13 Configuring Oracle Database Native Network Encryption and Data Integrity

You can configure native Oracle Net Services data encryption and data integrity for both servers and clients.

• About Oracle Database Native Network Encryption and Data Integrity
  Oracle Database enables you to encrypt data that is sent over a network.
• Oracle Database Native Network Encryption Data Integrity
  Encrypting network data provides data privacy so that unauthorized parties cannot view plaintext data as it passes over the network.
• Improving Native Network Encryption Security
  Oracle provides a patch that will strengthen native network encryption security for both Oracle Database servers and clients.
• Data Integrity Algorithms Support
  A keyed, sequenced implementation of the Message Digest 5 (MD5) algorithm or the Secure Hash Algorithm (SHA-1 and SHA-2) protect against these attacks.
• Diffie-Hellman Based Key Negotiation
  You can use the Diffie-Hellman key negotiation algorithm to secure data in a multiuser environment.
• Configuration of Data Encryption and Integrity
  Oracle Database native Oracle Net Services encryption and integrity presumes the prior installation of Oracle Net Services.

13.1 About Oracle Database Native Network Encryption and Data Integrity

Oracle Database enables you to encrypt data that is sent over a network.

• How Oracle Database Native Network Encryption and Integrity Works
  Oracle Database provides native data network encryption and integrity to ensure that data is secure as it travels across the network.
• Advanced Encryption Standard
  Oracle Database supports the Federal Information Processing Standard (FIPS) encryption algorithm, Advanced Encryption Standard (AES).
• ARIA
  Oracle Database supports the Academia, Research Institute, and Agency (ARIA) algorithm.
• GOST
  Oracle Database supports the GOsudarstvennyy STandart (GOST) algorithm.
• SEED
  Oracle Database supports the Korea Information Security Agency (KISA) encryption algorithm, SEED.
• Triple-DES Encryption
  Triple-DES encryption (3DES) encrypts message data with three passes of the DES algorithm.
• Choosing Between Native Network Encryption and Transport Layer Security
  Oracle offers two ways to encrypt data over the network, native network encryption and Transport Layer Security (TLS).

13.1.1 How Oracle Database Native Network Encryption and Integrity Works

Oracle Database provides native data network encryption and integrity to ensure that data is secure as it travels across the network.

The purpose of a secure cryptosystem is to convert plaintext data into unintelligible ciphertext based on a key, in such a way that it is very hard (computationally infeasible) to convert ciphertext back into its corresponding plaintext without knowledge of the correct key.

In a symmetric cryptosystem, the same key is used both for encryption and decryption of the same data. Oracle Database provides the Advanced Encryption Standard (AES) symmetric cryptosystem for protecting the confidentiality of Oracle Net Services traffic.

13.1.2 Advanced Encryption Standard

Oracle Database supports the Federal Information Processing Standard (FIPS) encryption algorithm, Advanced Encryption Standard (AES).

AES can be used by all U.S. government organizations and businesses to protect sensitive data over a network. This encryption algorithm defines three standard key lengths, which are 128-bit, 192-bit, and 256-bit. All versions operate in outer Cipher Block Chaining (CBC) mode. CBC mode is an encryption method that protects against block replay attacks by making the encryption of a cipher block dependent on all blocks that precede it; it is designed to make unauthorized decryption incrementally more difficult. Oracle Database employs outer cipher block chaining because it is more secure than inner cipher block chaining, with no material performance penalty.

Note:

The AES algorithms have been improved. To transition your Oracle Database environment to use stronger algorithms, download and install the patch described in My Oracle Support note 2118136.2.

13.1.3 ARIA

Oracle Database supports the Academia, Research Institute, and Agency (ARIA) algorithm.

This algorithm acknowledges the cooperative efforts of Korean researchers in designing the algorithm.

ARIA defines three standard key lengths, which are 128-bit, 192-bit, and 256-bit. All versions operate in outer cipher Cipher Block Chaining (CBC) mode.

13.1.4 GOST

Oracle Database supports the GOsudarstvennyy STandart (GOST) algorithm.

The GOST algorithm was created by the Euro-Asian Council for Standardization, Metrology and Certification (EACS).

GOST defines a key size of 256-bits. In Oracle Database, outer Cipher Block Chaining (CBC) mode is used.

