-
-
- Client extension output
-
None
- Authenticator extension processing
-
None
5.1. CollectedClientPaymentData
Dictionary
dictionary CollectedClientPaymentData :CollectedClientData {required CollectedClientAdditionalPaymentData payment ; };
The CollectedClientPaymentData
dictionary inherits from CollectedClientData
. It contains the following additional field:
payment
member, of type CollectedClientAdditionalPaymentData-
The additional payment information to sign.
5.2. CollectedClientAdditionalPaymentData
Dictionary
dictionary CollectedClientAdditionalPaymentData {required USVString rpId ;required USVString topOrigin ;USVString payeeName ;USVString payeeOrigin ;required PaymentCurrencyAmount total ;required PaymentCredentialInstrument instrument ; };
The CollectedClientAdditionalPaymentData
dictionary contains the following
fields:
rpId
member, of type USVString-
The id of the Relying Party that created the credential.
NOTE: For historical reasons, some implementations may additionally include this parameter with the name
rp
. The values ofrp
andrpId
must be the same if both are present. topOrigin
member, of type USVString-
The origin of the top level context that requested to sign the transaction details.
payeeName
member, of type USVString-
The name of the payee, if present, that was displayed to the user.
payeeOrigin
member, of type USVString-
The origin of the payee, if present, that was displayed to the user.
total
member, of type PaymentCurrencyAmount-
The
PaymentCurrencyAmount
of the [payment-request]total
field. instrument
member, of type PaymentCredentialInstrument-
The instrument information that was displayed to the user.
Note that there is no paymentRequestOrigin
field in CollectedClientAdditionalPaymentData
, because the origin of the calling
frame is already included in CollectedClientData
of [webauthn-3].
6. Common Data Structures
The following data structures are shared between registration and authentication.
6.1. PaymentCredentialInstrument
Dictionary
dictionary PaymentCredentialInstrument {required USVString displayName ;required USVString icon ;boolean iconMustBeShown =true ; };
The PaymentCredentialInstrument
dictionary contains the information to be
displayed to the user and signed together with the transaction details. It
contains the following members:
displayName
member, of type USVString-
The name of the payment instrument to be displayed to the user.
NOTE: See § 13 Internationalization Considerations for discussion about internationalization of the
displayName
. icon
member, of type USVString-
The URL of the icon of the payment instrument.
NOTE: The
icon
URL may either identify an image on an internet-accessible server (e.g.,https://bank.com/card.png
), or directly encode the icon data via a Data URL [RFC2397]. Between the two types of URLs, Data URLs offer several benefits to the Relying Party. They can improve reliability (e.g., in the case that the icon hosting server may be unavailable). They can also enhance validation because the Relying Party has cryptographic evidence of what the browser displayed to the user: the icon URL is signed as part of theCollectedClientAdditionalPaymentData
structure.NOTE: See related accessibility considerations.
iconMustBeShown
member, of type boolean, defaulting totrue
-
Indicates whether the specified icon must be successfully fetched and shown for the request to succeed.
7. Permissions Policy integration
This specification uses the "payment" policy-identifier string from [payment-request] to control access to both registration and authentication. This extends the WebAuthn Permission Policy.
Note: Algorithms specified in [CREDENTIAL-MANAGEMENT-1] perform the actual
permissions policy evaluation. This is because such policy evaluation needs to
occur when there is access to the current settings object. The [[Create]](origin, options, sameOriginWithAncestors)
and [[DiscoverFromExternalSource]](origin, options,
sameOriginWithAncestors)
internal methods do not have such access since
they are invoked in parallel (by algorithms specified in [CREDENTIAL-MANAGEMENT-1]).
8. SPC Relying Party Operations
8.1. Verifying an Authentication Assertion
In order to perform an authentication ceremony for Secure Payment Confirmation, the Relying Party MUST proceed as follows:
-
Let credential be a
PublicKeyCredential
returned from a successful invocation of the Secure Payment Confirmation payment handler by the SPC caller.Note: As SPC is designed to enable merchant control of authentication, the entity that invokes SPC may not be the Relying Party. This first step presumes that the SPC caller has returned a credential obtained via SPC to the Relying Party.
