## 21.5. Password Authentication

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`.