diff --git a/README.md b/README.md
index 70012a8..0d5751b 100644
--- a/README.md
+++ b/README.md
@@ -6,43 +6,112 @@ The goal is to improve payment confirmation experience with the help of WebAuthn
 
 ## Creating a credential
 
-A new credential type is introduced for `navigator.credentials.create()` that stores the name and the icons of a payment instrument.
+A new `SecurePaymentCredential` credential type is introduced for `navigator.credentials.create()` to bind descriptions of a payment instrument, i.e. a name and an icon, with a vanilla [PublicKeyCredential].
 
+Proposed new spec that extends [Web Authentication]:
+```webidl
+[SecureContext, Exposed=Window]
+interface SecurePaymentCredential : PublicKeyCredential {
+};
+
+partial dictionary CredentialCreationOptions {
+  SecurePaymentCredentialCreationOptions securePayment;
+};
+
+dictionary SecurePaymentCredentialCreationOptions {
+  required PublicKeyCredentialRpEntity rp;
+  required SecurePaymentCredentialInstrument instrument;
+  required BufferSource challenge;
+  required sequece<PublicKeyCredentialParameters> pubKeyCredParams;
+  unsigned long timeout;
+  
+  // PublicKeyCredentialCreationOption attributes that are intentionall omitted:
+  // user: For a SecurePaymentCredential, |instrument| is analogous to |user|.
+  // excludeCredentials: No payment use case has been proposed for this field.
+  // attestation: Authenticator attestation is considered an anti-pattern for adoption so will not be supported.
+  // extensions: No payment use case has been proposed for this field.
+};
+
+dictionary SecurePaymentCredentialInstrument {
+  required DOMString instrumentId;
+  required DOMString displayName;
+  required USVString icon;
+};
+```
+
+Example usage:
 ```javascript
-const publicKeyCredentialCreationOptions = {
-  paymentInstrument: {
-    name: 'Mastercard****4444',
-    icons: [{
-  	  'src': 'icon.png',
-  	  'sizes': '48x48',
-  	  'type': 'image/png',
-    }],
+const securePaymentConfirmationCredentialCreationOptions = {
+  instrument: {
+    instrumentId: "Q1J4AwSWD4Dx6q1DTo0MB21XDAV76",
+    displayName: 'Mastercard****4444',
+    icon: 'icon.png'
   },
   challenge,
   rp,
-  user,
   pubKeyCredParams,
-  authenticatorSelection,
   timeout,
-  attestation,
 };
 
+// This returns a SecurePaymentCredential, which is a subtype of PublicKeyCredential.
 const credential = await navigator.credentials.create({
-  publicKey: publicKeyCredentialCreationOptions
+  securePayment: securePaymentCredentialCreationOptions
 });
 ```
 
 See the [Guide to Web Authentication](https://webauthn.guide/) for mode details about the `navigator.credentials` API.
 
+### [Future] Register an existing PublicKeyCredential for Secure Payment Confirmation
+
+The relying party of an existing PublicKeyCredential can bind it for use in Secure Payment Confirmation.
+
+Proposed new spec that extends [Web Authentication]:
+```webidl
+partial dictionary SecurePaymentCredentialCreationOptions {
+  PublicKeyCredentialDescriptor existingCredential;
+};
+
+partial interface SecurePaymentCredential {
+  readonly attribute boolean createdCredential;
+};
+```
+
+Example usage:
+```javascript
+const securePaymentConfirmationCredentialCreationOptions = {
+  instrument: {
+    instrumentId: "Q1J4AwSWD4Dx6q1DTo0MB21XDAV76",
+    displayName: 'Mastercard****4444',
+    icon: 'icon.png'
+  },
+  existingCredential: {
+    type: "public-key",
+    id: Uint8Array.from(credentialId, c => c.charCodeAt(0))
+  },
+  challenge,
+  rp,
+  pubKeyCredParams,
+  timeout
+};
+
+// Bind |instrument| to |credentialId|, or create a new credential if |credentialId| doesn't exist.
+const credential = await navigator.credentials.create({
+  securePayment: securePaymentCredentialCreationOptions
+});
+
+// |credential.createdCredential| is true if the specified credential does not exist and a new one is created instead.
+```
+
+
 ## Querying the credential
 
-Only the creator of the credential can query it through the `navigator.credentials.get()` API.
+The creator of the credential can query it through the `navigator.credentials.get()` API, as if it is a vanilla PublicKeyCredential.
 
 ```javascript
 const publicKeyCredentialRequestOptions = {
   challenge,
   allowCredentials: [{
-    id: ['ADSUllKQmbqdGtpu4sjseh4cg2TxSvrbcHDTBsv4NSSX9...'],
+    id: Uint8Array.from(credentialId, c => c.charCodeAt(0)),
     type,
     transports,
   }],
@@ -58,26 +127,56 @@ const credential = await navigator.credentials.get({
 
 Any origin may invoke the [Payment Request API](https://w3c.github.io/payment-request/) to prompt the user to verify a credential created by any other origin. The `PaymentRequest.show()` method must require a user gesture and display user interface with the amount of payment and the hostname of the top-level context where the `PaymentRequest` API was invoked.
 
+Proposed new `secure-payment-confirmation` payment method:
+
+```webidl
+dictionary SecurePaymentConfirmationRequest {
+  SecurePaymentConfirmationAction action;
+  required DOMString instrumentId;
+  required BufferSource networkData;
+  unsigned long timeout;
+  required USVString fallbackUrl;
+};
+
+enum SecurePaymentConfirmationAction {
+  "authenticate",
+};
+
+dictionary SecurePaymentConfirmationResponse {
+  required SecurePaymentConfirmationMerchantData merchantData;
+  required BufferSource networkData;
+  required AuthenticatorAssertionResponse assertion;
+};
+
+dictionary SecurePaymentConfirmationMerchantData {
+  required USVString merchantOrigin;
+  required PaymentCurrencyAmount total;
+};
+```
+
+Example usage:
+
 ```javascript
-const publicKeyCredentialRequestOptions = {
+const securePaymentConfirmationRequest = {
+  action: "authenticate",
+  instrumentId: "Q1J4AwSWD4Dx6q1DTo0MB21XDAV76",
   challenge,
-  allowCredentials: [{
-    id: ['ADSUllKQmbqdGtpu4sjseh4cg2TxSvrbcHDTBsv4NSSX9...'],
-    type,
-    transports,
-  }],
   timeout,
+  fallbackUrl: "https://fallback.example/url"
 };
 
 const request = new PaymentRequest(
   [{supportedMethods: 'secure-payment-confirmation',
-    data: {
-      publicKey: publicKeyCredentialRequestOptions,
-      fallback: 'https://fallback.example/url',
-    },
+    data: securePaymentConfirmationRequest
   }],
   {total: {label: 'total', amount: {currency: 'USD', value: '20.00'}}});
 const response = await request.show();
+
+// Merchant validates |response.merchantData| and sends |response| to issuer for authentication.
 ```
 
-If none of the credentials from `allowCredentials.id` are available, then the user agent will invoke the payment handler for the payment method specified in the `fallback` method. It is recommended that the `fallback` payment method supports just-in-time installation of a payment handler, so the user agent may install it just-in-time. This enables the payment handler to open a fallback URL in the payment handler window.
+If the payment instrument specified by `instrumentId` is not available or if the user failed to authenticate, then the
+user agent will open `fallbackUrl` inside the Secure Modal Window.
+
+[PublicKeyCredential]: https://www.w3.org/TR/webauthn/#iface-pkcredential
+[WebAuthentication]: https://www.w3.org/TR/webauthn/