13.1.5 SEED

Oracle Database supports the Korea Information Security Agency (KISA) encryption algorithm, SEED.

SEED defines a key size of 128-bits. There are extensions to the standard that defines additional key sizes of 192- and 256-bits, but Oracle Database does not support these extensions. In the Oracle Database, SEED operates in outer Cipher Block Chaining (CBC) mode.

13.1.6 Triple-DES Encryption

Triple-DES encryption (3DES) encrypts message data with three passes of the DES algorithm.

Note:

The DES, DES40, 3DES112, and 3DES168 algorithms are deprecated in this release. To transition your Oracle Database environment to use stronger algorithms, download and install the patch described in My Oracle Support note 2118136.2.

3DES provides a high degree of message security, but with a performance penalty. The magnitude of the performance penalty depends on the speed of the processor performing the encryption. 3DES typically takes three times as long to encrypt a data block when compared to the standard DES algorithm.

3DES is available in two-key and three-key versions, with effective key lengths of 112-bits and 168-bits, respectively. Both versions operate in outer Cipher Block Chaining (CBC) mode.

The DES40 algorithm, available with Oracle Database and Secure Network Services, is a variant of DES in which the secret key is preprocessed to provide 40 effective key bits. It was designed to provide DES-based encryption to customers outside the U.S. and Canada at a time when the U.S. export laws were more restrictive. Currently DES40, DES, and 3DES are all available for export. DES40 is still supported to provide backward-compatibility for international customers.

13.1.7 Choosing Between Native Network Encryption and Transport Layer Security

Oracle offers two ways to encrypt data over the network, native network encryption and Transport Layer Security (TLS).

There are advantages and disadvantages to both methods.

Table 13-1 Comparison of Native Network Encryption and Transport Layer Security

-	Native Network Encryption	Transport Layer Security
Advantages	It is configured with parameters in the sqlnet.ora configuration file. In most cases, no client configuration changes are required. No certificates are required. Clients that do not support native network encryption can fall back to unencrypted connections while incompatibility is mitigated.	It is an industry standard for encrypting data in motion. It provides non-repudiation for server connections to prevent third-party attacks. It can be used for database user authentication.
Disadvantages	It uses a non-standard, Oracle proprietary implementation. It provides no non-repudiation of the server connection (that is, no protection against a third-party attack).	It requires client and server changes. Certificates are required for server and are optional for the client. However, the client must have the trusted root certificate for the certificate authority that issued the server’s certificate. Certificates eventually expire.

13.2 Oracle Database Native Network Encryption Data Integrity

Encrypting network data provides data privacy so that unauthorized parties cannot view plaintext data as it passes over the network.

Oracle Database also provides protection against two forms of active attacks.

Table 13-2 provides information about these attacks.

Table 13-2 Two Forms of Network Attacks

Type of Attack	Explanation
Data modification attack	An unauthorized party intercepting data in transit, altering it, and retransmitting it is a data modification attack. For example, intercepting a $100 bank deposit, changing the amount to $10,000, and retransmitting the higher amount is a data modification attack.
Replay attack	Repetitively retransmitting an entire set of valid data is a replay attack, such as intercepting a $100 bank withdrawal and retransmitting it ten times, thereby receiving $1,000.

13.3 Improving Native Network Encryption Security

Oracle provides a patch that will strengthen native network encryption security for both Oracle Database servers and clients.

• About Improving Native Network Encryption Security
  The Oracle patch will update encryption and checksumming algorithms and deprecate weak encryption and checksumming algorithms.
• Applying Security Improvement Updates to Native Network Encryption
  In addition to applying a patch to the Oracle Database server and client, you must set the server and client sqlnet.ora parameters.

13.3.1 About Improving Native Network Encryption Security

The Oracle patch will update encryption and checksumming algorithms and deprecate weak encryption and checksumming algorithms.

This patch, which you can download from My Oracle Support note 2118136.2, strengthens the connection between servers and clients, fixing a vulnerability in native network encryption and checksumming algorithms. It adds two parameters that make it easy to disable older, less secure encryption and checksumming algorithms. Oracle strongly recommends that you apply this patch to your Oracle Database server and clients.

This patch applies to Oracle Database releases 11.2 and later. You can apply this patch in the following environments: standalone, multitenant, primary-standby, Oracle Real Application Clusters (Oracle RAC), and environments that use database links.

