Minimum-viable identity provider
When I talk about viability I really mean "competitiveness" and I suppose what I'm really asking is what does it take to get RPs (Relying Parties) to integrate and users to authenticate with an IDP?
The list of requirements below was first published in a presentation I gave at Over The Air 2013. It's the result of hundreds of conversations with RPs and user over the last few years.
Valuable accounts. Are the accounts attached to real people who have been SMS verified? Does the IDP fight off attempts to create fake and spammy accounts?
Security. Are the accounts stored using salting and hashing? Do users authenticate using multiple factors? Are precautions being taken to ensure that user accounts are protected?
Rich profiles. Does the IDP offer data that you can use to personalise your service such as profile data, photos and social/interest graphs?
Ubiquitous APIs. Does the IDP offer ReSTful APIs, native SDKs, client libraries in various languages and support for RTL languages?
Escape hatches. Does the IDP lock-in RPs and/or users? Can the RP obtain the user's verified email address so that the user has the option of using a different IDP with the same RP account? Is the RP forced to build their own post-registration flow?
Business model. Does the IDP make money or otherwise benefit from providing this service? Do they have a compelling incentive to stay in this business?
The final and most controversial ingredient is scale. Most people would say that all other things being equal an IDP with more accounts is better than an IDP with fewer accounts. I'd suggest that it's better to have accounts that are appropriate to the service the RP is trying to provide. For instance for Statigram the best IDP is Instagram but for Favstar.fm the best IDP is Twitter and, of course, for any service that uses Google services (like Youtube, Android, Drive, etc) the best IDP is Google+.
The items listed above are just the necessary but not sufficient features of a viable IDP. Successful IDPs will still have to identify and provide additional value in order to get widespread adoption by users and RPs.