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Azure Hybrid Identity Authentication Methods

The move to the cloud is picking up steam.  As such, many corporations are beginning to find themselves supporting a mixture of on-prem apps as well as cloud apps. Users are finding that they need access to this mix of applications as well.  As one would expect, this can become a challenge to implement.

To improve access to solutions that span on-prem and cloud-based capabilities and platforms, an organization needs to create a common user identity that can be used for authentication and authorization to all such resources, including those on-prem and those in the cloud.  

This is called hybrid identity.

A successful hybrid identity solution requires the deployment of any one of three different authentication methods.  Which authentication method is deployed is dependent on the specific scenario being addressed.

In this article, we are going to discuss the three authentication methods, including Password Hash Synchronization, Pass-Through Authentication, and Federation.  Password Hash Synchronization is also referred to as PHS. Pass-Through Authentication is referred to as PTA. Federation is referred to as, well, Federation.

Password hash synchronization (PHS)

Password hash synchronization is a sign-in method that’s used as part of a hybrid identity solution.  To accomplish a hybrid identity solution with PHS, a hash of a user’s on-prem Active Directory (AD) password is synchronized to a cloud-based Azure AD instance.  This feature is typically used for signing in to Azure services such as Office 365 with the same password as an on-prem AD account.  This is a preferable solution for end users because it creates a pleasant end user experience.

Using a password hash synchronization strategy reduces the number of passwords that a user needs to remember to one.  As such, it often results in reduced help desk costs.

Password hash synchronization is the most widely used hybrid identity solution.  It requires the installation of Azure AD Connect. Azure AD Connect then performs an ongoing directory synchronization between the on-prem Active Directory and the Azure Active Directory instance.

Part of the synchronization configuration is password hash synchronization.

How Password Hash Synchronization works

The on-prem Active Directory instance stores each password in the form of a hash value representation of the actual user password.  The hash value is calculated from a one-way mathematical function or hashing algorithm. For security reasons, there is no way to reverse-engineer the hash back to the plain text version of a password.

To synchronize a password, Azure AD Connect extracts the password hash from the on-premises AD instance.  Additional security processing is also applied to the password hash before the hash is synchronized to the Azure Active Directory authentication service. Passwords are synchronized on a per-user basis and they are synchronized in chronological order.

Data flow of the password hash synchronization process is quite similar to the synchronization of user data. However, because passwords change often, they are synchronized more frequently than the standard directory synchronization window for other AD attributes. Password hash synchronization runs every 2 minutes and the frequency of this cycle cannot be modified.  As you would expect, when a password is synchronized, it overwrites the existing cloud password.

When the password hash synchronization feature is enabled for the first time, it performs an initial synchronization of the passwords for all users that are in-scope.  It’s not possible to explicitly define a subset of user passwords to synchronize.

When an on-prem password is changed, the updated password is synchronized, typically within in a matter of minutes. The password hash synchronization process will automatically retry failed synchronization attempts. When an error does occur during a synchronization attempt, an error is logged in the event viewer.

The synchronization of a password has no impact on a user who is currently signed in. While the current cloud service session is not immediately affected by a synchronized password change that occurs, when the cloud service requires a signed-in user to authenticate again, the user will need to provide the new password.

Generally speaking, users must provide their corporate credentials a second time when authenticating to Azure AD, whether they’re signed in to their corporate network or not.  However, this requirement can be minimized if users select the “Keep me signed in” checkbox at sign-in. When this box is checked, a session cookie is set. This cookie bypasses authentication for 180 days. The Azure admin can enable/disable the “keep me signed in” behavior.  Additionally, password prompts can be reduced by turning on Seamless SSO, which, in turn, automatically signs users in when they are on corporate devices and connected to the corporate network.

Pass-through authentication (PTA)

Similar to Password Hash Synchronization, Azure AD Pass-through Authentication allows users to sign in to on-prem apps as well as cloud-based apps, using the same password.  However, pass-through authentication validates user passwords directly against the on-premises Active Directory. It doesn’t use a synced password hash. Using pass-through authentication offers organizations the ability to enforce on-prem Active Directory security policies and password policies since it leverages the on-prem credentials.  

