KeychainItemWrapper.m

/*
 File: KeychainItemWrapper.m
 Abstract:
 Objective-C wrapper for accessing a single keychain item.
 
 Version: 1.2
 
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 */

#import "KeychainItemWrapper.h"
#import <Security/Security.h>

/*
 
 These are the default constants and their respective types,
 available for the kSecClassGenericPassword Keychain Item class:
 
 kSecAttrAccessGroup         -       CFStringRef
 kSecAttrCreationDate        -       CFDateRef
 kSecAttrModificationDate    -       CFDateRef
 kSecAttrDescription         -       CFStringRef
 kSecAttrComment             -       CFStringRef
 kSecAttrCreator             -       CFNumberRef
 kSecAttrType                -       CFNumberRef
 kSecAttrLabel               -       CFStringRef
 kSecAttrIsInvisible         -       CFBooleanRef
 kSecAttrIsNegative          -       CFBooleanRef
 kSecAttrAccount             -       CFStringRef
 kSecAttrService             -       CFStringRef
 kSecAttrGeneric             -       CFDataRef
 
 See the header file Security/SecItem.h for more details.
 
 */

@interface KeychainItemWrapper (PrivateMethods)
/*
 The decision behind the following two methods (secItemFormatToDictionary and dictionaryToSecItemFormat) was
 to encapsulate the transition between what the detail view controller was expecting (NSString *) and what the
 Keychain API expects as a validly constructed container class.
 */
- (NSMutableDictionary *)secItemFormatToDictionary:(NSDictionary *)dictionaryToConvert;
- (NSMutableDictionary *)dictionaryToSecItemFormat:(NSDictionary *)dictionaryToConvert;

// Updates the item in the keychain, or adds it if it doesn't exist.
- (void)writeToKeychain;

@end

@implementation KeychainItemWrapper

@synthesize keychainItemData, genericPasswordQuery;

- (id)initWithIdentifier: (NSString *)identifier accessGroup:(NSString *) accessGroup;
{
    if (self = [super init])
    {
        // Begin Keychain search setup. The genericPasswordQuery leverages the special user
        // defined attribute kSecAttrGeneric to distinguish itself between other generic Keychain
        // items which may be included by the same application.
        genericPasswordQuery = [[NSMutableDictionary alloc] init];
        
        [genericPasswordQuery setObject:(id)kSecClassGenericPassword forKey:(id)kSecClass];
        [genericPasswordQuery setObject:identifier forKey:(id)kSecAttrGeneric];
        
        // 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];
            
            // Add the generic attribute and the keychain access group.
            [keychainItemData setObject:identifier forKey:(id)kSecAttrGeneric];
            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;
}

- (void)dealloc
{
    [keychainItemData release];
    [genericPasswordQuery release];
    
    [super dealloc];
}

- (void)setObject:(id)inObject forKey:(id)key
{
    if (inObject == nil) return;
    id currentObject = [keychainItemData objectForKey:key];
    if (![currentObject isEqual:inObject])
    {
        [keychainItemData setObject:inObject forKey:key];
        [self writeToKeychain];
    }
}

- (id)objectForKey:(id)key
{
    return [keychainItemData objectForKey:key];
}

- (void)resetKeychainItem
{
    OSStatus junk = noErr;
    if (!keychainItemData)
    {
        self.keychainItemData = [[[NSMutableDictionary alloc] init] autorelease];
    }
    else if (keychainItemData)
    {
        NSMutableDictionary *tempDictionary = [self dictionaryToSecItemFormat:keychainItemData];
        junk = SecItemDelete((CFDictionaryRef)tempDictionary);
        NSAssert( junk == noErr || junk == errSecItemNotFound, @"Problem deleting current dictionary." );
    }
    
    // Default attributes for keychain item.
    [self.keychainItemData setObject:@"" forKey:(id)kSecAttrAccount];
    [self.keychainItemData setObject:@"" forKey:(id)kSecAttrLabel];
    [self.keychainItemData setObject:@"" forKey:(id)kSecAttrDescription];
    
    // Default data for keychain item.
    [keychainItemData setObject:@"" forKey:(id)kSecValueData];
}

- (NSMutableDictionary *)dictionaryToSecItemFormat:(NSDictionary *)dictionaryToConvert
{
    // The assumption is that this method will be called with a properly populated dictionary
    // containing all the right key/value pairs for a SecItem.
    