-
Perform steps 3-21 as specified in WebAuthn, with the following changes:
-
In step 5, verify that credential.
id
identifies one of the public key credentials provided to the SPC caller by the Relying Party. -
In step 11, verify that the value of C["
type
"] is the stringpayment.get
. -
In step 12, verify that the value of C["
challenge
"] equals the base64url encoding of the challenge provided to the SPC caller by the Relying Party. -
In step 13, verify that the value of C["
origin
"] matches the origin that the Relying Party expects SPC to have been called from. -
After step 13, insert the following steps:
-
Verify that the value of C["
payment
"]["rpId
"] matches the Relying Party's origin. -
Verify that the value of C["
payment
"]["topOrigin
"] matches the top-level origin that the Relying Party expects. -
Verify that the value of C["
payment
"]["payeeName
"] matches the name of the payee that should have been displayed to the user, if any. -
Verify that the value of C["
payment
"]["payeeOrigin
"] matches the origin of the payee that should have been displayed to the user, if any. -
Verify that the value of C["
payment
"]["total
"] matches the transaction amount that should have been displayed to the user. -
Verify that the value of C["
payment
"]["instrument
"] matches the payment instrument details that should have been displayed to the user.
-
-
9. User Agent Automation
For the purposes of user agent automation and website testing, this document defines the below [WebDriver2] extension commands. Interested parties should also consult the equivalent automation section in [webauthn-3].
9.1. Set SPC Transaction Mode
The Set SPC Transaction Mode WebDriver extension command instructs the user agent to place Secure Payment Confirmation into a mode where it will automatically simulate a user either accepting or rejecting the transaction confirmation UX.
The current transaction automation mode tracks what automation mode
is currently active for SPC. It defaults to "none
".
HTTP Method | URI Template |
---|---|
POST | /session/{session id}/secure-payment-confirmation/set-mode
|
The remote end steps are:
-
If parameters is not a JSON Object, return a WebDriver error with WebDriver error code invalid argument.
-
Let mode be the result of getting a property named
"mode"
from parameters. -
If mode is undefined or is not one of "
autoAccept
", "autoReject
", or "autoOptOut
", return a WebDriver error with WebDriver error code invalid argument. -
Set the current transaction automation mode to mode.
-
Return success with data
null
.
10. Security Considerations
As this specification builds on top of WebAuthn, the WebAuthn Security Considerations are applicable. The below subsections comprise the current Secure Payment Confirmation-specific security considerations, where this specification diverges from WebAuthn.
10.1. Cross-origin authentication ceremony
A significant departure that Secure Payment Confirmation makes from WebAuthn is in allowing a third-party to initiate an authentication ceremony using credentials for a different Relying Party, and returning the assertion to the third party. This feature can expose Relying Parties to both login and payment attacks, which are discussed here.
10.1.1. Login Attack
As credentials created for Secure Payment Confirmation are valid WebAuthn credentials, it is possible that a Relying Party may wish to use the same credential for a given user for both login and payment. This allows a potential attack on the Relying Party’s login system, if they do not carefully verify the assertion they receive.
The attack is as follows:
-
The user visits
attacker.com
, which is or pretends to be a merchant site. -
attacker.com
obtains credentials for the user fromrelyingparty.com
, either legitimately or by stealing them fromrelyingparty.com
or another party with whomrelyingparty.com
had shared the credentials. -
attacker.com
initiates SPC authentication, and the user agrees to the transaction (which may or may not be legitimate). -
attacker.com
takes the payment assertion that they received from the API call, and sends it to the login endpoint forrelyingparty.com
, e.g. by sending a POST tohttps://relyingparty.com/login
. -
relyingparty.com
is employing faulty assertion validation code, which checks the signature but fails to validate the necessary fields (see below), and believes the login attempt to be legitimate. -
relyingparty.com
returns e.g. a login cookie toattacker.com
. The user’s account atrelyingparty.com
has now been compromised.
Relying Parties can guard against this attack in two ways.
Firstly, a Relying Party must always follow the correct assertion validation steps either for WebAuthn login or SPC payment as appropriate. In particular, the following fields can all be used to detect an inappropriate use of a credential:
-
CollectedClientData
["type
"]- "webauthn.get" for login, "payment.get" for SPC. -
CollectedClientData
["challenge
"] - this value should be provided by the Relying Party server to the site ahead of any call to either WebAuthn or SPC, and should be verified as matching an expected, appropriate, previously-provided value. -
CollectedClientData
["origin
"] - if SPC is being performed cross-origin, this value will contain the origin of the caller (e.g.attacker.com
in the above example).
Secondly, a Relying Party can consider keeping their payment and login
credentials separate. If doing this, the Relying Party should only register
credentials for Secure Payment Confirmation on a subdomain (e.g. https//payment.relyingparty.com
), and should keep payment credentials and
login credentials separate in their database.
payment
extension specified to participate in SPC authentication, and the
specification may be updated to reflect that in the future.
