6.1.5 SHA-256 Pluggable Authentication

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

• sha256_password: Implements basic SHA-256 authentication.

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

This section describes the original noncaching SHA-2 authentication plugin. For information about the caching plugin, see Section 6.1.4, “Caching SHA-2 Pluggable Authentication”.

Important

To connect to the server using an account that authenticates with the sha256_password plugin, you must use either a TLS connection or an unencrypted connection that supports password exchange using an RSA key pair, as described later in this section. Either way, the sha256_password plugin uses MySQL' 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 following table shows the plugin names on the server and client sides.

Table 6.4 Plugin and Library Names for SHA-256 Authentication

Plugin or File	Plugin or File Name
Server-side plugin	sha256_password
Client-side plugin	sha256_password
Library file	None (plugins are built in)

The following sections provide installation and usage information specific to SHA-256 pluggable authentication:

• Installing SHA-256 Pluggable Authentication

• Using SHA-256 Pluggable Authentication

For general information about pluggable authentication in MySQL, see Section 4.13, “Pluggable Authentication”.

Installing SHA-256 Pluggable Authentication

The sha256_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.

Using SHA-256 Pluggable Authentication

To set up an account that uses the sha256_password plugin for SHA-256 password hashing, use the following statement, where password is the desired account password:

CREATE USER 'sha256user'@'localhost'
IDENTIFIED WITH sha256_password BY 'password';

The server assigns the sha256_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 sha256_password is the default authentication plugin. If sha256_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 sha256_password, put these lines in the server option file:

[mysqld]
default_authentication_plugin=sha256_password

That causes the sha256_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 'sha256user'@'localhost' IDENTIFIED BY 'password';

Another consequence of setting default_authentication_plugin to sha256_password is that, to use some other plugin for account creation, you must specify that plugin explicitly. For example, to use the mysql_native_password plugin, use this statement:

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

sha256_password supports connections over secure transport. sha256_password also supports encrypted password exchange using RSA over unencrypted connections if these conditions are satisfied:

• MySQL is compiled using OpenSSL, not yaSSL. sha256_password works with distributions compiled using either package, but RSA support requires OpenSSL.

  Note

  It is possible to compile MySQL using yaSSL as an alternative to OpenSSL only prior to MySQL 5.7.28. As of MySQL 5.7.28, support for yaSSL is removed and all MySQL builds use OpenSSL.

• The MySQL server to which you wish to connect is configured to support RSA (using the RSA configuration procedure given later in this section).

RSA support has these characteristics:

• On the server side, two system variables name the RSA private and public key-pair files: sha256_password_private_key_path and sha256_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 sha256_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 Rsa_public_key status variable displays the RSA public key value used by the sha256_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 using sha256_password and RSA public key pair-based password exchange, the server sends the RSA public key to the client as needed. However, if a copy of the public key is available on the client host, the client can use it to save a round trip in the client/server protocol:

  • For these command-line clients, use the --server-public-key-path option to specify the RSA public key file: mysql, mysqltest, and (as of MySQL 5.7.23) mysqladmin, mysqlbinlog, mysqlcheck, mysqldump, mysqlimport, mysqlpump, mysqlshow, mysqlslap, mysqltest.

  • 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.

  • For replicas, RSA key pair-based password exchange cannot be used to connect to source servers for accounts that authenticate with the sha256_password plugin. For such accounts, only secure connections can be used.

For clients that use the sha256_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 connections encrypted using TLS. The password is sent as cleartext but cannot be snooped because the connection is secure.

  Note

  Unlike caching_sha2_password, the sha256_password plugin does not treat shared-memory connections as secure, even though share-memory transport is secure by default.

• If the connection is not secure, and an RSA key pair is available, the connection remains unencrypted. This applies to connections not encrypted using TLS. 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.

• If a secure connection is not used and RSA encryption is not available, the connection attempt fails because the password cannot be sent without being exposed as cleartext.

As mentioned previously, RSA password encryption is available only if MySQL was compiled using OpenSSL. The implication for MySQL distributions compiled using yaSSL is that, to use SHA-256 passwords, clients must use an encrypted connection to access the server. See Section 5.1, “Configuring MySQL to Use Encrypted Connections”.

Note

To use RSA password encryption with sha256_password, the client and server both must be compiled using OpenSSL, not just one of them.

Assuming that MySQL has been compiled using OpenSSL, use the following procedure to enable use of an RSA key pair for password exchange during the client connection process:

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 sha256_password_private_key_path and sha256_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]
   sha256_password_private_key_path=myprivkey.pem
   sha256_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]
   sha256_password_private_key_path=/usr/local/mysql/myprivkey.pem
   sha256_password_public_key_path=/usr/local/mysql/mypubkey.pem

3. Restart the server, then connect to it and check the Rsa_public_key status variable value. The actual value differs from that shown here, but should be nonempty:

   mysql> SHOW STATUS LIKE 'Rsa_public_key'\G
   *************************** 1. row ***************************
   Variable_name: 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 sha256_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 sha256user -p
Enter password: password

For this connection attempt by sha256user, the server determines that sha256_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. In this case, the plugin 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.

The server sends the RSA public key to the client as needed. However, 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 sha256user -p --server-public-key-path=file_name
Enter password: password

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 sha256_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. In this case, the sha256_password plugin sends the public key to the client as if no --server-public-key-path option had been specified.

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 'Rsa_public_key' statement and save the returned key value in a file.