The supported algorithms that have been improved are as follows:

• Encryption algorithms: AES128, AES192 and AES256
• Checksumming algorithms: SHA1, SHA256, SHA384, and SHA512

Weak algorithms that are deprecated and should not be used after you apply the patch are as follows:

• Encryption algorithms: DES, DES40, 3DES112, 3DES168, RC4_40, RC4_56, RC4_128, and RC4_256
• Checksumming algorithm: MD5

The general procedure that you will follow is to first replace references to desupported algorithms in your Oracle Database environment with supported algorithms, patch the server, patch the client, and finally, set sqlnet.ora parameters to re-enable a proper connection between the server and clients.

The patch affects the following areas including, but not limited to, the following:

• JDBC network encryption-related configuration settings
• Encryption and integrity parameters that you have configured using Oracle Net Manager
• Secure Sockets Layer (SSL) SSL_CIPHER_SUITE parameter settings
• SecureFiles LOB encrypted columns
• Database Resident Connection Pooling (DRCP) configurations
• Encryption settings used for the configuration of Oracle Call Interface (Oracle OCI), ODP.NET

13.3.2 Applying Security Improvement Updates to Native Network Encryption

In addition to applying a patch to the Oracle Database server and client, you must set the server and client sqlnet.ora parameters.

Ensure that you perform the following steps in the order shown:

1. Back up the servers and clients to which you will install the patch.
2. Log in to My Oracle Support and then download patch described in My Oracle Support note 2118136.2.
   My Oracle Support is located at the following URL:

   https://support.oracle.com

3. Patch the server.
   Follow the instructions in My Oracle Support note 2118136.2 to apply the patch to the server. You will apply the same patch to the client in a later step.
4. Patch the clients.

   Determine which clients you need to patch.

   Follow the instructions in My Oracle Support note 2118136.2 to apply the patch to each client.

5. In each client sqlnet.ora file, remove all deprecated algorithms if they are defined.
   You can bypass this step if the following parameters are not defined or have no algorithms listed.

   • SQLNET.ENCRYPTION_TYPES_CLIENT
   • SQLNET.CRYPTO_CHECKSUM_TYPES_CLIENT
6. In the server sqlnet.ora file, remove all deprecated algorithms if they are defined.
   You can bypass this step if the following parameters are not defined or have no algorithms listed.

   • SQLNET.ENCRYPTION_TYPES_SERVER
   • SQLNET.CRYPTO_CHECKSUM_TYPES_SERVER
7. For maximum security on the server, set the following sqlnet.ora parameters:
   • SQLNET.ENCRYPTION_SERVER = REQUIRED
   • SQLNET.ENCRYPTION_TYPES_SERVER = (AES256)
   • SQLNET.CRYPTO_CHECKSUM_SERVER = REQUIRED
   • SQLNET.CRYPTO_CHECKSUM_TYPES_SERVER = (SHA512)
   • SQLNET.ALLOW_WEAK_CRYPTO_CLIENTS = FALSE
8. For maximum security on the client, set the following sqlnet.ora parameters:
   • SQLNET.ENCRYPTION_CLIENT = REQUIRED
   • SQLNET.ENCRYPTION_TYPES_CLIENT = (AES256)
   • SQLNET.CRYPTO_CHECKSUM_CLIENT = REQUIRED
   • SQLNET.CRYPTO_CHECKSUM_TYPES_CLIENT = (SHA512)
   • SQLNET.ALLOW_WEAK_CRYPTO = FALSE
9. In each client sqlnet.ora file, after you have removed all the deprecated algorithms from the server and the clients per steps 5 and 6, set the parameter SQLNET.ALLOW_WEAK_CRYPTO = FALSE so that the clients can be prevented from communicating with unpatched servers.
   If the SQLNET.ALLOW_WEAK_CRYPTO parameter is set to FALSE, then a client attempting to use a weak algorithm will produce an ORA-12269: client uses weak encryption/crypto-checksumming version error at the server. A client connecting to a server (or proxy) that is using weak algorithms will receive an ORA-12268: server uses weak encryption/crypto-checksumming version error.
10. In the server sqlnet.ora file, after you have updated all the clients with SQLNET.ALLOW_WEAK_CRYPTO = FALSE per step 9, set the parameter SQLNET.ALLOW_WEAK_CRYPTO_CLIENTS = FALSE. This parameter prevents a patched server from communicating with unpatched clients.

