KeychainWrapper Objective C Identifier and Key questions

Question

I’m new to both in app purchases and using the keychain. I am using Objective C and KeychainItemWrapper.

For now I want to simply store a number in the keychain. I don’t want anyone outside the app to be able to change it, and while I don’t much care if anyone can see it, I would like to understand what’s encrypted and what’s not as I may want to store private data later.

I have successfully stored a number and retrieved, even after deleting and reinstalling the app, but I’m not at all sure I’m doing it right. How does one choose an identifier and which keys are appropriate?

As you’ll see, I’m pretty unclear on the concepts.

First question: What is the “identifier” and why can’t I use more than one?

Here’s the code:

@property (nonatomic, strong) KeychainItemWrapper *myChain;
    
     . . .

if (myChain == nil)
{
    // first question: what identifier should I use?        
    myChain = [[KeychainItemWrapper alloc] initWithIdentifier:@"test" accessGroup:nil];
}

So once I’ve used the identifier “test”, it appears that I’m stuck with it on this phone. If I use any other value then KeychainItemWrapper fails “unable to add item”. I don’t understand this at all. How do I create different identifiers? Do I want to or need to? What exactly is this identifying?

Moving on to storing. Per the code below, I can store a number, description and comment.

// storing:
// let’s start with the number 10
NSString *testNum = @"10";
NSString *testDescr = @"this is a line of text";
[myChain setObject:testDescr forKey:(__bridge id)(kSecAttrDescription)];
[myChain setObject:testNum forKey:(__bridge id)(kSecValueData)];
[myChain setObject:@"This is a comment" forKey:(__bridge id)(kSecAttrComment)];


// retrieving:
NSString *descOut = [myChain objectForKey: (__bridge id)kSecAttrDescription];
NSData *numberOut = [myChain objectForKey: (__bridge id)(kSecValueData)];

And this all works fine.

But what are the appropriate keys for this usage? I just picked a few at random for testing.

Thank you, I know I’m asking a lot, but I’ve been through the docs and stack overflow and various tutorials and I haven’t really got a good understanding of this.