asn.1 - Flutter, in_app_purchase: way to show subscription expiration date or status

Question

Using the in_app_purchase: ^0.3.5+1 package, I'm trying to verify that a user has an active subscription on iOS.

I can get the productID and transactionDate, with purchaseDetails.productID & purchaseDetails.transactionDate. But I can't seem to get an expiration date or status. Watching the apple videos they talk about "status" heaps. I'm not sure if someone starts a free 3 day trial if the transaction date will know if they cancelled, so not confident to base everything off this transaction date.

I can get the purchaseDetails.verificationData.localVerificationData, which seems to be a ANS1 string, this may have more information but I can't seem to decode it. Does anyone now how to decode this for iOS? Or know another way to get subscription status?

I'm trying not to use a platform like RevenueCat (as they take a cut), they seem to offer features like this though.

Below is the ASN.1 string:

MIIbiQYJKoZIhvcNAQcCoIIbejCCG3YCAQExCzAJBgUrDgMCGgUAMIILKgYJKoZI
hvcNAQcBoIILGwSCCxcxggsTMAoCAQgCAQEEAhYAMAoCARQCAQEEAgwAMAsCAQEC
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question from:https://stackoverflow.com/questions/65648375/flutter-in-app-purchase-way-to-show-subscription-expiration-date-or-status

Answer 1

The string is indeed ASN.1 data. You can decode the string online, for example using https://lapo.it/asn1js/. The decoded data contains some dates fields, so maybe the information you need is there. You will need an ASN.1 schema in order to really understand which field means what.

In order to decode the ASN.1 data in your app, you need to use some ASN.1 decoding library You could also try to write a TLV decoder (tag-length-value) for ASN.1 BER (basic encoding rules), which will help you to get individual fields from the data. This should not be overly complicated to achieve. After you decode the ASN.1 BER data into TLVs, you could further decode only some specific fields that are useful to you.