    If the SQLNET.ALLOW_WEAK_CRYPTO parameter is set to FALSE, then a client attempting to use a weak algorithm will produce an ORA-12269: client uses weak encryption/crypto-checksumming version error at the server. A client connecting to a server (or proxy) that is using weak algorithms will receive an ORA-12268: server uses weak encryption/crypto-checksumming version error.

    If you use the database links, then the first database server acts as a client and connects to the second server. Therefore, ensure that all servers are fully patched and unsupported algorithms are removed before you set SQLNET.ALLOW_WEAK_CRYPTO to FALSE

13.4 Data Integrity Algorithms Support

A keyed, sequenced implementation of the Message Digest 5 (MD5) algorithm or the Secure Hash Algorithm (SHA-1 and SHA-2) protect against these attacks.

Note:

MD5 is deprecated in this release. To transition your Oracle Database environment to use stronger algorithms, download and install the patch described in My Oracle Support note 2118136.2.

Both of these hash algorithms create a checksum that changes if the data is altered in any way. This protection operates independently from the encryption process so you can enable data integrity with or without enabling encryption.

13.5 Diffie-Hellman Based Key Negotiation

You can use the Diffie-Hellman key negotiation algorithm to secure data in a multiuser environment.

Secure key distribution is difficult in a multiuser environment. Oracle Database uses the well known Diffie-Hellman key negotiation algorithm to perform secure key distribution for both encryption and data integrity.

When encryption is used to protect the security of encrypted data, keys must be changed frequently to minimize the effects of a compromised key. Accordingly, the Oracle Database key management function changes the session key with every session.

The client and the server begin communicating using the session key generated by Diffie-Hellman. When the client authenticates to the server, they establish a shared secret that is only known to both parties. Oracle Database combines the shared secret and the Diffie-Hellman session key to generate a stronger session key designed to defeat a man-in-the-middle attack.

13.6 Configuration of Data Encryption and Integrity

Oracle Database native Oracle Net Services encryption and integrity presumes the prior installation of Oracle Net Services.

• About Activating Encryption and Integrity
  In any network connection, both the client and server can support multiple encryption algorithms and integrity algorithms.
• About Negotiating Encryption and Integrity
  The sqlnet.ora file on systems using data encryption and integrity must contain some or all the REJECTED, ACCEPTED, REQUESTED, and REQUIRED parameters.
• Configuring Encryption and Integrity Parameters Using Oracle Net Manager
  You can set up or change encryption and integrity parameter settings using Oracle Net Manager.

13.6.1 About Activating Encryption and Integrity

In any network connection, both the client and server can support multiple encryption algorithms and integrity algorithms.

When a connection is made, the server selects which algorithm to use, if any, from those algorithms specified in the sqlnet.ora files.The server searches for a match between the algorithms available on both the client and the server, and picks the first algorithm in its own list that also appears in the client list. If one side of the connection does not specify an algorithm list, all the algorithms installed on that side are acceptable. The connection fails with error message ORA-12650 if either side specifies an algorithm that is not installed.

Encryption and integrity parameters are defined by modifying a sqlnet.ora file on the clients and the servers on the network.

You can choose to configure any or all of the available encryption algorithms, and either or both of the available integrity algorithms. Only one encryption algorithm and one integrity algorithm are used for each connect session.

Note:

Oracle Database selects the first encryption algorithm and the first integrity algorithm enabled on the client and the server. Oracle recommends that you select algorithms and key lengths in the order in which you prefer negotiation, choosing the strongest key length first.

See Also:

• Table 13-4 for a listing of valid encryption algorithms

• Oracle Database Advanced Security Guide for a listing of available integrity algorithms

• Data Encryption and Integrity Parameters

13.6.2 About Negotiating Encryption and Integrity

The sqlnet.ora file on systems using data encryption and integrity must contain some or all the REJECTED, ACCEPTED, REQUESTED, and REQUIRED parameters.

