The Cyrus SASL (Simple Authentication and Security Layer)

SASL is the Simple Authentication and Security Layer, a method
for adding authentication support to connection-based protocols.
To use SASL, a protocol includes a command for identifying and
authenticating a user to a server and for optionally negotiating
protection of subsequent protocol interactions. If its use is
negotiated, a security layer is inserted between the protocol
and the connection.

WWW: https://www.cyrusimap.org/sasl/