How to write a website application for VK?

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I am writing an IFrame application for a website, for vk.com And I ran into such a problem. https://vk.com/dev/rulesClause 1.5 says that you can not ask users for such data as Email AND password. Of course, it is understandable why this cannot be done. But one of the initial tasks that I put in the development of this application is the convenience for users. Namely, if the user registered through the main site and subsequently began to use the application of this site for vk.com, then the application would understand that the user already has a profile on the site and would not create a new profile for him, but would open the old page. The solutions to this problem were as follows. Read the user's cookies from the browser, but this option immediately disappeared because several people can use one PC and if one did not leave the main site, and at the same time, another user went into the application on vk. com, then confusion may occur, or if one person uses 2 or more browsers, then everything may not be smooth either. I also want to add that it would be ideal to make such a scheme according to which an attacker who hacked into the user's page in VK could not get data from the application. Or if the user changes the page in VK, then he could access his old site page through the adjusted ones. There was also an authorization option via sms, but I also can’t ask for a phone number according to VK rules. And the last thing I would like is that those who created their profiles through the transcription could also log into their accounts through the main site if they wish. In general, maybe someone had a similar problem, unsubscribe what solutions you applied. PS Of course, in my situation there are many more nuances that I did not take into account in this post. But I believe that having solved the main tasks, I can eliminate the remaining nuances too. Although, for example, WebMoney Keeper for VKontakte calmly asks for a mobile phone number. But probably it is understandable webmoney is a large company and there is much more trust in it than in ordinary, albeit honest, developers

Answer 1

When you first enter through VK, display a BIG RED BUTTON with an offer to go to the site and tie a profile and a small one with an offer to refuse. You can come up with some kind of ID that you can get in the personal account of your service and insert it into the field in VK, but this is somehow wild