Safely wrapping keychain passwords with NSString/CFString

gistfile1.m

void freeKeychainContent(void *ptr, void *info)
{
    SecKeychainItemFreeContent(NULL, ptr);
}

- (NSString *)passwordFromKeychainItem:(SecKeychainItemRef)keychainItem
{
    void *passwordData;
    UInt32 passwordLength;
    OSStatus status = SecKeychainItemCopyContent(keychainItem,
                                                 NULL, NULL,
                                                 &passwordLength, &passwordData);
    
    if (status != errSecSuccess) return nil;
    
    // Password data must be freed using special keychain APIs
    // Do so with a specially crafted CFString
    CFAllocatorContext context = {
        0,
        NULL,
        NULL, NULL, // don't do retain/release
        NULL,
        NULL, NULL, // no need to alloc or realloc
        freeKeychainContent,
        NULL };
    
    CFAllocatorRef deallocator = CFAllocatorCreate(NULL, &context);

    NSString *result = CFStringCreateWithBytesNoCopy(NULL,
                                                     passwordData, passwordLength,
                                                     kCFStringEncodingUTF8, false,
                                                     deallocator);

    CFRelease(deallocator);    
    return [(NSString *)result autorelease];
}