• About the Values for Negotiating Encryption and Integrity
  Oracle Net Manager can be used to specify four possible values for the encryption and integrity configuration parameters.
• REJECTED Configuration Parameter
  The REJECTED value disables the security service, even if the other side requires this service.
• ACCEPTED Configuration Parameter
  The ACCEPTED value enables the security service if the other side requires or requests the service.
• REQUESTED Configuration Parameter
  The REQUESTED value enables the security service if the other side permits this service.
• REQUIRED Configuration Parameter
  The REQUIRED value enables the security service or preclude the connection.

13.6.2.1 About the Values for Negotiating Encryption and Integrity

Oracle Net Manager can be used to specify four possible values for the encryption and integrity configuration parameters.

The following four values are listed in the order of increasing security, and they must be used in the profile file (sqlnet.ora) for the client and server of the systems that are using encryption and integrity.

The value REJECTED provides the minimum amount of security between client and server communications, and the value REQUIRED provides the maximum amount of network security:

• REJECTED

• ACCEPTED

• REQUESTED

• REQUIRED

The default value for each of the parameters is ACCEPTED.

Oracle Database servers and clients are set to ACCEPT encrypted connections out of the box. This means that you can enable the desired encryption and integrity settings for a connection pair by configuring just one side of the connection, server-side or client-side.

So, for example, if there are many Oracle clients connecting to an Oracle database, you can configure the required encryption and integrity settings for all these connections by making the appropriate sqlnet.ora changes at the server end. You do not need to implement configuration changes for each client separately.

Table 13-3 shows whether the security service is enabled, based on a combination of client and server configuration parameters. If either the server or client has specified REQUIRED, the lack of a common algorithm causes the connection to fail. Otherwise, if the service is enabled, lack of a common service algorithm results in the service being disabled.

Table 13-3 Encryption and Data Integrity Negotiations

Client Setting	Server Setting	Encryption and Data Negotiation
REJECTED	REJECTED	OFF
ACCEPTED	REJECTED	OFF
REQUESTED	REJECTED	OFF
REQUIRED	REJECTED	Connection fails
REJECTED	ACCEPTED	OFF
ACCEPTED	ACCEPTED	OFFFoot 1
REQUESTED	ACCEPTED	ON
REQUIRED	ACCEPTED	ON
REJECTED	REQUESTED	OFF
ACCEPTED	REQUESTED	ON
REQUESTED	REQUESTED	ON
REQUIRED	REQUESTED	ON
REJECTED	REQUIRED	Connection fails
ACCEPTED	REQUIRED	ON
REQUESTED	REQUIRED	ON
REQUIRED	REQUIRED	ON

^Footnote 1

This value defaults to OFF. Cryptography and data integrity are not enabled until the user changes this parameter by using Oracle Net Manager or by modifying the sqlnet.ora file.

13.6.2.2 REJECTED Configuration Parameter

The REJECTED value disables the security service, even if the other side requires this service.

In this scenario, this side of the connection specifies that the security service is not permitted. If the other side is set to REQUIRED, the connection terminates with error message ORA-12650. If the other side is set to REQUESTED, ACCEPTED, or REJECTED, the connection continues without error and without the security service enabled.

13.6.2.3 ACCEPTED Configuration Parameter

The ACCEPTED value enables the security service if the other side requires or requests the service.

In this scenario, this side of the connection does not require the security service, but it is enabled if the other side is set to REQUIRED or REQUESTED. If the other side is set to REQUIRED or REQUESTED, and an encryption or integrity algorithm match is found, the connection continues without error and with the security service enabled. If the other side is set to REQUIRED and no algorithm match is found, the connection terminates with error message ORA-12650.

If the other side is set to REQUESTED and no algorithm match is found, or if the other side is set to ACCEPTED or REJECTED, the connection continues without error and without the security service enabled.

13.6.2.4 REQUESTED Configuration Parameter

The REQUESTED value enables the security service if the other side permits this service.

In this scenario, this side of the connection specifies that the security service is desired but not required. The security service is enabled if the other side specifies ACCEPTED, REQUESTED, or REQUIRED. There must be a matching algorithm available on the other side, otherwise the service is not enabled. If the other side specifies REQUIRED and there is no matching algorithm, the connection fails.

13.6.2.5 REQUIRED Configuration Parameter

The REQUIRED value enables the security service or preclude the connection.

In this scenario, this side of the connection specifies that the security service must be enabled. The connection fails if the other side specifies REJECTED or if there is no compatible algorithm on the other side.

