Promotional Offers not Working - Unable to Purchase (Please contact developer for support)

[tonytang0114]

Relevant issue is below
hyochan/react-native-iap#2715

I have read the documentation on apple developer regarding promotional offer. I have successfully generated signature, nonce, keyId from backend and passed to client and apply to withOffer section.

It returns a dialog "Unable to Purchase (Please contact developer for support)" After clicking the "Purchase button". It does show the correct promotion info from app store dialog side.

I have already done the following checklist.

1. Create Promotional offer from a base plan
2. Make sure the sandbox user is an active or elipsed subscriber under the same subscription group
3. Make sure I am using the In-App-Purchase key with correct key Id.
4. Make sure everything from the server side and client side are synchronized (signature, offer Id, timestamp, productId, nonce, keyId)
5. I have double checked the signature I generated. I am using C# web api with ECDSA and SHA256

I am not sure if I have done something wrong or there is a bug here
Do anyone face the same issues here? Thanks

Client code

        var  applePromotionSignature =
                    await subscriptionRepository.getAppleSubscriptionSignature(
                  constants.diamondMemberYearlyPlan,
                  promotionalOffers.first.id,
                );

        var offerId =
            productIOS!.subscriptionInfoIOS!.promotionalOffers!.first.id;

        await sameIAp.requestPurchase(
          RequestPurchaseProps.subs((
            android: null,
            ios: RequestSubscriptionIosProps(
              sku: constants.diamondMemberYearlyPlan,
              appAccountToken: '',
              withOffer: DiscountOfferInputIOS(
                identifier: offerId,
                keyIdentifier: applePromotionSignature!.keyId,
                nonce: applePromotionSignature!.nonce,
                signature: applePromotionSignature!.signature,
                timestamp: applePromotionSignature!.timestamp,
              ),
            ),
            useAlternativeBilling: null,
          )),
        );

Server code in C#

Controller

        [HttpGet("Signature/productId/{productId}/offerId/{offerId}")]
        public async Task<IActionResult> GetAppleSubscriptionSignature(string productId, string offerId)
        {
            var keyFileName = _configuration.GetValue<string>("AppleIAPSubscriptionKeyFileName");
            var bundleId = _configuration.GetValue<string>("AppleClientId");
            var keyId = _configuration.GetValue<string>("AppleIAPPromotionKeyId");
            var nonce = Guid.NewGuid().ToString().ToLowerInvariant();
            // var nowMs = DateTimeOffset.Now.ToUnixTimeSeconds();
            long timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();


            var signature = await _subscriptionService.GenerateSignature(keyFileName, bundleId, keyId, productId, offerId, "", nonce, timestamp);
            if (signature != null)
            {
                var appleSubscriptionSignature = new AppleSubscriptionSignature()
                {
                    offerId = offerId,
                    productId = productId,
                    keyId = keyId,
                    nonce = nonce,
                    signature = signature,
                    timestamp = timestamp
                };
                return Ok(appleSubscriptionSignature);
            }
            return NotFound();
        }

Service class

        public async Task<string> GenerateSignature(
            string keyFileName,
            string bundleId,
            string keyId,
            string productId,
            string offerId,
            string appAccountTokenOrEmpty,
            string nonce,
            long timestampMs)
        {
            // 1) Build the string to sign joined by U+2063 (INVISIBLE SEPARATOR)
            const char SEP = '\u2063';
            var stringToSign = string.Join(SEP, [
                    bundleId,
                    keyId,
                    productId,
                    offerId,
                    appAccountTokenOrEmpty ?? string.Empty,
                    nonce,
                    timestampMs.ToString(CultureInfo.InvariantCulture)
                ]);

            // 2) Load P-256 private key from PKCS#8 PEM
            using var ecdsa = ECDsa.Create();
            var p8PrivateKeyPem = await File.ReadAllTextAsync(keyFileName);
            ImportPkcs8Pem(ecdsa, p8PrivateKeyPem);

