Understanding Pass-Through Authentication (PTA) and Password Hash Synchronization (PHS)

In hybrid environments where on-premises Active Directory (AD) integrates with Azure Active Directory (Azure AD), two primary methods are used to authenticate users: Pass-Through Authentication (PTA) and Password Hash Synchronization (PHS).

Pass-Through Authentication (PTA)
PTA allows users to authenticate directly against the on-premises AD. When a user attempts to sign in, their password is validated by the on-premises AD domain controller. Unlike other methods, PTA does not store or sync the password hash to Azure AD. Instead, it relies on an agent installed on the on-premises server to handle authentication requests. This ensures that the authentication process remains within the on-premises environment, providing a higher level of security for organizations that prefer to keep their authentication processes local.

Password Hash Synchronization (PHS)
PHS, on the other hand, synchronizes a hash of the user’s password from the on-premises AD to Azure AD. This hash is further hashed using a secure SHA256 algorithm before being stored in Azure AD. When a user attempts to sign in, Azure AD validates the password against the stored hash. This method allows for seamless Single Sign-On (SSO) experiences and reduces dependency on the on-premises infrastructure for authentication.

Key Differences

• Authentication Location:
  • PTA: Authentication occurs on-premises.
  • PHS: Authentication occurs in Azure AD.
• Password Storage:
  • PTA: No password hashes are stored in Azure AD.
  • PHS: A hash of the password hash is stored in Azure AD.
• Dependency:
  • PTA: Requires an on-premises agent to handle authentication requests.
  • PHS: Does not require an on-premises agent for authentication.
• Security:
  • PTA: Keeps authentication within the on-premises environment, which may be preferred for security reasons.
  • PHS: Provides a secure way to store password hashes in Azure AD using SHA256.

Use Cases

• PTA is suitable for organizations that want to maintain control over their authentication processes and prefer not to store password hashes in the cloud.
• PHS is ideal for organizations looking for a simpler setup with reduced dependency on on-premises infrastructure and a seamless SSO experience.