iOS Keychain Services: only specific values allowed for kSecAttrGeneric Key?

Question

I am trying to use the KeychainWrapper class provided in this Apple sample code: https://developer.apple.com/library/content/samplecode/GenericKeychain/

In the sampl

Answer 1

Simon almost fixed my issue because after changing the KeychainItemWrapper.m, I had issues getting and setting data to and from the keychain. So after adding this to the KeychainItemWrapper.m, I used this to get and store items:

KeychainItemWrapper *keychainItem = [[KeychainItemWrapper alloc] initWithAccount:@"Identfier" service:@"AppName" accessGroup:nil];
[keychainItem setObject:@"some value" forKey:(__bridge id)kSecAttrGeneric];
NSString *value = [keychainItem objectForKey: (__bridge id)kSecAttrGeneric];

Because [keychainItem objectForKey: (__bridge id)kSecAttrService] is returning the Account (in this example @"Identifier") which makes sense but it took me some time before I realized I needed to use kSecAttrGeneric to fetch data from the wrapper.

Answer 2

Okay, I found the solution in this blog post Keychain duplicate item when adding password

To sum it up, the issue is that the GenericKeychain sample app uses the value stored in kSecAttrGeneric key as the identifier for the keychain item when in fact that is not what the API uses to determine a unique keychain item. The keys you need to set with unique values are the kSecAttrAccount key and/or the kSecAttrService key.

You can rewrite the initilizer of KeychainItemWrapper so you don't need to change any other code by changing these lines:

Change:

[genericPasswordQuery setObject:identifier forKey:(id)kSecAttrGeneric];

to:

[genericPasswordQuery setObject:identifier forKey:(id)kSecAttrAccount];

and change:

[keychainItemData setObject:identifier forKey:(id)kSecAttrGeneric];

to:

[keychainItemData setObject:identifier forKey:(id)kSecAttrAccount];

Or, you could do what I did and write a new initilizer that takes both of the identifying keys:

Edit: For people using ARC (you should be nowadays) check nycynik's answer for all the correct bridging notation

- (id)initWithAccount:(NSString *)account service:(NSString *)service accessGroup:(NSString *) accessGroup;
{
    if (self = [super init])
    {
        NSAssert(account != nil || service != nil, @"Both account and service are nil.  Must specifiy at least one.");
        // Begin Keychain search setup. The genericPasswordQuery the attributes kSecAttrAccount and 
       // kSecAttrService are used as unique identifiers differentiating keychain items from one another
        genericPasswordQuery = [[NSMutableDictionary alloc] init];

        [genericPasswordQuery setObject:(id)kSecClassGenericPassword forKey:(id)kSecClass];

        [genericPasswordQuery setObject:account forKey:(id)kSecAttrAccount];
        [genericPasswordQuery setObject:service forKey:(id)kSecAttrService];

        // The keychain access group attribute determines if this item can be shared
        // amongst multiple apps whose code signing entitlements contain the same keychain access group.
        if (accessGroup != nil)
        {
#if TARGET_IPHONE_SIMULATOR
            // Ignore the access group if running on the iPhone simulator.
            // 
            // Apps that are built for the simulator aren't signed, so there's no keychain access group
            // for the simulator to check. This means that all apps can see all keychain items when run
            // on the simulator.
            //
            // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
            // simulator will return -25243 (errSecNoAccessForItem).
#else            
            [genericPasswordQuery setObject:accessGroup forKey:(id)kSecAttrAccessGroup];
#endif
        }

        // Use the proper search constants, return only the attributes of the first match.
        [genericPasswordQuery setObject:(id)kSecMatchLimitOne forKey:(id)kSecMatchLimit];
        [genericPasswordQuery setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnAttributes];

        NSDictionary *tempQuery = [NSDictionary dictionaryWithDictionary:genericPasswordQuery];

        NSMutableDictionary *outDictionary = nil;

        if (! SecItemCopyMatching((CFDictionaryRef)tempQuery, (CFTypeRef *)&outDictionary) == noErr)
        {
            // Stick these default values into keychain item if nothing found.
            [self resetKeychainItem];