            // 3) Sign with ECDSA-SHA256 and return Base64 (DER)
            var data = Encoding.UTF8.GetBytes(stringToSign);
            var derSig = ecdsa.SignData(data, HashAlgorithmName.SHA256);
            return Convert.ToBase64String(derSig);
        }

        private static void ImportPkcs8Pem(ECDsa ecdsa, string pem)
        {

            var pkcs8 = pem
                .Replace("-----BEGIN PRIVATE KEY-----", string.Empty)
                .Replace("-----END PRIVATE KEY-----", string.Empty)
                .Replace("\r", string.Empty)
                .Replace("\n", string.Empty)
                .Trim();
            var keyBytes = Convert.FromBase64String(pkcs8);
            ecdsa.ImportPkcs8PrivateKey(keyBytes, out _);
        }

📱 version

flutter_inapp_purchase: ^7.0.1

[hyochan]

Thanks for the detailed report! I investigated this issue and the signature handling looks correct in the code, but there are several common causes for this error.

Most Likely Cause: Sandbox Subscription State

In Apple's sandbox environment, you must wait for the current subscription to expire before testing promotional offers.
👉 This is a known Apple limitation that often causes the “Unable to Purchase” error.

---

Other Things to Check

• Nonce format
  → Verify it's a valid UUID format. (Your code looks correct, but double-check the actual value.)

• Signature string
  → Ensure the concatenation order is exact:

  bundleId, keyId, productId, offerId, appAccountToken (empty string if not used), nonce, timestamp
  

• Environment key
  → Make sure you're using the Sandbox IAP key for sandbox testing, not the production key.

• Offer eligibility
  → The sandbox user must be eligible (current or lapsed subscriber).

---

Quick Debugging Tip

Try logging the actual values being sent:

print('Nonce: ${applePromotionSignature.nonce}');
print('Signature length: ${applePromotionSignature.signature.length}');

---

Workaround for Testing

Wait for the sandbox subscription to expire (they expire quickly in sandbox),
then retry the promotional offer purchase.

👉 Can you verify if your sandbox subscription is currently active or expired when you're testing?

---

Update: Improved Error Handling in v7.1.2

I’ve improved the error handling for promotional offers in version 7.1.2,
which provides more detailed error messages to help identify the exact cause.

👉 Please try upgrading and let me know what specific error you see.

[tonytang0114]

Hi @hyochan

Thanks for your help. Appreciate a lot.

I have two subscriptions in the same group. I first subscribe to one lower grade. Wait 5 minutes until it expires in sandbox. Then try to subscribe to the one that contains promotion offer.

---

I have double checked the values returned from the server matches the values I sent to client. The same error occurs.

Value return server
[log] applePromotionSignature: keyId: keyId1, nonce: 41323e82-462d-4f91-ae67-1a53bd43da3c, signature: W6QH41eAFuB4Fz+tJHnV0SE4K12xHV5G5PSqYQ4v9C20AWbSz8zweyyyJKU2fkymK43HQJ5Dx3Af6dRrW82HPg==, timestamp: 1759991826055.0

Value in client before passed to requestSubscription
[log] productId: ProductId1, offerId: OfferId1, keyId: keyId1, nonce: 41323e82-462d-4f91-ae67-1a53bd43da3c, signature: W6QH41eAFuB4Fz+tJHnV0SE4K12xHV5G5PSqYQ4v9C20AWbSz8zweyyyJKU2fkymK43HQJ5Dx3Af6dRrW82HPg==, timestamp: 1759991826055.0

I have hided the ProductId and OfferId1 name

---

I am using flutter_inapp_purchase 7.1.2 and these are the logs returned from _purchaseErrorSubscription listener

[log] errorCode: purchase-error
[log] errorDetails:
[log] errorSource:
[log] errorMessage: Promotional offer purchase failed: (Unable to Purchase)
      
      Common causes:
      1. Invalid signature - verify server generates correct signature
      2. Sandbox testing - ensure current subscription has expired before testing offers
      3. Offer eligibility - user may not be eligible for this promotional offer
      4. Key mismatch - verify using correct In-App Purchase key for environment

---

Environment key
→ Make sure you're using the Sandbox IAP key for sandbox testing, not the production key.
I created the key in "Users and Access" -> Integrations -> In App Purchase -> Plus sign and create the key
Do I miss anything? How do I know if it is Sandbox IAP key or product key?

