SASL

SASL, or Simple Authentication and Security Layer, is a framework that provides authentication and, optionally, a security layer for connection-oriented application protocols. It does not define a specific authentication method; rather, it defines a common interface and negotiation process that allows a client and server to choose from a set of authentication mechanisms and, if supported, establish transport protection for subsequent data.

In SASL, the client and server negotiate a mechanism during an initial exchange. The chosen mechanism dictates how credentials are exchanged and whether a security layer is established to protect later communication. Some mechanisms perform only authentication, while others also enable integrity and/or confidentiality protection for the data that follows.

Examples of SASL mechanisms include simple methods such as PLAIN, which transmits credentials in a straightforward form, and challenge-response methods such as CRAM-MD5 and DIGEST-MMD5. More modern mechanisms include SCRAM-SHA-1 and SCRAM-SHA-256, which use salted passwords and server proofs. Other mechanisms support strong authentication through external systems (GSSAPI/Kerberos, EXTERNAL), or token-based approaches (OAUTHBEARER, OAUTH2). The availability of mechanisms varies by protocol and deployment.

SASL is widely used to add authentication and optional security to a range of application protocols, including email protocols like SMTP, IMAP, and POP3; directory services such as LDAP; and real-time messaging protocols like XMPP. It sits between the application protocol and the transport layer, enabling protocols to authenticate in a pluggable, protocol-agnostic way while allowing optional security features to be negotiated as part of the session.