promotional_offer.ts

// Copyright (c) 2023 Apple Inc. Licensed under MIT License.

import { KeyObject, createPrivateKey, createSign } from "crypto";

export class PromotionalOfferSignatureCreator {

    private signingKey: KeyObject;
    private keyId: string;
    private bundleId: string;

    public constructor(signingKey: string, keyId: string, bundleId: string) {
        this.signingKey = createPrivateKey(signingKey);
        this.keyId = keyId
        this.bundleId = bundleId
    }
    
    /**
     * Create a promotional offer signature
     *
     * {@link https://developer.apple.com/documentation/storekit/in-app_purchase/original_api_for_in-app_purchase/subscriptions_and_offers/generating_a_signature_for_promotional_offers Generating a signature for promotional offers}
     * @param productIdentifier The subscription product identifier
     * @param subscriptionOfferID The subscription discount identifier
     * @param applicationUsername An optional string value that you define; may be an empty string
     * @param nonce A one-time UUID value that your server generates. Generate a new nonce for every signature.
     * @param timestamp A timestamp your server generates in UNIX time format, in milliseconds. The timestamp keeps the offer active for 24 hours.
     * @return The Base64 encoded signature
     */
    public createSignature(productIdentifier: string, subscriptionOfferID: string, applicationUsername: string, nonce: string, timestamp: number): string {
        const payload = this.bundleId + '\u2063' +
            this.keyId + '\u2063' +
            productIdentifier + '\u2063' +
            subscriptionOfferID + '\u2063' +
            applicationUsername.toLowerCase()  + '\u2063'+
            nonce.toLowerCase() + '\u2063' +
            timestamp;
        const sign = createSign('SHA256')
        sign.update(payload)
        sign.end()
        return sign.sign(this.signingKey).toString('base64')
    }
}