6.1.2 Caching SHA-2 Pluggable Authentication

MySQL provides two authentication plugins that implement SHA-256 hashing for user account passwords:

• caching_sha2_password: Implements SHA-256 authentication (like the deprecated sha256_password), but uses caching on the server side for better performance and has additional features for wider applicability.

• sha256_password: Implements basic SHA-256 authentication. This is deprecated as of MySQL 8.0.16 and subject to removal in the future.

This section describes the caching SHA-2 authentication plugin. For information about the original basic (noncaching) plugin, see Section 6.1.3, “SHA-256 Pluggable Authentication”.

Important

In MySQL 8.0, caching_sha2_password is the default authentication plugin rather than mysql_native_password. For information about the implications of this change for server operation and compatibility of the server with clients and connectors, see caching_sha2_password as the Preferred Authentication Plugin.

Important

To connect to the server using an account that authenticates with the caching_sha2_password plugin, you must use either a secure connection or an unencrypted connection that supports password exchange using an RSA key pair, as described later in this section. Either way, the caching_sha2_password plugin uses MySQL's encryption capabilities. See Chapter 5, Using Encrypted Connections.

Note

In the name sha256_password, “sha256” refers to the 256-bit digest length the plugin uses for encryption. In the name caching_sha2_password, “sha2” refers more generally to the SHA-2 class of encryption algorithms, of which 256-bit encryption is one instance. The latter name choice leaves room for future expansion of possible digest lengths without changing the plugin name.

The caching_sha2_password plugin has these advantages, compared to sha256_password:

• On the server side, an in-memory cache enables faster reauthentication of users who have connected previously when they connect again.

• RSA-based password exchange is available regardless of the SSL library against which MySQL is linked.

• Support is provided for client connections that use the Unix socket-file and shared-memory protocols.

The following table shows the plugin names on the server and client sides.

Table 6.2 Plugin and Library Names for SHA-2 Authentication

Plugin or File	Plugin or File Name
Server-side plugin	caching_sha2_password
Client-side plugin	caching_sha2_password
Library file	None (plugins are built in)

The following sections provide installation and usage information specific to caching SHA-2 pluggable authentication:

• Installing SHA-2 Pluggable Authentication

• Using SHA-2 Pluggable Authentication

• Cache Operation for SHA-2 Pluggable Authentication

For general information about pluggable authentication in MySQL, see Section 4.17, “Pluggable Authentication”.

Installing SHA-2 Pluggable Authentication

The caching_sha2_password plugin exists in server and client forms:

• The server-side plugin is built into the server, need not be loaded explicitly, and cannot be disabled by unloading it.

• The client-side plugin is built into the libmysqlclient client library and is available to any program linked against libmysqlclient.

The server-side plugin uses the sha2_cache_cleaner audit plugin as a helper to perform password cache management. sha2_cache_cleaner, like caching_sha2_password, is built in and need not be installed.

Using SHA-2 Pluggable Authentication

To set up an account that uses the caching_sha2_password plugin for SHA-256 password hashing, use the following statement, where password is the desired account password:

CREATE USER 'sha2user'@'localhost'
IDENTIFIED WITH caching_sha2_password BY 'password';

The server assigns the caching_sha2_password plugin to the account and uses it to encrypt the password using SHA-256, storing those values in the plugin and authentication_string columns of the mysql.user system table.

The preceding instructions do not assume that caching_sha2_password is the default authentication plugin. If caching_sha2_password is the default authentication plugin, a simpler CREATE USER syntax can be used.

To start the server with the default authentication plugin set to caching_sha2_password, put these lines in the server option file:

[mysqld]
default_authentication_plugin=caching_sha2_password

That causes the caching_sha2_password plugin to be used by default for new accounts. As a result, it is possible to create the account and set its password without naming the plugin explicitly:

CREATE USER 'sha2user'@'localhost' IDENTIFIED BY 'password';

Another consequence of setting default_authentication_plugin to caching_sha2_password is that, to use some other plugin for account creation, you must specify that plugin explicitly. For example, to use the deprecated mysql_native_password plugin, use this statement:

CREATE USER 'nativeuser'@'localhost'
IDENTIFIED WITH mysql_native_password BY 'password';

caching_sha2_password supports connections over secure transport. If you follow the RSA configuration procedure given later in this section, it also supports encrypted password exchange using RSA over unencrypted connections. RSA support has these characteristics:

• On the server side, two system variables name the RSA private and public key-pair files: caching_sha2_password_private_key_path and caching_sha2_password_public_key_path. The database administrator must set these variables at server startup if the key files to use have names that differ from the system variable default values.