In both implementation and specification today, a credential created
with the payment
can be used
for login, if the Relying Party wishes. This is not expected to change.
10.1.2. Payment Attack
A Secure Payment Confirmation assertion is essentially useless unless it is part of an ongoing online transaction.
A variety of mechanisms protect against an attack where a malicious third-party, instead of attempting to hijack a user account, initiates an unauthorized payment using Secure Payment Confirmation credentials (obtained either legitimately or otherwise):
-
When the attacker initiates SPC, the user will be shown UI by the User Agent that clearly states the transaction details (including the payee and amount). The user is very likely to "cancel" in this scenario.
-
If the user does agree to the transaction, and completes the subsequent WebAuthn authentication ceremony, the attacker now has a signed SPC assertion for the Relying Party.
-
If the Relying Party is not expecting a transaction, it will reject the assertion.
-
If the Relying Party is expecting a transaction, it will detect at least one of the following and reject the assertion:
-
An incorrect
CollectedClientData
["challenge
"], if an attacker attempts to race against a valid ongoing payment. -
An incorrect
CollectedClientData
["origin
"], if an attacker attempts to sit between the user and a valid merchant site and forward the assertion.
-
10.2. Merchant-supplied authentication data
The bank can and should protect against spoofing by verifying the authentication assertion they receive to ensure it aligns with the transaction details provided by the merchant.
That is because a consequence of this specification’s third-party authentication ceremony is that even in a valid transaction (i.e. one that the Relying Party is expecting), a third-party provides the transaction details that are shown to the user:
-
Transaction amount and currency
-
Payment instrument name and icon
-
Payee name and origin
This could lead to a spoofing attack, in which a merchant presents incorrect data to the user. For example, the merchant could tell the bank (in the backend) that it is initiating a purchase of $100, but then pass $1 to the SPC API (and thus show the user a $1 transaction to verify). Or the merchant could provide the correct transaction details but pass Secure Payment Confirmation credentials that don’t match what the Relying Party expects.
Secure Payment Confirmation actually makes defeating this kind of attack easier than it currently is on the web. In online payments today, the bank has to trust that the merchant showed the user the correct amount in their checkout flow (and any fraud discoveries are post-payment, when the user checks their account statement).
10.3. Lack of user activation requirement
If the user agent does not require user activation, as outlined in § 4.1.4 Modification of user activation requirement, some additional security mitigations should be considered. Not requiring user activation increases the risk of spam and click-jacking attacks, by allowing a Secure Payment Confirmation flow to be initiated without the user interacting with the page immediately beforehand.
In order to mitigate spam, the user agent may decide to enforce a user activation requirement after some threshold, for example after the user has already been shown a Secure Payment Confirmation flow without a user activation on the current page. In order to mitigate click-jacking attacks, the user agent may implement a time threshold in which clicks are ignored immediately after a dialog is shown.
Another relevant mitigation exists in PaymentRequest.show()
: the Payment Request API
requires the document to be visible, and thus SPC cannot be triggered from a
background tab, minimized window, or other similar hidden situations.
11. Privacy Considerations
As this specification builds on top of WebAuthn, the WebAuthn Privacy Considerations are applicable. The below subsections comprise the current Secure Payment Confirmation-specific privacy considerations, where this specification diverges from WebAuthn.
11.1. Registration in a Cross-Origin iframe
Unlike WebAuthn, this specification allows the creation of credentials in a cross-origin iframe (as long as the appropriate Permission Policy is set on the iframe). That is, if site A embeds an iframe from site B, with the "payment" policy set, then site B may initiate a credential creation for site B within that iframe.
NOTE: Allowing credential creation in cross-origin iframes is currently under discussion in the WebAuthn Working Group, and thus may move from this specification to WebAuthn in the future.
Allowing credential creation in a cross-origin iframe presents a risk that an
iframe may attempt to trick a user into registering a credential. That
credential could then be used for tracking (see WebAuthn issue
1656). To mitigate such an
attack, this specification requires that a call to navigator.credentials.create()
inside a
cross-origin iframe may only be invoked when the iframe has transient
activation (e.g., via a click or press from the user).
NOTE: Requiring user activation for WebAuthn APIs in general is under discussion in the WebAuthn WG too; see issue #1293.
11.2. Probing for credential ids
As per WebAuthn’s section on Authentication Ceremony Privacy, implementors of Secure Payment Confirmation must make sure not to enable malicious callers (who now may not even be the Relying Party) to distinguish between these cases:
-
A credential is not available.
-
A credential is available, but the user does not consent to use it.
