receipt_utility.ts

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

import { ASN1HEX } from 'jsrsasign';

const IN_APP_TYPE_ID = 17;
const TRANSACTION_IDENTIFIER_TYPE_ID = 1703;
const ORIGINAL_TRANSACTION_IDENTIFIER_TYPE_ID = 1705;

export class ReceiptUtility {
    
    /**
     * Extracts a transaction id from an encoded App Receipt. Throws if the receipt does not match the expected format.
     * *NO validation* is performed on the receipt, and any data returned should only be used to call the App Store Server API.
     * @param appReceipt The unmodified app receipt
     * @returns A transaction id from the array of in-app purchases, null if the receipt contains no in-app purchases
     */
    extractTransactionIdFromAppReceipt(appReceipt: string): string | null {
        // Xcode receipts use indefinite length encoding, not supported by all parsers
        // Indefinite length encoding is only entered, but never left during parsing for receipts
        // We therefore round up indefinite length encoding to be the remaining length
        const prevGetVblenFunction = ASN1HEX.getVblen
        ASN1HEX.getVblen = function(s, idx) {
            const c = ASN1HEX.getL(s, idx)
            const oldResult = prevGetVblenFunction(s, idx)
            // Round up to the remaining length in the string, measured in bytes (2 hex values per byte)
            if (oldResult === 0 && c === '80') {
                return (s.length - idx) / 2
            }
            return oldResult
        }
        const prevGetLblen = ASN1HEX.getLblen
        ASN1HEX.getLblen = function(s, idx) {
            const oldResult = prevGetLblen(s, idx)
            // The length for the length byte for 80 00 is 1
            if (oldResult == -1) {
                return 1
            }
            return oldResult
        }
        try {
            let receiptInfo = ASN1HEX.getVbyList(Buffer.from(appReceipt, 'base64').toString('hex'), 0, [1, 0, 2, 1, 0]) as string
            if (receiptInfo.length > 2 && receiptInfo.startsWith('04')) {
                // We are still in an Octet String, Xcode wraps with an extra Octet, decode it here
                receiptInfo = ASN1HEX.getV(receiptInfo, 0)
            }
            let index = 0;
            while(ASN1HEX.getVbyList(receiptInfo, 0, [index, 0])) {
                const val = ASN1HEX.getVbyList(receiptInfo, 0, [index, 0]) as string
                if (IN_APP_TYPE_ID === parseInt(val, 16)) {
                    const inAppInfo = ASN1HEX.getVbyList(receiptInfo, 0, [index, 2]) as string
                    let inAppIndex = 0;
                    while(ASN1HEX.getVbyList(inAppInfo, 0, [inAppIndex, 0])) {
                        const val = ASN1HEX.getVbyList(inAppInfo, 0, [inAppIndex, 0]) as string
                        if (TRANSACTION_IDENTIFIER_TYPE_ID === parseInt(val, 16) || ORIGINAL_TRANSACTION_IDENTIFIER_TYPE_ID === parseInt(val, 16)) {
                            const transactionIdUTF8String = ASN1HEX.getVbyList(inAppInfo, 0, [inAppIndex, 2]) as string
                            const transactionId = ASN1HEX.getVbyList(transactionIdUTF8String, 0, []) as string
                            return Buffer.from(transactionId, 'hex').toString()
                        }
                        inAppIndex = inAppIndex + 1
                    }
                }
                index = index + 1
            }
            return null
        } finally {
            ASN1HEX.getLblen = prevGetLblen
            ASN1HEX.getVblen = prevGetVblenFunction
        }
    }

    /**
     * Extracts a transaction id from an encoded transactional receipt. Throws if the receipt does not match the expected format.
     * *NO validation* is performed on the receipt, and any data returned should only be used to call the App Store Server API.
     * @param transactionReceipt The unmodified transactionReceipt
     * @return A transaction id, or null if no transactionId is found in the receipt
     */
    extractTransactionIdFromTransactionReceipt(transactionReceipt: string): string | null {
        const topLevel = Buffer.from(transactionReceipt, 'base64').toString()
        const topLevelRegex = /"purchase-info"\s+=\s+"([a-zA-Z0-9+/=]+)";/
        const topLevelMatchResult = topLevel.match(topLevelRegex)        
        if (!topLevelMatchResult || topLevelMatchResult?.length !== 2) {
            return null
        }
        
        const purchaseInfo = Buffer.from(topLevelMatchResult[1], 'base64').toString()
        const purchaseInfoRegex = /"transaction-id"\s+=\s+"([a-zA-Z0-9+/=]+)";/
        const purchaseInfoMatchResult = purchaseInfo.match(purchaseInfoRegex)
        if (!purchaseInfoMatchResult || purchaseInfoMatchResult?.length !== 2) {
            return null
        }
        return purchaseInfoMatchResult[1]
    }
}