    // Create a dictionary to return populated with the attributes and data.
    NSMutableDictionary *returnDictionary = [NSMutableDictionary dictionaryWithDictionary:dictionaryToConvert];
    
    // Add the Generic Password keychain item class attribute.
    [returnDictionary setObject:(id)kSecClassGenericPassword forKey:(id)kSecClass];
    
    // Convert the NSString to NSData to meet the requirements for the value type kSecValueData.
    // This is where to store sensitive data that should be encrypted.
    NSString *passwordString = [dictionaryToConvert objectForKey:(id)kSecValueData];
    [returnDictionary setObject:[passwordString dataUsingEncoding:NSUTF8StringEncoding] forKey:(id)kSecValueData];
    
    return returnDictionary;
}

- (NSMutableDictionary *)secItemFormatToDictionary:(NSDictionary *)dictionaryToConvert
{
    // The assumption is that this method will be called with a properly populated dictionary
    // containing all the right key/value pairs for the UI element.
    
    // Create a dictionary to return populated with the attributes and data.
    NSMutableDictionary *returnDictionary = [NSMutableDictionary dictionaryWithDictionary:dictionaryToConvert];
    
    // Add the proper search key and class attribute.
    [returnDictionary setObject:(id)kCFBooleanTrue forKey:(id)kSecReturnData];
    [returnDictionary setObject:(id)kSecClassGenericPassword forKey:(id)kSecClass];
    
    // Acquire the password data from the attributes.
    NSData *passwordData = NULL;
    if (SecItemCopyMatching((CFDictionaryRef)returnDictionary, (CFTypeRef *)&passwordData) == noErr)
    {
        // Remove the search, class, and identifier key/value, we don't need them anymore.
        [returnDictionary removeObjectForKey:(id)kSecReturnData];
        
        // Add the password to the dictionary, converting from NSData to NSString.
        NSString *password = [[[NSString alloc] initWithBytes:[passwordData bytes] length:[passwordData length]
                                                     encoding:NSUTF8StringEncoding] autorelease];
        [returnDictionary setObject:password forKey:(id)kSecValueData];
    }
    else
    {
        // Don't do anything if nothing is found.
        NSAssert(NO, @"Serious error, no matching item found in the keychain.\n");
    }
    
    [passwordData release];
    
    return returnDictionary;
}

- (void)writeToKeychain
{
    NSDictionary *attributes = NULL;
    NSMutableDictionary *updateItem = NULL;
    OSStatus result;
    
    if (SecItemCopyMatching((CFDictionaryRef)genericPasswordQuery, (CFTypeRef *)&attributes) == noErr)
    {
        // First we need the attributes from the Keychain.
        updateItem = [NSMutableDictionary dictionaryWithDictionary:attributes];
        // Second we need to add the appropriate search key/values.
        [updateItem setObject:[genericPasswordQuery objectForKey:(id)kSecClass] forKey:(id)kSecClass];
        
        // Lastly, we need to set up the updated attribute list being careful to remove the class.
        NSMutableDictionary *tempCheck = [self dictionaryToSecItemFormat:keychainItemData];
        [tempCheck removeObjectForKey:(id)kSecClass];
        
#if TARGET_IPHONE_SIMULATOR
        // Remove 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).
        //
        // The access group attribute will be included in items returned by SecItemCopyMatching,
        // which is why we need to remove it before updating the item.
        [tempCheck removeObjectForKey:(id)kSecAttrAccessGroup];
#endif
        
        // An implicit assumption is that you can only update a single item at a time.
        
        result = SecItemUpdate((CFDictionaryRef)updateItem, (CFDictionaryRef)tempCheck);
        NSAssert( result == noErr, @"Couldn't update the Keychain Item." );
    }
    else
    {
        // No previous item found; add the new one.
        result = SecItemAdd((CFDictionaryRef)[self dictionaryToSecItemFormat:keychainItemData], NULL);
        NSAssert( result == noErr, @"Couldn't add the Keychain Item." );
    }
}

@end