Thanks

[hyochan]

Thanks for the detailed logs! The values look correct. Let me help narrow down the issue. Looking at your signature generation, I see a potential issue.

Can you verify these points in your C# code:

---

1. Empty appAccountToken handling

In your Flutter code, you're using:

appAccountToken: '',

Make sure your server signature generation uses exactly an empty string (not null) for the appAccountToken parameter when constructing the signature string.

---

2. Signature string construction

Your signature string should be:

bundleId + '\u2063' + keyId + '\u2063' + productId + '\u2063' + 
offerId + '\u2063' + '' + '\u2063' + nonce + '\u2063' + '1759991826055'

Notice the empty string between offerId and nonce (for appAccountToken).

---

3. Verify the exact signature input

Can you add logging in your C# code to print the exact string being signed before signing? Like:

Console.WriteLine($"String to sign: [{stringToSign}]");
Console.WriteLine($"String length: {stringToSign.Length}");

---

4. Common C# ECDSA issue

Make sure you're using SignData() not SignHash().
Your code looks correct, but double-check you're using:

var derSig = ecdsa.SignData(data, HashAlgorithmName.SHA256);

And NOT:

var hash = SHA256.HashData(data);
var sig = ecdsa.SignHash(hash);  // ❌ Wrong!

---

5. About the IAP Key

You're right — there's no separate "sandbox" vs "production" key.
The same key works for both environments.

That part of my previous answer was incorrect, sorry for the confusion!

[hyochan]

@tonytang0114 It looks very likely that the issue is caused by how appAccountToken is handled during signature generation.

In RequestPurchaseIosProps, there’s an optional field:

appAccountToken: String?

And in the your Flutter code, it’s explicitly set to an empty string:

ios: RequestSubscriptionIosProps(
  sku: constants.diamondMemberYearlyPlan,
  appAccountToken: '',  // ← empty string
  withOffer: DiscountOfferInputIOS(...)
)

However, in the C# server code, the signature string is constructed like this:

var stringToSign = string.Join(SEP, [
    bundleId,
    keyId,
    productId,
    offerId,
    appAccountTokenOrEmpty ?? string.Empty,  // ← here
    nonce,
    timestampMs.ToString(...)
]);

If the client sends appAccountToken: '' (empty string) but the server uses null or any other value instead of an empty string when building the signature,
the generated signature will not match, and Apple will return "Unable to Purchase" during the promotional offer purchase flow.

So, this seems to be the most likely root cause.

[hyochan]

@tonytang0114 Also just updated to 7.1.4 with fixed error messages!

[tonytang0114]

Hi @hyochan Thanks you so much for your reply. Sorry to take you time.
I have ensured to pass "" for appAccountToken. But same thing happened "Unable to purchase".

---

Logs on server side

2025-10-09 15:22:27.681 +00:00 [INF] offerId: OfferId1
2025-10-09 15:22:27.682 +00:00 [INF] appAccountTokenOrEmpty: 
2025-10-09 15:22:27.683 +00:00 [INF] nonce: 7305cd7c-e27c-4397-998f-1b50e74c5d42
2025-10-09 15:22:27.683 +00:00 [INF] timestampMs: 1760023347671
2025-10-09 15:22:27.684 +00:00 [INF] String to sign: com.company.app�FZ85522GLH�ProductId1�oOferId1��7305cd7c-e27c-4397-998f-1b50e74c5d42�1760023347671
2025-10-09 15:22:27.698 +00:00 [INF] String length: 102
2025-10-09 15:22:27.714 +00:00 [INF] Signature: IQh2FwLReFtJPWODuYqA9VIhA131izUCZ9RV6MTk2IGtajyGLCbhC/2YK5JhgIVaPIw3K2VLQSuGsZZH5DO2lg==