            //Adding the account and service identifiers to the keychain
            [keychainItemData setObject:account forKey:(id)kSecAttrAccount];
            [keychainItemData setObject:service forKey:(id)kSecAttrService];

            if (accessGroup != nil)
            {
#if TARGET_IPHONE_SIMULATOR
                // Ignore the access group if running on the iPhone simulator.
                // 
                // Apps that are built for the simulator aren't signed, so there's no keychain access group
                // for the simulator to check. This means that all apps can see all keychain items when run
                // on the simulator.
                //
                // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
                // simulator will return -25243 (errSecNoAccessForItem).
#else            
                [keychainItemData setObject:accessGroup forKey:(id)kSecAttrAccessGroup];
#endif
            }
        }
        else
        {
            // load the saved data from Keychain.
            self.keychainItemData = [self secItemFormatToDictionary:outDictionary];
        }

        [outDictionary release];
    }

    return self;
}

Hope this helps someone else out!

Answer 3

Same as above, but it works for ARC. Thanks simon

- (id)initWithAccount:(NSString *)account service:(NSString *)service accessGroup:(NSString *) accessGroup;

{
    if (self = [super init])
    {
        NSAssert(account != nil || service != nil, @"Both account and service are nil.  Must specifiy at least one.");
        // Begin Keychain search setup. The genericPasswordQuery the attributes kSecAttrAccount and
        // kSecAttrService are used as unique identifiers differentiating keychain items from one another
        genericPasswordQuery = [[NSMutableDictionary alloc] init];

        [genericPasswordQuery setObject:(__bridge id)kSecClassGenericPassword forKey:(__bridge id)kSecClass];

        [genericPasswordQuery setObject:account forKey:(__bridge id)kSecAttrAccount];
        [genericPasswordQuery setObject:service forKey:(__bridge id)kSecAttrService];

        // The keychain access group attribute determines if this item can be shared
        // amongst multiple apps whose code signing entitlements contain the same keychain access group.
        if (accessGroup != nil)
        {
#if TARGET_IPHONE_SIMULATOR
            // Ignore the access group if running on the iPhone simulator.
            //
            // Apps that are built for the simulator aren't signed, so there's no keychain access group
            // for the simulator to check. This means that all apps can see all keychain items when run
            // on the simulator.
            //
            // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
            // simulator will return -25243 (errSecNoAccessForItem).
#else
            [genericPasswordQuery setObject:accessGroup forKey:(__bridge id)kSecAttrAccessGroup];
#endif
        }

        // Use the proper search constants, return only the attributes of the first match.
        [genericPasswordQuery setObject:(__bridge id)kSecMatchLimitOne forKey:(__bridge id)kSecMatchLimit];
        [genericPasswordQuery setObject:(__bridge id)kCFBooleanTrue forKey:(__bridge id)kSecReturnAttributes];

        NSDictionary *tempQuery = [NSDictionary dictionaryWithDictionary:genericPasswordQuery];

        CFMutableDictionaryRef outDictionary = NULL;

        if (! SecItemCopyMatching((__bridge CFDictionaryRef)tempQuery, (CFTypeRef *)&outDictionary) == noErr)
        {
            // Stick these default values into keychain item if nothing found.
            [self resetKeychainItem];

            //Adding the account and service identifiers to the keychain
            [keychainItemData setObject:account forKey:(__bridge id)kSecAttrAccount];
            [keychainItemData setObject:service forKey:(__bridge id)kSecAttrService];

            if (accessGroup != nil)
            {
#if TARGET_IPHONE_SIMULATOR
                // Ignore the access group if running on the iPhone simulator.
                //
                // Apps that are built for the simulator aren't signed, so there's no keychain access group
                // for the simulator to check. This means that all apps can see all keychain items when run
                // on the simulator.
                //
                // If a SecItem contains an access group attribute, SecItemAdd and SecItemUpdate on the
                // simulator will return -25243 (errSecNoAccessForItem).
#else
                [keychainItemData setObject:accessGroup forKey:(__bridge id)kSecAttrAccessGroup];
#endif
            }
        }
        else
        {
            // load the saved data from Keychain.
            keychainItemData = [self secItemFormatToDictionary:(__bridge NSDictionary *)outDictionary];
        }

        if(outDictionary) CFRelease(outDictionary);
    }

    return self;
}