Google OpenID Connect User Authorization - google-oauth

We have a SPA web application that supports OpenID Connect login from external Identity providers. Our application supports different user types (roles). We want to support Sign in with Google using Google Identity service (https://developers.google.com/identity/protocols/oauth2/openid-connect) but the Access Token seems to be for Google APIs only (we want to protect our own APIs). The ID Token is a JWT we can validate, but there is nothing in the claims we can use for Authorization.
Is it possible to add custom claims to the Google ID token that we can use for Authorization?
From the link above...
This document describes our OAuth 2.0 implementation for authentication, which conforms to the OpenID Connect specification, and is OpenID Certified
I guess this means they ONLY support Authentication and not Authorization? Is this true? Or is there some other Google service we need to use to get user roles (Google Cloud IAM, etc.)?
How does Google expect us to Authorize users they Authenticate?


To get control over what the tokens contains, then my recommendation is to add a separate identity provider in-between your applications and Google. In this way, your applications only need to trust your local provider and it can provide the tokens that you need and you can add additional user information/claims to the tokens.
Just like this picture tries to show:

Related

How do I use Google's Identify Platform to authenticate against Cloud Endpoints?

In our organisation we use the Google Kubernetes Engine (GKE) to implement a micro-service architecture. As we are also G-Suite users, everyone in the organisation is guaranteed to have a Google account. In consequence we would like to use those accounts to manage authentication and authorization of micro services.
We have prototyped login using the angularfire2 client to authenticate against the Google Identity Platform. We also have Google Cloud Endpoints configured to control access to relevant services.
The piece we are missing is how to get from the identity in Google to an access token we can use on our services -- the access token coming back using the Firebase API has no claims in it, and the documentation on custom claims seems to make it quite clear that these go into the identity token.
My expectation would be to have JWTs with the appropriate audience (our backend), containing a sufficient set of claims to implement role based access control within the services. Ideally the infrastructure could validate a claim already -- some of our services are small enough to require only one role, which could be enforced outside the service. Or we could annotate our endpoints (Protobuf) with the required claims.
In the GCP environment, what is the standard process of creating access tokens to be used for accessing GKE services? Is there anything that supports this?

The piece we are missing is how to get from the identity in Google to
an access token we can use on our services -- the access token coming
back using the Firebase API has no claims in it, and the documentation
on custom claims seems to make it quite clear that these go into the
identity token.
Google OAuth Access Tokens do not have an identity in the sense that you want to use it. Identity is stored in the Identity Token. Add the scope "email" when authenticating the user. Google will return an ID Token. For some frameworks, you can request custom claims for the Identity Token.
In the GCP environment, what is the standard process of creating
access tokens to be used for accessing GKE services? Is there anything
that supports this?
There are two types of access excluding methods such as API keys. User Accounts and Service Accounts. Service-to-service typically uses service account Access Tokens (RBAC) or service account Identity Tokens (IBAC). In your case, you want to use Identity Platform which means User Accounts.
If I was designing this system, I would use User Accounts to authenticate with the system - Firebase is great for this purpose. I would look up what roles this identity supports/allows from my database (Firestore) and create a service account Access Token with the required scopes for GCP services. I would then use this Access Token for GCP service-to-service authorization. If I also required custom roles for my own services, I would create a custom Identity Token with my custom roles and include that as a custom HTTP header and include the Google Access Token in the standard HTTP "authorization: bearer" header. I would use the service account private key to sign my custom Identity Token or use a GCP IAM API to sign for me so that the other end can verify with the service account's public key. This method prevents data leakage at the client, no private keys are distributed, scopes/roles are not disclosed, etc.

I would suggest you follow this doc of authentication between services by using service account files.

OAuth and authentication

according to here (https://oauth.net/articles/authentication/) and many other things I have come across. OAuth is not meant to handle end user authentication. Reading through the article above and others while searching provides so much information at once from so many angles that it is hard to see through it all. ok...
Does this mean that...
A) The protocol itself is not intended to handle authentication, so therefore, OAuth client apps should inspect "who" can authorize users according to the OAuth providers?
If ONLY the user can authorize third party apps, then isn't the fact of receiving authorization from the OAuth provider in itself proof of authentication? (if this is the case, then can OAuth access tokens from places like Google and Facebook be trusted as authentications?)
B) OAuth client apps cannot trust authenticating users with OAuth, so therefore must provide another sound authentication mechanism alongside it?
If this is the case, then every site that I have clicked "Login With [provider]" (and no other complementary authentication scheme) has got authentication wrong?
It seems to me that if only trusted and specific OAuth providers are used, then this flow could infer authentication
App requests login with trusted providers
User is directed to provider to authorize (ONLY user can authorize)
App then requests and receives token from provider, and adds user to the app database if necessary.
Token is put into secure cookie or JWT and returned to the user to be presented on subsequent visits.

