Styjun

There are three ways to compare OAuth and OpenID:

1. Purposes

OpenID was created for federated authentication, that is, letting a third-party authenticate your users for you, by using accounts they already have. The term federated is critical here because the whole point of OpenID is that any provider can be used (with the exception of white-lists). You don't need to pre-choose or negotiate a deal with the providers to allow users to use any other account they have.

OAuth was created to remove the need for users to share their passwords with third-party applications. It actually started as a way to solve an OpenID problem: if you support OpenID on your site, you can't use HTTP Basic credentials (username and password) to provide an API because the users don't have a password on your site.

The problem is with this separation of OpenID for authentication and OAuth for authorization is that both protocols can accomplish many of the same things. They each provide a different set of features which are desired by different implementations but essentially, they are pretty interchangeable. At their core, both protocols are an assertion verification method (OpenID is limited to the 'this is who I am' assertion, while OAuth provides an 'access token' that can be exchanged for any supported assertion via an API).

2. Features

Both protocols provide a way for a site to redirect a user somewhere else and come back with a verifiable assertion. OpenID provides an identity assertion while OAuth is more generic in the form of an access token which can then be used to "ask the OAuth provider questions". However, they each support different features:

OpenID - the most important feature of OpenID is its discovery process. OpenID does not require hard coding each the providers you want to use ahead of time. Using discovery, the user can choose any third-party provider they want to authenticate. This discovery feature has also caused most of OpenID's problems because the way it is implemented is by using HTTP URIs as identifiers which most web users just don't get. Other features OpenID has is its support for ad-hoc client registration using a DH exchange, immediate mode for optimized end-user experience, and a way to verify assertions without making another round-trip to the provider.

OAuth - the most important feature of OAuth is the access token which provides a long lasting method of making additional requests. Unlike OpenID, OAuth does not end with authentication but provides an access token to gain access to additional resources provided by the same third-party service. However, since OAuth does not support discovery, it requires pre-selecting and hard-coding the providers you decide to use. A user visiting your site cannot use any identifier, only those pre-selected by you. Also, OAuth does not have a concept of identity so using it for login means either adding a custom parameter (as done by Twitter) or making another API call to get the currently "logged in" user.

3. Technical Implementations

The two protocols share a common architecture in using redirection to obtain user authorization. In OAuth the user authorizes access to their protected resources and in OpenID, to their identity. But that's all they share.

Each protocol has a different way of calculating a signature used to verify the authenticity of the request or response, and each has different registration requirements.

OpenID is (mainly) for identification/authentication, so that stackoverflow.com knows that I own chris.boyle.name (or wherever) and therefore that I am probably the same person who owned chris.boyle.name yesterday and earned some reputation points.

OAuth is designed for authorization to take actions on your behalf, so that stackoverflow.com (or wherever) can ask permission to, say, Tweet on your behalf automatically, without knowing your Twitter password.

Many people still visit this so here's a very simple diagram to explain it

Courtesy Wikipedia

OAuth

Used for delegated authorization only -- meaning you are authorizing a third-party service access to use personal data, without giving out a password. Also OAuth "sessions" generally live longer than user sessions. Meaning that OAuth is designed to allow authorization

i.e. Flickr uses OAuth to allow third-party services to post and edit a persons picture on their behalf, without them having to give out their flicker username and password.

OpenID

Used to authenticate single sign-on identity. All OpenID is supposed to do is allow an OpenID provider to prove that you say you are. However many sites use identity authentication to provide authorization (however the two can be separated out)

i.e. One shows their passport at the airport to authenticate (or prove) the person's who's name is on the ticket they are using is them.

Use OAuth if your users might just want to login with Facebook, or Twitter. Use OpenID if your users are neckbeards that run their own OpenID providers because they "don't want anyone else owning their identity".

OpenID and OAuth are each HTTP-based protocols for authentication and/or authorization. Both are intended to allow users to perform actions without giving authentication credentials or blanket permissions to clients or third parties. While they are similar, and there are proposed standards to use them both together, they are separate protocols.

OpenID is intended for federated authentication. A client accepts an identity assertion from any provider (although clients are free to whitelist or blacklist providers).

OAuth is intended for delegated authorization. A client registers with a provider, which provides authorization tokens which it will accept to perform actions on the user's behalf.

OAuth is currently better suited for authorization, because further interactions after authentication are built into the protocol, but both protocols are evolving. OpenID and its extensions could be used for authorization, and OAuth can be used for authentication, which can be thought of as a no-op authorization.

I believe it makes sense revisit this question as also pointed out in the comments, the introduction of OpenID Connect may have brought more confusion.

