Dovecot supports various authentication mechanisms that can be used to verify the identity of the users who connect to the mail server. Authentication mechanisms are protocols that define how the client and the server exchange the user credentials, such as the username and the password. Some authentication mechanisms are plaintext, which means that the user credentials are sent without any encryption. Others are non-plaintext, which means that the user credentials are protected from eavesdropping or tampering by using some form of encryption or hashing. Dovecot supports the following authentication mechanisms:
B. digest-md5: This is a non-plaintext mechanism that uses a challenge-response scheme based on the MD5 hash function. The client and the server exchange a series of messages that include a nonce (a random number), a realm (a domain name), and a digest (a hashed combination of the username, password, nonce, and realm). This mechanism prevents replay attacks and supports mutual authentication, meaning that both the client and the server can verify each other’s identity. However, this mechanism is not widely supported by clients and has some security weaknesses12.
C. cram-md5: This is another non-plaintext mechanism that uses a challenge-response scheme based on the MD5 hash function. The server sends a nonce to the client, and the client responds with the username and a digest of the password and the nonce. This mechanism protects the password from eavesdropping, but does not prevent replay attacks or support mutual authentication. It also requires the server to have access to the plaintext password or a special hashed version of it. This mechanism has somewhat good support in clients12.
D. plain: This is the simplest and most common plaintext mechanism. The client simply sends the username and the password to the server without any encryption. This mechanism is supported by all clients, but it is vulnerable to eavesdropping and tampering. Therefore, it should only be used with SSL/TLS encryption to secure the connection12.
The other options are not supported by Dovecot as authentication mechanisms. A. ldap is not an authentication mechanism, but a protocol for accessing directory services. E. krb5 is not an authentication mechanism, but a network authentication protocol based on Kerberos. Dovecot supports Kerberos authentication through the GSSAPI mechanism