• The server uses the caching_sha2_password_auto_generate_rsa_keys system variable to determine whether to automatically generate the RSA key-pair files. See Section 5.3, “Creating SSL and RSA Certificates and Keys”.

• The Caching_sha2_password_rsa_public_key status variable displays the RSA public key value used by the caching_sha2_password authentication plugin.

• Clients that are in possession of the RSA public key can perform RSA key pair-based password exchange with the server during the connection process, as described later.

• For connections by accounts that authenticate with caching_sha2_password and RSA key pair-based password exchange, the server does not send the RSA public key to clients by default. Clients can use a client-side copy of the required public key, or request the public key from the server.

  Use of a trusted local copy of the public key enables the client to avoid a round trip in the client/server protocol, and is more secure than requesting the public key from the server. On the other hand, requesting the public key from the server is more convenient (it requires no management of a client-side file) and may be acceptable in secure network environments.

  • For command-line clients, use the --server-public-key-path option to specify the RSA public key file. Use the --get-server-public-key option to request the public key from the server. The following programs support the two options: mysql, mysqlsh, mysqladmin, mysqlbinlog, mysqlcheck, mysqldump, mysqlimport, mysqlpump, mysqlshow, mysqlslap, mysqltest, mysql_upgrade.

  • For programs that use the C API, call mysql_options() to specify the RSA public key file by passing the MYSQL_SERVER_PUBLIC_KEY option and the name of the file, or request the public key from the server by passing the MYSQL_OPT_GET_SERVER_PUBLIC_KEY option.

  • For replicas, use the CHANGE REPLICATION SOURCE TO statement (from MySQL 8.0.23) or CHANGE MASTER TO statement (before MySQL 8.0.23) with the SOURCE_PUBLIC_KEY_PATH | MASTER_PUBLIC_KEY_PATH option to specify the RSA public key file, or the GET_SOURCE_PUBLIC_KEY | GET_MASTER_PUBLIC_KEY option to request the public key from the source. For Group Replication, the group_replication_recovery_public_key_path and group_replication_recovery_get_public_key system variables serve the same purpose.

  In all cases, if the option is given to specify a valid public key file, it takes precedence over the option to request the public key from the server.

For clients that use the caching_sha2_password plugin, passwords are never exposed as cleartext when connecting to the server. How password transmission occurs depends on whether a secure connection or RSA encryption is used:

• If the connection is secure, an RSA key pair is unnecessary and is not used. This applies to TCP connections encrypted using TLS, as well as Unix socket-file and shared-memory connections. The password is sent as cleartext but cannot be snooped because the connection is secure.

• If the connection is not secure, an RSA key pair is used. This applies to TCP connections not encrypted using TLS and named-pipe connections. RSA is used only for password exchange between client and server, to prevent password snooping. When the server receives the encrypted password, it decrypts it. A scramble is used in the encryption to prevent repeat attacks.

To enable use of an RSA key pair for password exchange during the client connection process, use the following procedure:

1. Create the RSA private and public key-pair files using the instructions in Section 5.3, “Creating SSL and RSA Certificates and Keys”.

2. If the private and public key files are located in the data directory and are named private_key.pem and public_key.pem (the default values of the caching_sha2_password_private_key_path and caching_sha2_password_public_key_path system variables), the server uses them automatically at startup.

   Otherwise, to name the key files explicitly, set the system variables to the key file names in the server option file. If the files are located in the server data directory, you need not specify their full path names:

   [mysqld]
   caching_sha2_password_private_key_path=myprivkey.pem
   caching_sha2_password_public_key_path=mypubkey.pem

   If the key files are not located in the data directory, or to make their locations explicit in the system variable values, use full path names:

   [mysqld]
   caching_sha2_password_private_key_path=/usr/local/mysql/myprivkey.pem
   caching_sha2_password_public_key_path=/usr/local/mysql/mypubkey.pem

3. If you want to change the number of hash rounds used by caching_sha2_password during password generation, set the caching_sha2_password_digest_rounds system variable. For example:

   [mysqld]
   caching_sha2_password_digest_rounds=10000

4. Restart the server, then connect to it and check the Caching_sha2_password_rsa_public_key status variable value. The value actually displayed differs from that shown here, but should be nonempty:

   mysql> SHOW STATUS LIKE 'Caching_sha2_password_rsa_public_key'\G
   *************************** 1. row ***************************
   Variable_name: Caching_sha2_password_rsa_public_key
           Value: -----BEGIN PUBLIC KEY-----
   MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDO9nRUDd+KvSZgY7cNBZMNpwX6
   MvE1PbJFXO7u18nJ9lwc99Du/E7lw6CVXw7VKrXPeHbVQUzGyUNkf45Nz/ckaaJa
   aLgJOBCIDmNVnyU54OT/1lcs2xiyfaDMe8fCJ64ZwTnKbY2gkt1IMjUAB5Ogd5kJ
   g8aV7EtKwyhHb0c30QIDAQAB
   -----END PUBLIC KEY-----

   If the value is empty, the server found some problem with the key files. Check the error log for diagnostic information.