OpenID Connect is an authentication protocol like OpenID 1.0/2.0 but it is actually built on top of OAuth 2.0, so you'll get authorization features along with authentication features. The difference between the two is pretty well explained in detail in this (relatively recent, but important) article: http://oauth.net/articles/authentication/

The explanation of the difference between OpenID, OAuth, OpenID Connect:

OpenID is a protocol for authentication while OAuth is for
authorization. Authentication is about making sure that the guy you
are talking to is indeed who he claims to be. Authorization is about
deciding what that guy should be allowed to do.

In OpenID, authentication is delegated: server A wants to authenticate
user U, but U's credentials (e.g. U's name and password) are sent to
another server, B, that A trusts (at least, trusts for authenticating
users). Indeed, server B makes sure that U is indeed U, and then tells
to A: "ok, that's the genuine U".

In OAuth, authorization is delegated: entity A obtains from entity B
an "access right" which A can show to server S to be granted access; B
can thus deliver temporary, specific access keys to A without giving
them too much power. You can imagine an OAuth server as the key master
in a big hotel; he gives to employees keys which open the doors of the
rooms that they are supposed to enter, but each key is limited (it
does not give access to all rooms); furthermore, the keys
self-destruct after a few hours.

To some extent, authorization can be abused into some
pseudo-authentication, on the basis that if entity A obtains from B an
access key through OAuth, and shows it to server S, then server S may
infer that B authenticated A before granting the access key. So some
people use OAuth where they should be using OpenID. This schema may or
may not be enlightening; but I think this pseudo-authentication is
more confusing than anything. OpenID Connect does just that: it abuses
OAuth into an authentication protocol. In the hotel analogy: if I
encounter a purported employee and that person shows me that he has a
key which opens my room, then I suppose that this is a true employee,
on the basis that the key master would not have given him a key which
opens my room if he was not.

(source)

How is OpenID Connect different than OpenID 2.0?

OpenID Connect performs many of the same tasks as OpenID 2.0, but does
so in a way that is API-friendly, and usable by native and mobile
applications. OpenID Connect defines optional mechanisms for robust
signing and encryption. Whereas integration of OAuth 1.0a and OpenID
2.0 required an extension, in OpenID Connect, OAuth 2.0 capabilities are integrated with the protocol itself.

(source)

OpenID connect will give you an access token plus an id token. The id
token is a JWT and contains information about the authenticated user.
It is signed by the identity provider and can be read and verified
without accessing the identity provider.

In addition, OpenID connect standardizes quite a couple things that
oauth2 leaves up to choice. for instance scopes, endpoint discovery,
and dynamic registration of clients.

This makes it easier to write code that lets the user choose between
multiple identity providers.

(source)

Google's OAuth 2.0

Google's OAuth 2.0 APIs can be used for both authentication and
authorization. This document describes our OAuth 2.0 implementation
for authentication, which conforms to the OpenID Connect
specification, and is OpenID Certified. The documentation found in
Using OAuth 2.0 to Access Google APIs also applies to this service. If
you want to explore this protocol interactively, we recommend the
Google OAuth 2.0 Playground.

(source)

More an extension to the question than an answer, but it may add some perspective to the great technical answers above. I'm an experienced programmer in a number of areas, but a total noob to programming for the web. Now trying to build a web-based application using Zend Framework.

Definitely will implement an application-specific basic username/password authentication interface, but recognize that for a growing number of users the thought of yet another username and password is a deterrent. While not exactly social networking, I know that a very large percentage of the application's potential users already have facebook or twitter accounts. The application doesn't really want or need to access information about the user's account from those sites, it just wants to offer the convenience of not requiring the user to set up new account credentials if they don't want to. From a functionality point of view, that would seem a poster child for OpenID. But it seems that neither facebook nor twitter are OpenID providers as such, though they do support OAuth authentication to access their user's data.

In all the articles I've read about the two and how they differ, it wan't until I saw Karl Anderson's observation above, that "OAuth can be used for authentication, which can be thought of as a no-op authorization" that I saw any explicit confirmation that OAuth was good enough for what I wanted to do.

In fact, when I went to post this "answer", not being a member at the time, I looked long and hard at the bottom of this page at the options for identifying myself. The option for using an OpenID login or obtaining one if I didn't have one, but nothing about twitter or facebook, seemed to suggest that OAuth wasn't adequate for the job. But then I opened another window and looked for the general signup process for stackoverflow - and lo and behold there's a slew of 3rd-party authentication options including facebook and twitter. In the end I decided to use my google id (which is an OpenID) for exactly the reason that I didn't want to grant stackoverflow access to my friends list and anything else facebook likes to share about its users - but at least it's a proof point that OAuth is adequate for the use I had in mind.