I have hided offerId, bundleId and productId. I could send to you If you need. Assume offerId to be OfferId1, bundleId to be com.company.app and productId to be ProductId1

---

Logs on client side after getting the apple promotion signature
[log] applePromotionSignature: productId: ProductId1, offerId: OfferId1, keyId: FZ85522GLH, nonce: 7305cd7c-e27c-4397-998f-1b50e74c5d42, signature: IQh2FwLReFtJPWODuYqA9VIhA131izUCZ9RV6MTk2IGtajyGLCbhC/2YK5JhgIVaPIw3K2VLQSuGsZZH5DO2lg==, timestamp: 1760023347671.0

Logs on client side before requesting Purchase
[log] productId: ProductId1, offerId: OfferId1, keyId: FZ85522GLH, nonce: 7305cd7c-e27c-4397-998f-1b50e74c5d42, signature: IQh2FwLReFtJPWODuYqA9VIhA131izUCZ9RV6MTk2IGtajyGLCbhC/2YK5JhgIVaPIw3K2VLQSuGsZZH5DO2lg==, timestamp: 1760023347671.0

---

Logs after purchasing error

[log] productId: ProductId1, offerId: OfferId1, keyId: FZ85522GLH, nonce: 7305cd7c-e27c-4397-998f-1b50e74c5d42, signature: IQh2FwLReFtJPWODuYqA9VIhA131izUCZ9RV6MTk2IGtajyGLCbhC/2YK5JhgIVaPIw3K2VLQSuGsZZH5DO2lg==, timestamp: 1760023347671.0
[log] requestPromotionOfferPurchase error: [flutter_inapp_purchase]: PurchaseError: Failed to request purchase: PlatformException(purchase-error, Promotional offer purchase failed: Unable to purchase
      Common causes:
      1. Invalid signature - verify server generates correct signature with exact parameter order
      2. Empty appAccountToken - ensure empty string ('') is used in signature, not null
      3. Sandbox testing - ensure current subscription has expired before testing offers
      4. Offer eligibility - user may not be eligible for this promotional offer, ProductId1, null)
      5. 

---

New server code (Controller) I have passed "" to be appAccountTokenOrEmpty so it will be empty.

        [HttpGet("AppleSubscriptionSignature/productId/{productId}/offerId/{offerId}")]
        public async Task<IActionResult> GetAppleSubscriptionSignature(string productId, string offerId)
        {
            if (!int.TryParse(_userProvider.GetUserId(), out int userId))
            {
                return StatusCode(500);
            }
            var keyFileName = _configuration.GetValue<string>("AppleIAPSubscriptionKeyFileName");
            var bundleId = _configuration.GetValue<string>("AppleClientId");
            var keyId = _configuration.GetValue<string>("AppleIAPPromotionKeyId");
            var nonce = Guid.NewGuid().ToString().ToLowerInvariant();
            long timestamp = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();

            var signature = await _subscriptionService.GenerateSignature(keyFileName, bundleId, keyId, productId, offerId, "", nonce, timestamp);
            if (signature != null)
            {
                Log.Information($"Signature: {signature}");
                var appleSubscriptionSignature = new AppleSubscriptionSignature()
                {
                    offerId = offerId,
                    productId = productId,
                    keyId = keyId,
                    nonce = nonce,
                    signature = signature,
                    timestamp = timestamp
                };
                return Ok(appleSubscriptionSignature);
            }
            return NotFound();
        }

Service class

?? is a nullable operator if appAccountTokenOrEmpty is null it will be assigned string.Empty.
appAccountTokenOrEmpty ?? string.Empty,

        public async Task<string> GenerateSignature(
            string keyFileName,
            string bundleId,
            string keyId,
            string productId,
            string offerId,
            string appAccountTokenOrEmpty,
            string nonce,
            long timestampMs)
        {
            // 1) Build the string to sign joined by U+2063 (INVISIBLE SEPARATOR)
            const char SEP = '\u2063';
            var stringToSign = string.Join(SEP, [
                    bundleId,
                    keyId,
                    productId,
                    offerId,
                    appAccountTokenOrEmpty ?? string.Empty,
                    nonce,
                    timestampMs.ToString(CultureInfo.InvariantCulture)
                ]);