Combining Pass-through Authentication with Seamless Single Sign-On allows an organization’s users to access applications on corporate machines inside the network, without needing to type in their passwords again.

Azure AD Pass-through Authentication offers a better end-user experience because it provides users the ability to complete self-service password management tasks.  It’s relatively easy to deploy and administer pass-through authentication because all it requires is a lightweight agent to be installed on-prem. Because the agent automatically receives updates, there isn’t any management to be concerned with.

Pass-through authentication provides better security than password hash synchronization because, with pass-through authentication, on-prem passwords are never stored in the cloud.  Additionally, pass-through authentication offers more account protection because it works with Azure AD Conditional Access policies, including multi-factor authentication. Another key benefit of pass-through authentication is the fact that the agent only makes outbound connections from the network. Because there are no inbound connections required, all requirements for a DMZ as part of the solution are removed.

Communications between the on-prem agent and Azure Active Directory is secured with certificate-based authentication, which adds yet another layer of security.  Azure AD automatically renews the certificates that are used every few months; thus, removing any requirement to manually maintain them.

In addition to offering high security, Azure AD Pass-through Authentication offers high availability, which is attained by installing additional agents on multiple on-prem servers.

How Pass-Through Authentication Works

Assuming a user is not already signed in when they launch an application (e.g. Outlook Web App), the user is redirected to the Azure AD User Sign-in page.

At the Azure AD sign-in page, the user provides their username and then clicks the Next button. After providing their password, the user clicks the sign-in button.

When it receives the sign-in request, Azure AD places the username and password, which has been encrypted with the public key of the Authentication Agents, in a queue.  One of the on-premises Authentication Agents retrieves the username and encrypted password from the queue and then decrypts the password by using its private key.

After decrypting the password, the agent validates the username and password against Active Directory, using standard Windows APIs (similar to what ADFS does). The on-premises Active Directory domain controller evaluates the request and then it returns the appropriate response to the agent. Such a response would be success, failure, password expired, or user locked out.  The Authentication Agent then returns this response back to Azure AD.

Next, Azure AD evaluates the response and responds to the user as appropriate. If the user sign-in is successful, the user can access the application.

Federation

The third authentication method, Federation, is a bit different from the other two methods.  Federation consists of a collection of domains with an established trust. The trust typically includes authentication and almost always includes authorization.  A typical federation configuration would include several organizations that have established trust for shared access to a set of resources.

How Federation Works

Federating an on-prem AD environment with Azure Active Directory allows an organization to use the federation for authentication and authorization.  By using federation, the organization can ensure all user authentication is performed on-prem. As such, federation offers administrators the ability to maintain more rigorous levels of access control.  Federation is available with both ADFS and PingFederate.

To protect itself against a failure of the ADFS infrastructure when using federation, an organization can leverage password hash synchronization as a backup.  By doing so, authentication can continue, despite a failure of the ADFS infrastructure.

Summary

As you can see, there are several options available to an organization looking for a hybrid identity solution from Microsoft Azure.  To determine which solution (or solutions) to consider, it is critical to understand the problem you are trying to solve. While all three hybrid identity authentication methods offer single sign-on capabilities, other considerations must be taken into account as well.

An organization looking for a simple implementation with fewer moving parts might consider the password hash sync method.  However, a really security-conscious organization that wants to retain all authentication on-premises might want to consider federation or maybe pass-through authentication.

At the end of the day, choosing a hybrid identity authentication method starts with understanding the needs of the business.  With three identity authentication methods available, a solution likely isn’t far away.

Written by

Thomas Mitchell

Tom is not only a Cloud Platform & Infrastructure MCSE but also an IT industry veteran with 20+ years of experience in multiple technologies. An Active Directory specialist, Tom has never met an AD problem that he couldn't solve. He also speaks Microsoft Exchange fluently.

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