It would really be great if someone could either post info or pointers to info about supporting this kind of multiple 3rd-part authorization setup, and how you deal with users that revoke authorization or lose access to their 3rd party site. I also get the impression that my username here identifies a unique stackoverflow account that I could access with basic authentication if I wanted to set it up, and also access this same account through other 3rd-party authenticators (e.g. so that I would be considered logged in to stackoverflow if I was logged in to any of google, facebook, or twitter...). Since this site is doing it, somebody here probably has some pretty good insight on the subject. :-)

Sorry this was so long, and more a question than an answer - but Karl's remark made it seem like the most appropriate place to post amidst the volume of threads on OAuth and OpenID. If there's a better place for this that I didn't find, I apologize in advance, I did try.

• 
  OpenID is an open standard and decentralized authentication protocol controlled by the OpenID Foundation.


• 
  OAuth is an open standard for access delegation.


• 
  OpenID Connect (OIDC) Combines the features of OpenID and OAuth i.e. does both Authentication and Authorization.

OpenID take the form of a unique URI managed by some "OpenID provider" i.e identity provider (idP).

OAuth can be used in conjunction with XACML where OAuth is used for ownership consent and access delegation whereas XACML is used to define the authorization policies.

OIDC uses simple JSON Web Tokens (JWT), which you can obtain using flows conforming to the OAuth 2.0 specifications. OAuth is directly related to OIDC since OIDC is an authentication layer built on top of OAuth 2.0.

For example, if you chose to sign in to Auth0 using your Google account then you used OIDC. Once you successfully authenticate with Google and authorize Auth0 to access your information, Google will send back to Auth0 information about the user and the authentication performed. This information is returned in a JSON Web Token (JWT). You'll receive an Access Token and, if requested, an ID Token. Types of Token : Source: OpenID Connect

Analogy:

An Organisation use ID card for identification purpose and it contains chips, it stores details about Employee along with Authorization i.e. Campus/Gate/ODC access. ID Card act as a OIDC and Chip act as a OAuth. more examples and form wiki

OpenID proves who you are.

OAuth grants access to the features provided by the authorizing party.

I am currently working on OAuth 2.0 and OpenID connect spec. So here is my understanding:
Earlier they were:

1. OpenID was proprietary implementation of Google allowing third party applications like for newspaper websites you can login using google and comment on an article and so on other usecases. So essentially, no password sharing to newspaper website. Let me put up a definition here, this approach in enterprise approach is called Federation. In Federation, You have a server where you authenticate and authorize (called IDP, Identity Provider) and generally the keeper of User credentials. the client application where you have business is called SP or Service Provider. If we go back to same newspaper website example then newspaper website is SP here and Google is IDP. In enterprise this problem was earlier solved using SAML. that time XML used to rule the software industry. So from webservices to configuration, everything used to go to XML so we have SAML, a complete Federation protocol


2. 
   

   OAuth: OAuth saw it's emergence as an standard looking at all these kind of proprietary approaches and so we had OAuth 1.o as standard but addressing only authorization. Not many people noticed but it kind of started picking up. Then we had OAuth 2.0 in 2012. CTOs, Architects really started paying attention as world is moving towards Cloud computing and with computing devices moving towards mobile and other such devices. OAuth kind of looked upon as solving major problem where software customers might give IDP Service to one company and have many services from different vendors like salesforce, SAP, etc. So integration here really looks like federation scenario bit one big problem, using SAML is costly so let's explore OAuth 2.o. Ohh, missed one important point that during this time, Google sensed that OAuth actually doesn't address Authentication, how will IDP give user data to SP (which is actually wonderfully addressed in SAML) and with other loose ends like:

   
   
   

   a. OAuth 2.o doesn't clearly say, how client registration will happen
   b. it doesn't mention anything about the interaction between SP (Resource Server) and client application (like Analytics Server providing data is Resource Server and application displaying that data is Client)

   
   

There are already wonderful answers given here technically, I thought of giving of giving brief evolution perspective

OpenId uses OAuth to deal with authentication.

By analogy, it's like .NET relies on Windows API. You could directly call Windows API but it's so wide, complex and method arguments so vast, you could easily make mistakes/bugs/security issue.

Same with OpenId/OAuth. OpenId relies on OAuth to manage Authentication but defining a specific Token (Id_token), digital signature and particular flows.

