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* [Identity] On Behalf Of sample

* once again, I forgot to remove en-us

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Co-authored-by: Scott Addie <10702007+scottaddie@users.noreply.github.com>

* extra line

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* Apply suggestions from code review

Co-authored-by: Scott Addie <10702007+scottaddie@users.noreply.github.com>

Co-authored-by: Scott Addie <10702007+scottaddie@users.noreply.github.com>
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118 changes: 59 additions & 59 deletions sdk/identity/identity/samples/AzureIdentityExamples.md
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- [Authenticating with a pre-fetched access token](#authenticating-with-a-pre-fetched-access-token).
- [Authenticating with MSAL directly](#authenticating-with-msal-directly).
- [Authenticating with the @azure/msal-node Confidential Client](#authenticating-with-the-@azure/msal-node-confidential-client).
- [Authenticating with the @azure/msal-node On Behalf Flow](#authenticating-with-the-@azure/msal-node-on-behalf-of-flow).
- [Authenticating with the @azure/msal-browser Public Client](#authenticating-with-the-@azure/msal-browser-public-client).
- [Authenticating with Key Vault Certificates](#authenticating-with-key-vault-certificates)
- [Rolling Certificates](#rolling-certificates)
- [Authenticate on behalf of](#authenticate-on-behalf-of)
- [Control user interaction](#control-user-interaction)
- [Persist user authentication data](#persist-user-authentication-data)
- [Persist the token cache](#persist-the-token-cache)
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}
```

#### Authenticating with the @azure/msal-node On Behalf Of Flow

Currently, the `@azure/identity` library doesn't provide a credential type for clients which need to authenticate via the [On Behalf of Flow](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/master/lib/msal-common/docs/request.md#on-behalf-of-flow). While we may add support for this feature in the future, users currently requiring this will have to implement their own `TokenCredential` class.

In this example, the `OnBehalfOfCredential` accepts a client ID, client secret, and a user's access token. It then creates an instance of `ConfidentialClientApplication` from MSAL to obtain an OBO token that can authenticate client requests.

**Prerequisites**

Install the [@azure/msal-node][msal_node_npm] and [@azure/core-auth][core_auth].

> For more information about MSAL for Node.js, see [the README of the `@azure/msal-node` package][msal_node_readme].
> For more information about working with the Confidential Client of MSAL, see [Initialization of MSAL (Node.js)](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/master/lib/msal-node/docs/initialize-confidential-client-application.md).
> For more information about working with the On Behalf Flow with MSAL, see [On Behalf of Flow](https://github.com/AzureAD/microsoft-authentication-library-for-js/blob/master/lib/msal-common/docs/request.md#on-behalf-of-flow).
```ts
import { TokenCredential, AccessToken } from "@azure/core-auth";
import * as msalNode from "@azure/msal-node";

class OnBehalfOfCredential implements TokenCredential {
private confidentialApp: msalNode.ConfidentialClientApplication;

constructor(
private clientId: string,
private clientSecret: string,
private userAccessToken: string
) {
this.confidentialApp = new msalNode.ConfidentialClientApplication({
auth: {
clientId,
clientSecret
}
});
}
async getToken(scopes: string | string[]): Promise<AccessToken> {
const result = await this.confidentialApp.acquireTokenOnBehalfOf({
scopes: Array.isArray(scopes) ? scopes : [scopes],
oboAssertion: this.userAccessToken
});
return {
token: result.accessToken,
expiresOnTimestamp: result.expiresOn.getTime()
};
}
}
```

The following example shows an how the `OnBehalfOfCredential` could be used to authenticate a `SecretClient`:

```ts
import { SecretClient } from "@azure/keyvault-secrets";

async function main() {
const oboCredential = new OnBehalfOfCredential(clientId, clientSecret, userAccessToken);

const client = new SecretClient("https://myvault.vault.azure.net/", oboCredential);
}
```

#### Authenticating with the @azure/msal-browser Public Client

While `@azure/identity` provides some browser support, for users that need the complete feature set offered by `@azure/msal-browser`, it's possible to implement a `TokenCredential` on top of MSAL's public API for the browsers.
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In this example, the custom credential type `RotatingCertificateCredential` again uses a `ClientCertificateCredential` instance to retrieve tokens. However, in this case, it will attempt to refresh the certificate before obtaining the token. The method `RefreshCertificate` will query to see if the certificate has changed. If so, it will replace `this.credential` with a new instance of the certificate credential using the same certificate path.

### Authenticate on behalf of

Many multi-user apps use the [On-Behalf-Of (OBO) flow](https://docs.microsoft.com/azure/active-directory/develop/v2-oauth2-on-behalf-of-flow) to make authenticated requests between two services that would otherwise be unreachable. The Identity SDK provides an `OnBehalfOfCredential` that supports this form of authentication.

Two accounts participate in the OBO flow:

- A user, which aims to obtain a special access level.
- An app registration, which will act as the provider of the special access level.

Both accounts must belong to the same Azure AD tenant.

For this authentication flow to work, app registrations must be configured with a custom scope. To create a scope through the Azure portal:

1. Select **Active Directory** > **Enterprise Applications**.
2. Go to the app you want to authenticate against.
3. On the left menu, select **Expose an API** > **Add a scope**.

While other credentials authenticate requesting access to a set of resources, the OBO flow requires the user token to have access specifically to the scope of the Azure AD app that will delegate its access to the users.

```ts
const credential = new InteractiveBrowserCredential();

// Make sure to use the custom scope created on your app registration.
const token = await credential.getToken("api://AAD_APP_CLIENT_ID/CUSTOM_SCOPE_NAME");
```

Once the token is retrieved, pass it in the `userAssertionToken` property of the `OnBehalfOfCredentialOptions`, along with `tenantId`, `clientId`, and `clientSecret`. Once initialized, this credential will have granted the user access to the resources available to the app registration.

```ts
import { InteractiveBrowserCredential } from "@azure/identity";

async function main(): Promise<void> {
// One would use AuthCodeCredential in real life.
const credential = new InteractiveBrowserCredential();

const token = await deviceCred.getToken("api://AAD_APP_CLIENT_ID/Read");

const oboCred = new identity.OnBehalfOfCredential({
tenantId: "TENANT",
clientId: "AAD_APP_CLIENT_ID",
clientSecret: "AAD_APP_CLIENT_SECRET",
userAssertionToken: token.token
});

// Now, the originally authenticated user will be granted access by the app registration
// to previously inaccessible resources.
const token2 = await oboCred.getToken("https://storage.azure.com/.default");
console.log({ token, token2 });
}

main().catch((err) => {
console.log("error code: ", err.code);
console.log("error message: ", err.message);
console.log("error stack: ", err.stack);
process.exit(1);
});
```

### Control user interaction

In many cases, applications require tight control over user interaction. In these applications, automatically blocking on required user interaction is often undesired or impractical. For this reason, credentials in the `@azure/identity` library that interact with the user offer mechanisms to fully control user interaction. These settings are available under `InteractiveCredentialOptions` in both Node.js and the browser.
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