The purpose of OAuth2 access token is to delegate some access rights (scopes) from a user to a client application. So the application redirects the user to an authentication provider (OAuth2 server), which authenticates the user and asks the user (consent step) whether he/she wants to delegate some access rights (the scopes requested by the application) to the application.
If a client application receives an access token, it can get its meta data at the OAuth2 introspection endpoint - such as username of the user (resource owner). So this way, the OAuth2 can be used for authentication. But the main purpose of access tokens is to delegate some rights. For example if a third party application wants to save its data to a user's Google Drive, it needs an access token issued by Google with scopes that allow it to access Google Drive.
If you want to use OAuth2 only for authentication in your client application (to get identity of a user), you can use OpenId Connect (OAuth2 extension) and its ID token, which is in JWT format and contains information about the user that was authenticated the authentication provider. This is better suited for the "Login With ..." functionality.

Web api 2 security - key for api

I'm building an API that my public mobile app (xamarin forms) will call to get information. However to delimit the users of the API to just my app for security reasons, I want the public mobile app (xamarin forms) to pass a key to be able to call the API. Is basic authentication the best option then ? Or is there another approach ?

You can use any Auth based mechanism in your web api project. The best would be the latest Identity 2.0 using Owin.
OAuth is an open standard for authentication, and enables a resource
owner to notify a resource provider that permission should be granted
to a third party in order to access their information without sharing
the resource owners identity.
In your Xamarin client project (mobile app) use the Xamarin.Auth SDK to authenticate your users. It provides many features and also helps you to scale and use Social authentication as well.
Xamarin.Auth is a cross-platform API for authenticating users and
storing their accounts. It includes OAuth authenticators that provide
support for consuming identity providers such as Google, Microsoft,
Facebook, and Twitter.
The authentication flow when consuming an OAuth identity provider is as follows:
The application redirects a browser to an identity provider URL. The
URL query parameters indicate the type of access being requested.
The identity provider handles user authentication, and returns an
authorization code to the application.
The application exchanges the authorization code, client ID, and
client secret for an access token from the identity provider.
The application uses the access token to access APIs on the identity
provider, such as requesting basic user data.
The application uses the OAuth2Authenticator and OAuth2Request classes, provided by Xamarin.Auth, to implement the application side of the authentication flow.
The detailed explanation of Authenticating Users with an Identity Provider
Using the Xamarin.Auth component for authentication implementing is available here.

What's the difference between OpenID and OAuth?

I'm really trying to understand the difference between OpenID and OAuth? Maybe they're two totally separate things?

OpenID is about authentication (ie. proving who you are), OAuth is about authorisation (ie. to grant access to functionality/data/etc.. without having to deal with the original authentication).
OAuth could be used in external partner sites to allow access to protected data without them having to re-authenticate a user.
The blog post "OpenID versus OAuth from the user’s perspective" has a simple comparison of the two from the user's perspective and "OAuth-OpenID: You’re Barking Up the Wrong Tree if you Think They’re the Same Thing" has more information about it.

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.

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

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".

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)

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/

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.

After reading and doing some work, I figured the things I needed to know, these are: OpenID Connect, OAuth, JWT and SAML.
I will give a summary, it might help someone:
OpenID Connect (OIDC): If we can login a website using google account, then you are using OIDC.
OAuth: an application wants to access my facebook contact list and do some stuff on my behalf. If I authorise this application, then probably I am using OAuth.
JWT: OAuth uses JWT, JWT (JSON Web Tokens)- It is just a token format. JWT tokens are JSON encoded data structures contains information about issuer, subject (claims), expiration time etc. It is signed for tamper proof and authenticity and it can be encrypted to protect the token information using symmetric or asymmetric approach. JWT is simpler than SAML 1.1/2.0 and supported by all devices and it is more powerful than SWT(Simple Web Token).
Authorization flow in OAuth:
OAuth 2.0 protocol provides several workflows for authorizing a user and obtaining an access token. It depends on the type and architecture of the client which flow is the most suitable.
Below are 2 most used Authorizations flows:
Authorization Code: Suitable for third-party websites which contain a client and a server component.
The user enters credentials to a secure login webpage.
After logging in, the browser is redirected to a special URL (defined by the client), passing an authorization code in the URL.
The third-party server obtains the access token with another HTTP request in the background, using the authorization code.
From https://developers.video.ibm.com/api-basics-authentication/
Note: if you have a frontend application and the server sets a cookie in the browser, then you already have the cookie in your browser and can accees the website.
Client Credentials: Best choice for users developing server-side applications to manage their content or settings.
IBM has a good guide here: https://developers.video.ibm.com/api-basics-authentication
To know all other flows with pros and cons: here: https://www.geeksforgeeks.org/workflow-of-oauth-2-0/
SAML: Also used as an alternative of openid, but it is xml based. because developers find OIDC much easier to work with and because it is more flexible (e.g. working with mobile app is easier than xml based SAML), OIDC looks like it will become the winner.
OpenID Connect (OIDC) vs SAML: there are main differences:
SAML transmits user data in XML format. OIDC transmits user data in JSON format.
SAML calls the user data it sends a SAML Assertion. OIDC calls the
data Claims.
SAML calls the application or system the user is trying to get into
the Service Provider. OIDC calls it the Relying Party.
SAML is old, has more features, but OpenID is gaining more popularity as it is easier to implement, easier to use than XML based SAML
But not all identity providers support OpenID or SAML, if the identity provider you want to integrate only supports SAML, then you have no choice.
Want more OpenID vs SAML? read below:
https://www.onelogin.com/blog/real-difference-saml-oidc
https://auth0.com/intro-to-iam/saml-vs-openid-connect-oidc/
Want more? You can read this OAuth and OpenID analogy:
http://cakebaker.42dh.com/2008/04/01/openid-versus-oauth-from-the-users-perspective/