I'd like to address a particular aspect of this question, as captured in this comment:

OAuth: before granting access to some feature, authentication must be done, right ?. so OAuth = what OpenId does + grants access to some features ? – Hassan Makarov Jun 21 at 1:57

Yes... and no. The answer is subtle, so bear with me.

When the OAuth flow redirects you to a target service (the OAuth provider, that is), it is likely that you'll need to authenticate with that service before a token will be handed back to the client application/service. The resulting token then allows the client app to make requests on behalf of a given user.

Note the generality of that last sentence: specifically, I wrote "on behalf of a given user", not "on behalf of you". It's a common error to assume that "having a capability to interact with a resource owned by a given user" implies "you and the owner of the target resource(s) are one in the same".

Don't make this mistake.

While it's true that you authenticate with the OAuth provider (say, by user name and password, or maybe SSL client certs, or some other means), what the client gets in return should not necessarily be taken as proof of identity. An example would be a flow in which access to another user's resources was delegated to you (and by proxy, the OAuth client). Authorization does not imply authentication.

To handle authentication, you'll likely want to look into OpenID Connect, which is essentially another layer on top of the foundation set by OAuth 2.0. Here's a quote that captures (in my opinion) the most salient points regarding OpenID Connect (from https://oauth.net/articles/authentication/):

OpenID Connect is an open standard published in early 2014 that defines an interoperable way to use OAuth 2.0 to perform user authentication. In essence, it is a widely published recipe for chocolate fudge that has been tried and tested by a wide number and variety of experts. Instead of building a different protocol to each potential identity provider, an application can speak one protocol to as many providers as they want to work with. Since it's an open standard, OpenID Connect can be implemented by anyone without restriction or intellectual property concerns.

OpenID Connect is built directly on OAuth 2.0 and in most cases is deployed right along with (or on top of) an OAuth infrastructure. OpenID Connect also uses the JSON Object Signing And Encryption (JOSE) suite of specifications for carrying signed and encrypted information around in different places. In fact, an OAuth 2.0 deployment with JOSE capabilities is already a long way to defining a fully compliant OpenID Connect system, and the delta between the two is relatively small. But that delta makes a big difference, and OpenID Connect manages to avoid many of the pitfalls discussed above by adding several key components to the OAuth base: [...]

The document then goes on to describe (among other things) token IDs and a UserInfo endpoint. The former provides a set of claims (who you are, when the token was issued, etc, and possibly a signature to verify the authenticity of the token via a published public key without having to ask the upstream service), and the latter provides a means of e.g. asking for the user's first/last name, email, and similar bits of info, all in a standardized way (as opposed to the ad-hoc extensions to OAuth that people used before OpenID Connect standardized things).

Both protocols were created for different reasons. OAuth was created to authorize third parties to access resources. OpenID was created to perform decentralize identity validation. This website states the following:

OAuth is a protocol designed to verify the identity of an end-user and to grant permissions to a third party. This verification results in a token. The third party can use this token to access resources on the user’s behalf. Tokens have a scope. The scope is used to verify whether a resource is accessible to a user, or not

OpenID is a protocol used for decentralised authentication. Authentication is about identity; Establishing the user is in fact the person who he claims to be. Decentralising that, means this service is unaware of the existence of any resources or applications that need to be protected. That’s the key difference between OAuth and OpenID.

OAuth 2.0 is a Security protocol. It is NEITHER an Authentication NOR an Authorization protocol.

Authentication by definition the answers two questions.

1. Who is the user?


2. Is the user currently present on the system?

OAuth 2.0 has the following grant types

• client_credentials: When one app needs to interact with another app and modify the data of multiple users.


• authorization_code: User delegates the Authorization server to issue an access_token that the client can use to access protected resource


• refresh_token: When the access_token expires, the refresh token can be leveraged to get a fresh access_token


• password: User provides their login credentials to a client that calls the Authorization server and receives an access_token

All 4 have one thing in common, access_token, an artifact that can be used to access protected resource.

The access_token does not provide the answer to the 2 questions that an "Authentication" protocol must answer.

An example to explain Oauth 2.0 (credits: OAuth 2 in Action, Manning publications)

Let's talk about chocolate. We can make many confections out of chocolate including, fudge, ice cream, and cake. But, none of these can be equated to chocolate because multiple other ingredients such as cream and bread are needed to make the confection, even though chocolate sounds like the main ingredient. Similarly, OAuth 2.0 is the chocolate, and cookies, TLS infrastucture, Identity Providers are other ingredients that are required to provide the "Authentication" functionality.

If you want Authentication, you may go for OpenID Connect, which provides an "id_token", apart from an access_token, that answers the questions that every authentication protocol must answer.