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AT88SC25616C

5017KS–SMEM–08/09

12.2.

Checksum

The AT88SC25616C implements a data validity check function in the form of a checksum, which may function in

standard, authentication or encryption modes.

In the standard mode, the checksum is implemented as a Modification Detection Code (MDC), in which the host may

read a MDC from the device in order to verify that the data sent was received correctly.

In the authentication and encryption modes, the checksum becomes more powerful since it provides a bidirectional

data integrity check and data origin authentication capability in the form of a Message Authentication Code (MAC).

Only the host/device that carried out a valid authentication is capable of computing a valid MAC. While operating in the

authentication or encryption modes, the use of a MAC is required. For an ingoing command, if the device calculates a

MAC different from the MAC transmitted by the host, not only is the command abandoned but the mode is also reset. A

new authentication and/or encryption activation will be required to reactivate the MAC.

12.3.

Encryption

The data exchanged between the device and the host during read, write and verify password commands may be

encrypted to ensure data confidentiality.

The issuer may choose to require encryption for a user zone by settings made in the configuration memory. Any one of

four keys may be selected for use with a user zone. In this case, activation of the encryption mode is required in order

to read/write data in the zone and only encrypted data will be transmitted. Even if not required, the host may elect to

activate encryption provided the proper keys are known.

12.4.

Supervisor Mode

Enabling this feature allows the holder of one specific password to gain full access to all eight password sets, including

the ability to change passwords.

12.5.

Modify Forbidden

No write access is allowed in a user zone protected with this feature at any time. The user zone must be written during

device personalization prior to blowing the security fuses.

12.6.

Program Only

For a user zone protected by this feature, data within the zone may be changed from a “1” to a “0”, but never from a “0”

to a “1”.

13.

Initial Device Programming

To enable the security features of CryptoMemory, the device must first be personalized to set up several registers and

load in the appropriate passwords and keys. This is accomplished through programming the configuration memory of

CryptoMemory using simple write and read commands. To gain access to the configuration memory, the secure code

must first be successfully presented. For the AT88SC25616C device, the secure code is $17 C3 3A. After writing and

verifying data in the configuration memory, the security fuses must be blown to lock this information in the device. For

additional information on personalizing CryptoMemory, please see the application notes Programming CryptoMemory

for Embedded Applications and Initializing CryptoMemory for Smart Card Applications