2.2.2 Command and Response ABNF Grammar
This section uses Augmented Backus-Naur Form (ABNF) (as specified in [RFC5234]) to define the format of commands and responses used by this extension, where CRLF, SP, and CHAR are specified in [RFC5234]. Note that the values of username and password MUST be encoded using base64 encoding, as specified in [RFC4648], before being transmitted.
username = 1*CHAR ; Base64-encoded username password = 1*CHAR ; Base64-encoded password auth_login_command = "AUTH LOGIN" [SP username] CRLF auth_login_username_challenge = "334 VXNlcm5hbWU6" CRLF auth_login_username_response = username CRLF auth_login_password_challenge = "334 UGFzc3dvcmQ6" CRLF auth_login_password_response = password CRLF
The auth_login_command ABNF rule is consistent with the AUTH command syntax specified in [RFC4954], where the mechanism parameter is "LOGIN" and the initial-response parameter is the base64-encoded username.