If the above cases are distinguishable, information is leaked by which a malicious Relying Party could identify the user by probing for which credentials are available.
Section § 4.1.9 Steps to check if a payment can be made gives normative steps to mitigate this risk.
11.3. Joining different payment instruments
If a Relying Party uses the same credentials for a given user across multiple payment instruments, this might allow a merchant to join information about payment instruments that might otherwise not be linked. That is, across two different transactions that a user U performs with payment instruments P1 and P2 (either on the same merchant M, or two colluding merchants M1 and M2), the merchant(s) may now be able to learn that P1 and P2 are for the same user.
For many current online payment flows this may not be a significant risk, as the user often provides sufficient information to do this joining anyway (e.g., name, email address, shipping address).
However, if payment methods that involve less identifying information (e.g., tokenization) become commonplace, it is important that ecosystem stakeholders take steps to preserve user privacy. For example:
-
Payment systems might establish rules that place limits on storage of credential ID(s) by third parties.
-
When a Relying Party assigns multiple instruments to a single SPC credential, it might choose not to share that credential ID with other parties. In this case, the Relying Party could still use the SPC credential itself (in either a first-party or third-party context) to authenticate the user.
-
A Relying Party (e.g., a bank) might enable the user to register a distinct SPC credential per payment instrument. This would not prevent the Relying Party from joining those accounts internally.
11.4. Credential ID(s) as a tracking vector
Even for a single payment instrument, the credential ID(s) returned by the Relying Party could be used by a malicious entity as a tracking vector, as they are strong, cross-site identifiers. However in order to obtain them from the Relying Party, the merchant already needs an as-strong identifier to give to the Relying Party (e.g., the credit card number).
11.5. User opt out
The API option showOptOut
tells the
user agent to provide a way for the user to indicate they wish to opt out of the
relying party’s storage of information. When the user invokes this opt out, an OptOutError
is returned to the caller to indicate the user’s intent to opt
out. It is then up to the caller to act on the opt out, e.g. by clearing payment
information stored for the user.
Implementors must make sure that the return of an OptOutError
does not
reveal that the user has credentials but did not complete an authentication.
This can be mitigated by similar means as § 11.2 Probing for credential ids, e.g. by also providing the user an
opportunity to opt out on the interstitial UX in the case where a credential
match is not found.
This is not intended to be a mechanism to delete browser data or credentials - it is for the developer to prompt for opt out via the user agent. The user agent should make this clear to the user, for example with some clarifying text: "This provider may have stored information about your payment method, which you can request to be deleted."
12. Accessibility Considerations
User agents render the icon
and displayName
together. Relying parties ensure the accessibility of the icon presentation by providing sufficient information via the displayName
(e.g., if the icon represents a bank, by including the bank name in the displayName
).
User Agents implementing this specification should follow both WebAuthn’s Accessibility Considerations and PaymentRequest’s Accessibility Considerations.
13. Internationalization Considerations
Callers of the API should express the desired locale of the
transaction dialog as well as the localization of any displayable
strings via the locale
member. In
general this member should match the localization of the page where
the request originates (such as by querying the lang
attribute of
the button triggering the request).
This specification does not (yet) include mechanisms for callers to
associate language or direction metadata with the displayable strings
they provide as input to the API (e.g., displayName
).
In the meantime, callers of the API should:
-
Aim for consistency between values of
locale
(when provided) and the language of displayable strings. -
Ensure that direction changes within a string will be correctly rendered when the string is displayed (see How to use Unicode controls for bidi text and Inline changes to base direction for more information).
Implementations (and other processes attempting to display values) should apply bidi isolation around displayable string values when inserting them into the user interface. They should set the direction when it is known, or default to first-strong ("auto") when it is not.
14. IANA Considerations
This section adds the below-listed extension identifier to the IANA "WebAuthn Extension Identifiers" registry [IANA-WebAuthn-Registries] established by [RFC8809].
-
WebAuthn Extension Identifier: payment
-
Description: This extension supports the following functionality defined by the Secure Payment Confirmation API: (1) it allows credential creation in a cross-origin iframe (2) it allows a party other than the Relying Party to use the credential to perform an authentication ceremony on behalf of the Relying Party, and (3) it allows the browser to identify and cache Secure Payment Confirmation credentials. For discussion of important ways in which SPC differs from Web Authentication, see in particular § 10 Security Considerations and § 11 Privacy Considerations
-
Specification Document: Section § 5 WebAuthn Extension - "payment" of this specification
-
Change Controller: W3C Web Payments Working Group
-
Notes: Registration follows 3 May 2023 discussion with the Web Authentication Working Group.