There are several password-based authentication methods. These methods operate similarly but differ in how the users' passwords are stored on the server and how the password provided by a client is sent across the connection.
scram-sha-256
The method scram-sha-256 performs SCRAM-SHA-256
authentication, as described in
RFC 7677. It
is a challenge-response scheme that prevents password sniffing on
untrusted connections and supports storing passwords on the server in a
cryptographically hashed form that is thought to be secure.
This is the most secure of the currently provided methods, but it is not supported by older client libraries.
md5
The method md5 uses a custom less secure challenge-response
mechanism. It prevents password sniffing and avoids storing passwords
on the server in plain text but provides no protection if an attacker
manages to steal the password hash from the server. Also, the MD5 hash
algorithm is nowadays no longer considered secure against determined
attacks.
The md5 method cannot be used with
the db_user_namespace feature.
To ease transition from the md5 method to the newer
SCRAM method, if md5 is specified as a method
in pg_hba.conf but the user's password on the
server is encrypted for SCRAM (see below), then SCRAM-based
authentication will automatically be chosen instead.
password
The method password sends the password in clear-text and is
therefore vulnerable to password “sniffing” attacks. It should
always be avoided if possible. If the connection is protected by SSL
encryption then password can be used safely, though.
(Though SSL certificate authentication might be a better choice if one
is depending on using SSL).
PostgreSQL database passwords are
separate from operating system user passwords. The password for
each database user is stored in the pg_authid system
catalog. Passwords can be managed with the SQL commands
CREATE ROLE and
ALTER ROLE,
e.g., CREATE ROLE foo WITH LOGIN PASSWORD 'secret',
or the psql
command \password.
If no password has been set up for a user, the stored password
is null and password authentication will always fail for that user.
The availability of the different password-based authentication methods
depends on how a user's password on the server is encrypted (or hashed,
more accurately). This is controlled by the configuration
parameter password_encryption at the time the
password is set. If a password was encrypted using
the scram-sha-256 setting, then it can be used for the
authentication methods scram-sha-256
and password (but password transmission will be in
plain text in the latter case). The authentication method
specification md5 will automatically switch to using
the scram-sha-256 method in this case, as explained
above, so it will also work. If a password was encrypted using
the md5 setting, then it can be used only for
the md5 and password authentication
method specifications (again, with the password transmitted in plain text
in the latter case). (Previous PostgreSQL releases supported storing the
password on the server in plain text. This is no longer possible.) To
check the currently stored password hashes, see the system
catalog pg_authid.
To upgrade an existing installation from md5
to scram-sha-256, after having ensured that all client
libraries in use are new enough to support SCRAM,
set password_encryption = 'scram-sha-256'
in postgresql.conf, make all users set new passwords,
and change the authentication method specifications
in pg_hba.conf to scram-sha-256.