How MACs are used

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When a message is sent, an application program can generate an authentication code for it using the MAC Generate verb.

CCA supports the ANSI X9.9-1 basic procedure and both the ANSI X9.19 basic procedure and optional double key MAC procedure. The MAC Generate verb computes the text of the Message Authentication Code using the algorithm and a key. The ANSI X9.9-1 or ANSI X9.19 basic procedures accept either a single-length MAC generation (MAC) key or a data-encrypting (DATA) key, and the message text. The ANSI X9.19 optional double key MAC procedure accepts a double-length MAC key and the message text. The originator of the message sends the MAC with the message text.

When the receiver gets the message, an application program calls the MAC Verify verb. The MAC Generate verb generates a MAC using the same algorithm as the sender and either the single-length or double-length MAC verification key, the single-length or double-length MAC generation key, or DATA key, and the message text. The MAC Verify verb compares the MAC it generates with the one sent with the message and issues a return code that indicates whether the MACs match. If the return code indicates that the MACs match, the receiver can accept the message as genuine and unaltered. If the return code indicates that the MACs do not match, the receiver can assume the message is either fraudulent or has been altered. The newly computed MAC is not revealed outside the cryptographic coprocessor.

In a similar manner, MACs can be used to ensure the integrity of data stored on the system or on removable media, such as tape.

Secure use of the MAC Generate and MAC Verify verbs requires the use of MAC and MACVER keys in these verbs, respectively. To accomplish this, the originator of the message generates a MAC/MACVER key pair, uses the MAC key in the MAC Generate verb, and exports the MACVER key to the receiver. The originator of the message enforces key separation on the link by encrypting the MACVER key under a transport key that is not an NOCV key before exporting the key to the receiver. With this type of key separation enforced, the receiver can receive only a MACVER key and can use only this key in the MAC Verify verb. This ensures that the receiver cannot alter the message and produce a valid MAC with the altered message. These security features are not present if DATA keys are used in the MAC Generate verb or if DATA or MAC keys are used in the MAC Verify verb.

By using MACs you get the following benefits:
  • For data transmitted over a network, you can validate the authenticity of the message as well as ensure the data has not been altered during transmission. For example, an active eavesdropper can tap into a transmission line and interject fraudulent messages or alter sensitive data being transmitted. If the data is accompanied by a MAC, the recipient can use a verb to detect whether the data has been altered. Because both the sender and receiver share a secret key, the receiver can use a verb that calculates a MAC on the received message and compares it to the MAC transmitted with the message. If the comparison is equal, the message could be accepted as unaltered. Furthermore, because the shared key is secret, when a MAC is verified it can be assumed that the sender was, in fact, the other person who knew the secret key.
  • For data stored on tape or DASD, you can ensure that the data read back onto the system was the same as the data written onto the tape or DASD. For example, someone might be able to bypass access controls. Such an access might escape the notice of auditors. However, if a MAC is stored with the data, and verified when the data is read, you can detect alterations to the data.