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Azure AD Connect Authentication (sign-in) Options:
Below are the four different authentication (sign-in) mechanisms provided by Azure AD when you are using Azure AD Connect, based on your feasibility from security and compliance perspective you can choose the one appropriate. During Azure AD Connect installation wizard you will have the ability to choose one of the authentication mechanism.

1. Password Hash Synchronization (PHS):
–When we install Azure AD Connect with “Express Settings” then Password Harsh Synchronization (PHS) authentication mechanism is the default configuration.
–AAD Connect synchronizes a hash, of the hash, of an AD user’s password from an on-premises AD to Azure AD.
–To synchronize user’s password, Azure AD Connect sync extracts user’s password hash from the on-premises Active Directory. Extra security processing is applied to the password hash before it is synchronized to the Azure Active Directory.
–PHS process runs every 2 minutes and we cannot modify the frequency of this process.

2.Pass-through Authentication (PTA):
–Users credentials are validated by on-premises Active Directory Domain Controller via AAD Connect Authentication Agent, On-premises AD user’s passwords are not stored in Azure AD in any form.
–For Pass-through Authentication to work, users need to be provisioned into Azure AD from on-premises Active Directory using Azure AD Connect. Pass-through Authentication does not apply to cloud-only users.
–Communication between Authentication Agent and Azure AD is uses certificate-based authentication. These certificates are automatically renewed every few months by Azure AD.
–Microsoft recommends to have more than one AAD Connect Authentication Agent to provide high availability of authentication requests.
–PTA can also be used in conjunction with PHS for high availability scenarios, As per Microsoft “Enabling Password Hash Synchronization gives you the option to failover authentication if your on-premises infrastructure is disrupted. This failover from Pass-through Authentication to Password Hash Synchronization is not automatic. You’ll need to switch the sign-in method manually using Azure AD Connect. If the server running Azure AD Connect goes down, you’ll require help from Microsoft Support to turn off Pass-through Authentication.”

3. Federation with ADFS:
–In ADFS federation scenario, Azure AD will be redirecting authentication request to ADFS.

4. Federation with PingFederate:
–If you are already using PingFederate in your environment then you may choose this method for authentication. AAD Connect natively supports PingFederate, please refer below official document from PingFederate regarding implementation of this.