            Log.Information($"keyFileName: {keyFileName}");
            Log.Information($"bundleId: {bundleId}");
            Log.Information($"keyId: {keyId}");
            Log.Information($"productId: {productId}");
            Log.Information($"offerId: {offerId}");
            Log.Information($"appAccountTokenOrEmpty: {appAccountTokenOrEmpty}");
            Log.Information($"nonce: {nonce}");
            Log.Information($"timestampMs: {timestampMs}");
            Log.Information($"String to sign: {stringToSign}");

            Log.Information($"String to sign: [{stringToSign}]");
            Log.Information($"String length: {stringToSign.Length}");
            // 2) Load P-256 private key from PKCS#8 PEM
            using var ecdsa = ECDsa.Create();
            var p8PrivateKeyPem = await File.ReadAllTextAsync(keyFileName);
            ImportPkcs8Pem(ecdsa, p8PrivateKeyPem);

            // 3) Sign with ECDSA-SHA256 and return Base64 (DER)
            var data = Encoding.UTF8.GetBytes(stringToSign);
            var derSig = ecdsa.SignData(data, HashAlgorithmName.SHA256);
            return Convert.ToBase64String(derSig);
        }

        private static void ImportPkcs8Pem(ECDsa ecdsa, string pem)
        {

            var pkcs8 = pem
                .Replace("-----BEGIN PRIVATE KEY-----", string.Empty)
                .Replace("-----END PRIVATE KEY-----", string.Empty)
                .Replace("\r", string.Empty)
                .Replace("\n", string.Empty)
                .Trim();
            var keyBytes = Convert.FromBase64String(pkcs8);
            ecdsa.ImportPkcs8PrivateKey(keyBytes, out _);
        }

---

Flutter client side code

  Future<void> requestPromotionOfferPurchase() async {
    if (Platform.isIOS) {
      if (applePromotionSignature == null || productIOS == null) return;
      Helper.showLoaderDialog(context);

      var productId = applePromotionSignature!.productId;
      var offerId = applePromotionSignature!.offerId;
      var keyId = applePromotionSignature!.keyId;
      var nonce = applePromotionSignature!.nonce;
      var signature = applePromotionSignature!.signature;
      var timestamp = applePromotionSignature!.timestamp;
      log('productId: $productId, offerId: $offerId, keyId: $keyId, nonce: $nonce, signature: $signature, timestamp: $timestamp');

      try {
        await sameIAp.requestPurchase(
          RequestPurchaseProps.subs((
            android: null,
            ios: RequestSubscriptionIosProps(
              sku: applePromotionSignature!.productId,
              appAccountToken: '',
              withOffer: DiscountOfferInputIOS(
                identifier: applePromotionSignature!.offerId,
                keyIdentifier: applePromotionSignature!.keyId,
                nonce: applePromotionSignature!.nonce,
                signature: applePromotionSignature!.signature,
                timestamp: applePromotionSignature!.timestamp,
              ),
            ),
            useAlternativeBilling: null,
          )),
        );
      } catch (ex) {
        log('requestPromotionOfferPurchase error: $ex');
        log('requestPromotionOfferPurchase error: ${ex.toString()}');
        if (Helper.isLoaderVisible) {
          Navigator.of(context).pop();
        }
        showCustomSnackbar(
            context: context,
            message: AppLocalizations.of(context).vip_subscribe_failed,
            type: SnackBarType.error);
      }
    } else {
      if (productAndroid == null) return;
      await sameIAp.requestPurchase(
        RequestPurchaseProps.subs((
          android: RequestSubscriptionAndroidProps(
            subscriptionOffers: [
              AndroidSubscriptionOfferInput(
                offerToken: productAndroid!
                    .subscriptionOfferDetailsAndroid.first.offerToken,
                sku: constants.diamondMemberYearlyPlan,
              )
            ],
            skus: [constants.diamondMemberYearlyPlan],
          ),
          ios: null,
          useAlternativeBilling: null,
        )),
      );
    }
  }

I am using flutter_inapp_purchase 7.1.4

[hyochan]

Hi @tonytang0114,

I apologize for the confusion in my previous responses.
I was investigating your issue further and found something that might be the root cause.

