Simple Authentication and Security Layer (SASL) decouples authentication mechanisms from application protocols, in theory allowing any authentication mechanism supported by SASL to be used in any application protocol that uses SASL

Where Does SASL Fit In?

In an application, we may use SMTP to send emails and use LDAP to access directory services. But each of these protocols may support another authentication mechanism, like Digest-MD5 or Kerberos.

What if there was a way for protocols to swap authentication mechanisms more declaratively? This is exactly where SASL comes into the picture. Protocols supporting SASL can invariably support any of the SASL mechanisms.

Hence, applications can negotiate a suitable mechanism and adopt that for authentication and secure communication.

How Does SASL Work?

SASL is a challenge-response framework. Here, the server issues a challenge to the client, and the client sends a response based on the challenge. The challenge and response are byte arrays of arbitrary length and, hence, can carry any mechanism-specific data.

This exchange can continue for multiple iterations and finally ends when the server issues no further challenge.

Furthermore, the client and server can negotiate a security layer post-authentication. All subsequent communication can then leverage this security layer. However, note that some of the mechanisms may only support authentication.

It's important to understand here that SASL only provides a framework for the exchange of challenge and response data. It does not mention anything about the data itself or how they are exchanged. Those details are left for the applications adopting to use of SASL