Implementing In-App Purchases with iOS Keychain Storage

Understanding In-App Purchases on iOS

In-app purchases are a popular feature used in mobile apps to offer additional content or functionality for purchase by users. This feature is particularly useful for developers who want to monetize their app without disrupting the user experience. In this article, we will explore how to implement in-app purchases on iOS using the iPhone’s keychain storage.

What are In-App Purchases?

In-app purchases allow users to buy and download additional content or features within an app. This can include digital goods such as music, movies, or e-books, as well as premium features like access to advanced tools or exclusive content. The process of in-app purchases involves the use of a secure token, which is used to authenticate the user’s purchase.

Storing Flags with In-App Purchases

When implementing in-app purchases on iOS, it’s essential to store flags that indicate whether the user has purchased certain features or content. These flags can be stored securely using the iPhone’s keychain storage. The keychain provides a secure and encrypted way to store sensitive data, such as passwords, credit card numbers, and other confidential information.

Choosing a Storage Method

There are several methods for storing flags with in-app purchases on iOS:

Using Plists: A plist is a file that stores configuration data for an app. While it’s possible to use plists to store flags, this method has security risks associated with jailbroken apps.
NSUserDefaults: NSUserDefaults is a simple way to store small amounts of data in an app. However, like plists, this method also poses security risks with jailbroken apps.
iOS Keychain: The iOS keychain provides a secure and encrypted storage solution for flags.

Why Use the iOS Keychain?

The iOS keychain offers several benefits when it comes to storing flags with in-app purchases:

Security: The keychain provides a secure storage solution that’s protected by encryption and authentication.
Ease of use: The keychain is easily integrated into an app using the security framework.

Implementing In-App Purchases with iOS Keychain

To implement in-app purchases on iOS using the keychain, follow these steps:

Create a flag in the keychain to indicate whether the user has purchased certain features or content.
When the user purchases the app, set the value of the flag in the keychain to YES.
In your app, use the security framework to retrieve the value of the flag from the keychain.

Setting a Flag in the Keychain

To set a flag in the keychain, follow these steps:

{< highlight shell >
# Import the security framework
#import <Security/Security.h>

// Create a new CFTypeRef for the flag
CFStringRef key = CFStringCreateWithFormat(kCFAllocatorDefault, (const char *) "contentHasBeenPurchased", kCFStringGetCFTypeIdentifier);

// Set the value of the flag to YES
NSDictionary *attributes = [NSDictionary dictionary];
AttributesDictionaryAddAttribute(attributes, (const void **)&keyRef, kSecAttrValueData, nil);
SECKeyRef keyRef = (SECKeyRef)CFRetain(key);
CFRelease(key);

 // Add the flag to the keychain
 SecAccessSynchronize(NULL);
OSStatus status = SECAddAccessWithFlags(NULL, NULL, &keyRef, kSecAttributekSecMatchFlagAnyValueOnly, attributes);
CFRelease(key);
SecTrustEvaluate(NULL, &status);
CFRelease(attributes);

Retrieving a Flag from the Keychain

To retrieve the value of a flag from the keychain, follow these steps:

{< highlight shell >
# Import the security framework
#import <Security/Security.h>

// Retrieve the flag from the keychain
NSDictionary *attributes = [NSDictionary dictionary];
SecAttributeRef attribute = NULL;
OSStatus status = SECFindAttributesWithFlags(NULL, kSecAttrKeyClassGeneric, &attribute);
if (status == noErr) {
    SecValueRef value = NULL;
    CFTypeRef result = NULL;
    OSStatus err = SECGetAttribute(attribute, kSecAttrValueData, &result);
    if (err == noErr) {
        CFArrayRef values = CFArrayCreate(NULL, (__const void **)result, 1, sizeof(void *));
        value = CFArrayGetValue(values, 0);
        CFRelease(result);
        CFArrayRelease(values);
    }
    CFRelease(attribute);
} else {
    CFRelease(attribute);
}

// Convert the result to a boolean
BOOL flag = YES;
if (value) {
    CFDictionaryRef dictionary = NULL;
    CFTypeRef result = NULL;
    OSStatus err = SECGetAttribute(value, kSecAttrValueData, &result);
    if (err == noErr) {
        dictionary = CFArrayGetValue(result, 0);
        flag = YES;
        CFRelease(result);
    }
    CFRelease(attribute);
}

Conclusion

In-app purchases are a popular feature used in mobile apps to offer additional content or functionality for purchase by users. Implementing in-app purchases on iOS using the keychain storage provides a secure and encrypted solution for storing flags that indicate whether the user has purchased certain features or content.

By following these steps, you can easily implement in-app purchases with the iPhone’s keychain storage, ensuring a secure and seamless experience for your app users.

Example Use Case

Suppose we want to create an e-book reader app where some features are available only after purchasing a premium version. We can use the security framework to store a flag in the keychain that indicates whether the user has purchased the premium version.

{< highlight shell >
// Create a new CFTypeRef for the flag
CFStringRef key = CFStringCreateWithFormat(kCFAllocatorDefault, (const char *) "premiumVersionHasBeenPurchased", kCFStringGetCFTypeIdentifier);

// Set the value of the flag to YES when the user purchases the app
NSDictionary *attributes = [NSDictionary dictionary];
AttributesDictionaryAddAttribute(attributes, (const void **)&keyRef, kSecAttrValueData, nil);
SECKeyRef keyRef = (SECKeyRef)CFRetain(key);
CFRelease(key);

 // Add the flag to the keychain
 SecAccessSynchronize(NULL);
OSStatus status = SECAddAccessWithFlags(NULL, NULL, &keyRef, kSecAttributekSecMatchFlagAnyValueOnly, attributes);
CFRelease(key);
SecTrustEvaluate(NULL, &status);
CFRelease(attributes);

When the user accesses a feature that requires the premium version, we can use the security framework to retrieve the value of the flag from the keychain.

{< highlight shell >
// Retrieve the flag from the keychain
NSDictionary *attributes = [NSDictionary dictionary];
SecAttributeRef attribute = NULL;
OSStatus status = SECFindAttributesWithFlags(NULL, kSecAttrKeyClassGeneric, &attribute);
if (status == noErr) {
    SecValueRef value = NULL;
    CFTypeRef result = NULL;
    OSStatus err = SECGetAttribute(attribute, kSecAttrValueData, &result);
    if (err == noErr) {
        CFArrayRef values = CFArrayCreate(NULL, (__const void **)result, 1, sizeof(void *));
        value = CFArrayGetValue(values, 0);
        CFRelease(result);
        CFArrayRelease(values);
    }
    CFRelease(attribute);
}

// Convert the result to a boolean
BOOL premiumVersionHasBeenPurchased = YES;
if (value) {
    CFDictionaryRef dictionary = NULL;
    CFTypeRef result = NULL;
    OSStatus err = SECGetAttribute(value, kSecAttrValueData, &result);
    if (err == noErr) {
        dictionary = CFArrayGetValue(result, 0);
        premiumVersionHasBeenPurchased = YES;
        CFRelease(result);
    }
    CFRelease(attribute);
}

If the flag is YES, we can provide access to the feature. Otherwise, we can display an error message or prompt the user to purchase a subscription.

By using the iOS keychain to store flags that indicate whether the user has purchased certain features or content, you can create a seamless and secure experience for your app users.

Last modified on 2024-04-03