I'm not very familiar with .NET, so I was a bit confused initially, but after diving deeper into the signature format requirements, I found a potential issue.

---

Possible Issue: Signature Format

Your C# code currently uses:

var derSig = ecdsa.SignData(data, HashAlgorithmName.SHA256);

By default, .NET’s ECDsa.SignData() returns signatures in IEEE P1363 format,
which is just r || s concatenated as 64 bytes for P-256.

However, Apple StoreKit requires signatures in DER/ASN.1 format (RFC 3279),
which has a completely different binary structure.

---

Suggested Fix

Try adding the DSASignatureFormat.Rfc3279DerSequence parameter:

var derSig = ecdsa.SignData(
    data, 
    HashAlgorithmName.SHA256, 
    DSASignatureFormat.Rfc3279DerSequence
);

Note:
This parameter is available in .NET 5 or later.
If you're using an earlier version, you might need to upgrade
or use a library like BouncyCastle to generate DER-formatted signatures.

---

Complete Updated Method

public async Task<string> GenerateSignature(
    string keyFileName,
    string bundleId,
    string keyId,
    string productId,
    string offerId,
    string appAccountTokenOrEmpty,
    string nonce,
    long timestampMs)
{
    const char SEP = '\u2063';
    var stringToSign = string.Join(SEP, [
        bundleId,
        keyId,
        productId,
        offerId,
        appAccountTokenOrEmpty ?? string.Empty,
        nonce,
        timestampMs.ToString(CultureInfo.InvariantCulture)
    ]);

    using var ecdsa = ECDsa.Create();
    var p8PrivateKeyPem = await File.ReadAllTextAsync(keyFileName);
    ImportPkcs8Pem(ecdsa, p8PrivateKeyPem);

    var data = Encoding.UTF8.GetBytes(stringToSign);
    var derSig = ecdsa.SignData(
        data, 
        HashAlgorithmName.SHA256, 
        DSASignatureFormat.Rfc3279DerSequence
    );
    
    return Convert.ToBase64String(derSig);
}

---

I believe this might be why Apple is rejecting the signature
even though all your parameters look correct.

Could you give this a try and let me know if it resolves the issue?

[hyochan]

I also found some references here. Hope this helps.

Reference

Microsoft Docs: ECDsa.SignData with DSASignatureFormat
Stack Overflow: ECDSA Signatures OpenSSL vs .NET

• .NET default: IEEE P1363 format (64 bytes for P-256: 32-byte r + 32-byte s)
• Apple expects: DER/ASN.1 format (variable length with ASN.1 structure)

[tonytang0114]

Hi @hyochan ,

Thanks so much! Your solution is working. I can no longer see the "Unable to Purchase" dialog and it said it is purchased. I will test the transaction right now.

I could summarize this ticket info into the documentation if you would like to.

[hyochan]

@tonytang0114 That would be wonderful! Thanks so much for the suggestion

[tonytang0114]

@hyochan Sure. I will have a look on the contribution.md

I have some other findings.

In apple documentations here. Promotional offer is eligible to existing or expired subscription users.

When I subscribe to a lower rank schedule and subscribe to the higher rank with promotional offer. The purchasedUpdateListener will listen to the lower rank subscription with the status "RENEWAL" and the new status for the one with promotional offer will be delayed.

Is it normal in sandbox environment that the subscription will be renewed every 5 minutes.? But in production it will be renewed every month so in this case it actually won't affect the production?

---

The way how I use flutter_inapp_purchase is to init connection at main.dart before the apps run. Whenever I go to a page that need subscription. I will open connection to purchase updated listener and purchase error listener. Then close these two listeners after leaving that page.

Is it the best practice of using this plugin? By Keeping the connection open all the time?

Thanks