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:
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
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

Nowadays OpenID connect is most relevant so I will explain the difference between OpenID connect and OAuth 2.
OpenID connect specifies the IDToken standard: https://openid.net/specs/openid-connect-core-1_0.html#IDToken
This is the primary contribution of OpenID connect. So it specifies what should be contained in the response after authentication is complete.
The IDToken needs to be a JWT token, and contains information of the user, such as user id, user name, etc. The information returned depends on the claims passed when authorizing. It also contains an expiration date of the token and it should contain the digital signature of the token. This signature is used to validate the token using a public key.
The second big difference is related to the public key. OpenID connect uses something called discovery or well known endpoint. It's an endpoint that is publicly open and just returns a JSON with values such as the public key and authorization endpoints.
https://openid.net/specs/openid-connect-core-1_0.html#SelfIssuedDiscovery
So essentially OpenID is related to authentication since it specifies the IDToken, which is necessary to authenticate a user by checking the digital signature and expiration date of the IDToken.
OAuth deals with authorization especially related to scopes and validating access tokens on the resource server.
However as written here, OpenID uses OAuth 2 authorization for authentication.
https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest
An Authentication Request is an OAuth 2.0 Authorization Request that requests that the End-User be authenticated by the Authorization Server.
In short, try to think of OpenID as authentication using the JWT token and OAuth as authorization with scopes.

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 gives you back the access token to access the resource from resource server, OpenID gives you back meta data details about resources in JWT / encrypted token

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.

OpenId - Used only for Authentication.
OAuth - Used for both Authentication and Authorization. Authorization depends on the access_token which comes as part of JWT token. It can have details of user permissions or any useful information.
Both can rely on 3rd party auth provider which maintains their accounts. For example OKTA identity provider, User provides the credentials on OKTA login page and on successful login the user is redirected on the consumer application with the JWT token in the header.

OAuth 2.0 is a Security protocol. It is NEITHER an Authentication NOR an Authorization protocol.
Authentication by definition the answers two questions.
Who is the user?
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.

OAuth builds authentication on top of authorization: The user delegates access to their identity to the application, which, then, becomes a consumer of the identity API, thereby finding out who authorized the client in the first place http://oauth.net/articles/authentication/

Use cases of OAuth2.0

I am building a muli-tenant saas(software as a service) architecture. I have to build the authentication system for the system.
From what I have studied, I think I need to the build the authentication system based on OAuth2.0 and the bearer token with JWT tokens.
After reading a lot about OAuth2.0 of how to build an OAuth2.0 server I still didn't understand the full concept of OAuth and also have confusion about whether I need it or not or I need some other Authentication system.
What my system need is we will provide an SDK to all our customer and each client will have an Application Id and a secret key using the SDK client will connect to his application present in our system.
The application ID will map the customer to his application present in our system and the client secret key will authenticate the client inside the application.Do I still need to build an authentication system based on OAuth2.0 or can I build my own authentication system based on our need?
What are the use cases of OAUTH2.0 and when we won't need it to implement?

First, as clearly indicated in OAuth authentication
OAuth 2.0 is not an authentication protocol.
Authentication in the context of a user accessing an application tells an application who the current user is and whether or not they're present. A full authentication protocol will probably also tell you a number of attributes about this user, such as a unique identifier, an email address, and what to call them when the application says "Good Morning".
However, OAuth tells the application none of that.
OAuth says absolutely nothing about the user, nor does it say how the user proved their presence or even if they're still there.
As far as an OAuth client is concerned, it asked for a token, got a token, and eventually used that token to access some API. It doesn't know anything about who authorized the application or if there was even a user there at all.
There is a standard for user authentication using OAuth: OpenID Connect, compatible with OAuth2.
The OpenID Connect ID Token is a signed JSON Web Token (JWT) that is given to the client application along side the regular OAuth access token.
The ID Token contains a set of claims about the authentication session, including an identifier for the user (sub), the identifier for the identity provider who issued the token (iss), and the identifier of the client for which this token was created (aud).
In Go, you can look at coreos/dex, an OpenID Connect Identity (OIDC) and OAuth 2.0 Provider with Pluggable Connector.