Authentication (SASL) Mechanisms
Cleartext Authentication
The simplest authentication mechanism is PLAIN. The client simply sends the password unencrypted to Dovecot. All clients support the PLAIN mechanism, but obviously there's the problem that anyone listening on the network can steal the password. For that reason (and some others) other mechanisms were implemented.
Today however many people use SSL, and there's no problem with sending unencrypted password inside SSL secured connections. So if you're using SSL, you probably don't need to bother worrying about anything else than the PLAIN mechanism.
Another cleartext mechanism is LOGIN. It's typically used only by SMTP servers to let Outlook clients perform SMTP authentication. Note that LOGIN mechanism is not the same as IMAP's LOGIN command. The LOGIN command is internally handled using PLAIN mechanism.
Non-Cleartext Authentication
Non-cleartext mechanisms have been designed to be safe to use even without SSL encryption. Because of how they have been designed, they require access to the cleartext password or their own special hashed version of it. This means that it's impossible to use non-cleartext mechanisms with password hashes.
If you want to use more than one non-cleartext mechanism, the passwords must be stored as cleartext so that Dovecot is able to generate the required special hashes for all the different mechanisms. If you want to use only one non-cleartext mechanism, you can store the passwords using the mechanism's own password schemes.
With success/failure password databases (see passdb, e.g. PAM authentication database), it's not possible to use non-cleartext mechanisms at all, because they only support verifying a known cleartext password.
Dovecot Support
| Mechanism | Summary |
|---|---|
| CRAM-MD5 | Protects the password in transit against eavesdroppers. Somewhat good support in clients. |
| DIGEST-MD5 | Somewhat stronger cryptographically than CRAM-MD5, but clients rarely support it. |
| SCRAM-SHA-1 | Salted Challenge Response Authentication Mechanism (SCRAM) SAS and GSS-API Mechanisms. Intended as DIGEST-MD5 replacement. |
| SCRAM-SHA-256 | Stronger replacement for SCRAM-SHA-1 RFC 7677. |
| APOP | This is a POP3-specific authentication. Similar to CRAM-MD5, but requires storing password in cleartext. |
| GSS-SPNEGO | A wrapper mechanism defined by RFC 4178. Can be accessed via GSSAPI. |
| GSSAPI | Kerberos v5 support. |
| ANONYMOUS | Support for logging in anonymously. This may be useful if you're intending to provide publicly accessible IMAP archive. |
| OTP | One time password mechanisms. |
| EXTERNAL | EXTERNAL SASL mechanism. |
| OAUTHBEARER | OAuth2 bearer authentication RFC 7628. |
| XOAUTH2 | Google flavor OAUTHBEARER |
Configuration
By default only PLAIN mechanism is enabled. To use more, see auth_mechanisms.