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AT88SC0104C

2021GS–SMEM–7/06

Checksum

The AT88SC0104C implements a data validity check function in the form of a check-

sum, 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 an 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 capa-

bility 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 authentica-

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

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 zone. 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.

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.

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.

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”.

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 zone of CryptoMemory using sim-

ple write and read commands. To gain access to the configuration zone, the secure

code must first be successfully presented. For the AT88SC0104C device, the secure

code is $DD 42 97. After writing and verifying data in the configuration zone, 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 CryptoM-

emory for Embedded Applications and Initializing CryptoMemory for Smart Card

Applications (at www.Atmel.com).