13.6.3 Configuring Encryption and Integrity Parameters Using Oracle Net Manager

You can set up or change encryption and integrity parameter settings using Oracle Net Manager.

• Configuring Encryption on the Client and the Server
  Use Oracle Net Manager to configure encryption on the client and on the server.
• Configuring Integrity on the Client and the Server
  You can use Oracle Net Manager to configure network integrity on both the client and the server.

13.6.3.1 Configuring Encryption on the Client and the Server

Use Oracle Net Manager to configure encryption on the client and on the server.

1. Start Oracle Net Manager.

   • (UNIX) From $ORACLE_HOME/bin, enter the following command at the command line:

     netmgr
     

   • (Windows) Select Start, Programs, Oracle - HOME_NAME, Configuration and Migration Tools, then Net Manager.

2. Expand Oracle Net Configuration, and from Local, select Profile.
3. From the Naming list, select Network Security.

   The Network Security tabbed window appears.

4. Select the Encryption tab.
   
   Description of the illustration asoencry_12102.png
5. Select CLIENT or SERVER option from the Encryption box.
6. From the Encryption Type list, select one of the following:

   • REQUESTED

   • REQUIRED

   • ACCEPTED

   • REJECTED

7. (Optional) In the Encryption Seed field, enter between 10 and 70 random characters. The encryption seed for the client should not be the same as that for the server.
8. Select an encryption algorithm in the Available Methods list. Move it to the Selected Methods list by choosing the right arrow (>). Repeat for each additional method you want to use.
9. Select File, Save Network Configuration. The sqlnet.ora file is updated.
10. Repeat this procedure to configure encryption on the other system. The sqlnet.ora file on the two systems should contain the following entries:

    • On the server:

      SQLNET.ENCRYPTION_SERVER = [accepted | rejected | requested | required]
      SQLNET.ENCRYPTION_TYPES_SERVER = (valid_encryption_algorithm [,valid_encryption_algorithm])

    • On the client:

      SQLNET.ENCRYPTION_CLIENT = [accepted | rejected | requested | required]
      SQLNET.ENCRYPTION_TYPES_CLIENT = (valid_encryption_algorithm [,valid_encryption_algorithm])
      

Table 13-4 lists valid encryption algorithms and their associated legal values.

Table 13-4 Valid Encryption Algorithms

Algorithm Name	Legal Value
AES 256-bit key	AES256
AES 192-bit key	AES192
AES 128-bit key	AES128

13.6.3.2 Configuring Integrity on the Client and the Server

You can use Oracle Net Manager to configure network integrity on both the client and the server.

1. Start Oracle Net Manager.

   • (UNIX) From $ORACLE_HOME/bin, enter the following command at the command line:

     netmgr
     

   • (Windows) Select Start, Programs, Oracle - HOME_NAME, Configuration and Migration Tools, then Net Manager.

2. Expand Oracle Net Configuration, and from Local, select Profile.
3. From the Naming list, select Network Security.

   The Network Security tabbed window appears.

4. Select the Integrity tab.
   
   Description of the illustration cfig0002.gif
5. Depending upon which system you are configuring, select the Server or Client from the Integrity box.
6. From the Checksum Level list, select one of the following checksum level values:

   • REQUESTED

   • REQUIRED

   • ACCEPTED

   • REJECTED

7. Select an integrity algorithm in the Available Methods list. Move it to the Selected Methods list by choosing the right arrow (>). Repeat for each additional method you want to use.
8. Select File, Save Network Configuration.

   The sqlnet.ora file is updated.

9. Repeat this procedure to configure integrity on the other system.

   The sqlnet.ora file on the two systems should contain the following entries:

   • On the server:

     SQLNET.CRYPTO_CHECKSUM_SERVER = [accepted | rejected | requested | required]
     SQLNET.CRYPTO_CHECKSUM_TYPES_SERVER = (valid_crypto_checksum_algorithm [,valid_crypto_checksum_algorithm])
     

   • On the client:

     SQLNET.CRYPTO_CHECKSUM_CLIENT = [accepted | rejected | requested | required]
     SQLNET.CRYPTO_CHECKSUM_TYPES_CLIENT = (valid_crypto_checksum_algorithm [,valid_crypto_checksum_algorithm])
     

Valid integrity/checksum algorithms that you can user are as follows:

• SHA1
• SHA256
• SHA384
• SHA512