After the server has been configured with the RSA key files, accounts that authenticate with the caching_sha2_password plugin have the option of using those key files to connect to the server. As mentioned previously, such accounts can use either a secure connection (in which case RSA is not used) or an unencrypted connection that performs password exchange using RSA. Suppose that an unencrypted connection is used. For example:

$> mysql --ssl-mode=DISABLED -u sha2user -p
Enter password: password

For this connection attempt by sha2user, the server determines that caching_sha2_password is the appropriate authentication plugin and invokes it (because that was the plugin specified at CREATE USER time). The plugin finds that the connection is not encrypted and thus requires the password to be transmitted using RSA encryption. However, the server does not send the public key to the client, and the client provided no public key, so it cannot encrypt the password and the connection fails:

ERROR 2061 (HY000): Authentication plugin 'caching_sha2_password'
reported error: Authentication requires secure connection.

To request the RSA public key from the server, specify the --get-server-public-key option:

$> mysql --ssl-mode=DISABLED -u sha2user -p --get-server-public-key
Enter password: password

In this case, the server sends the RSA public key to the client, which uses it to encrypt the password and returns the result to the server. The plugin uses the RSA private key on the server side to decrypt the password and accepts or rejects the connection based on whether the password is correct.

Alternatively, if the client has a file containing a local copy of the RSA public key required by the server, it can specify the file using the --server-public-key-path option:

$> mysql --ssl-mode=DISABLED -u sha2user -p --server-public-key-path=file_name
Enter password: password

In this case, the client uses the public key to encrypt the password and returns the result to the server. The plugin uses the RSA private key on the server side to decrypt the password and accepts or rejects the connection based on whether the password is correct.

The public key value in the file named by the --server-public-key-path option should be the same as the key value in the server-side file named by the caching_sha2_password_public_key_path system variable. If the key file contains a valid public key value but the value is incorrect, an access-denied error occurs. If the key file does not contain a valid public key, the client program cannot use it.

Client users can obtain the RSA public key two ways:

• The database administrator can provide a copy of the public key file.

• A client user who can connect to the server some other way can use a SHOW STATUS LIKE 'Caching_sha2_password_rsa_public_key' statement and save the returned key value in a file.

Cache Operation for SHA-2 Pluggable Authentication

On the server side, the caching_sha2_password plugin uses an in-memory cache for faster authentication of clients who have connected previously. Entries consist of account-name/password-hash pairs. The cache works like this:

1. When a client connects, caching_sha2_password checks whether the client and password match some cache entry. If so, authentication succeeds.

2. If there is no matching cache entry, the plugin attempts to verify the client against the credentials in the mysql.user system table. If this succeeds, caching_sha2_password adds an entry for the client to the hash. Otherwise, authentication fails and the connection is rejected.

In this way, when a client first connects, authentication against the mysql.user system table occurs. When the client connects subsequently, faster authentication against the cache occurs.

Password cache operations other than adding entries are handled by the sha2_cache_cleaner audit plugin, which performs these actions on behalf of caching_sha2_password:

• It clears the cache entry for any account that is renamed or dropped, or any account for which the credentials or authentication plugin are changed.

• It empties the cache when the FLUSH PRIVILEGES statement is executed.

• It empties the cache at server shutdown. (This means the cache is not persistent across server restarts.)

Cache clearing operations affect the authentication requirements for subsequent client connections. For each user account, the first client connection for the user after any of the following operations must use a secure connection (made using TCP using TLS credentials, a Unix socket file, or shared memory) or RSA key pair-based password exchange:

• After account creation.

• After a password change for the account.

• After RENAME USER for the account.

• After FLUSH PRIVILEGES.

FLUSH PRIVILEGES clears the entire cache and affects all accounts that use the caching_sha2_password plugin. The other operations clear specific cache entries and affect only accounts that are part of the operation.

Once the user authenticates successfully, the account is entered into the cache and subsequent connections do not require a secure connection or the RSA key pair, until another cache clearing event occurs that affects the account. (When the cache can be used, the server uses a challenge-response mechanism that does not use cleartext password transmission and does not require a secure connection.)