Anti-tearing
In the event of a power loss during a write cycle, the integrity of the device’s stored data may be recovered. This function is optional: the host may choose to activate the anti-tearing function, depending on application requirements. When anti-tearing is active, write commands take longer to execute, since more write cycles are required to complete them, and data are limited to eight bytes.
Data are written first to a buffer zone in EEPROM instead of the intended destination address, but with the same access conditions. The data are then written in the required location. If this second write cycle is interrupted due to a power loss, the device will automatically recover the data from the system buffer zone at the next power-up.
In 2-wire mode, the host is required to perform ACK polling for up to 8 ms after write commands when anti-tearing is active. At power-up, the host is required to perform ACK polling, in some cases for up to 2 ms, in the event that the device needs to carry out the data recovery process.
Write Lock
If a user zone is configured in the write lock mode, the lowest address byte of an 8-byte page constitutes a write access byte for the bytes of that page.
Password Verification
Passwords may be used to protect read and/or write access of any user zone. When a valid password is presented, it is memorized and active until power is turned off, unless a new password is presented or RST becomes active. There are eight password sets that may be used to protect any user zone. Only one password is active at a time, but write passwords give read access also.
Authentication Protocol
The access to a user zone may be protected by an authentication protocol. Any one of four keys may be selected to use with a user zone.
The authentication success is memorized and active as long as the chip is powered, unless a new authentication is initialized or RST becomes active. If the new authentication request is not validated, the card loses its previous authentication and it should be presented again. Only the last request is memorized d.
Note: Password and authentication may be presented at any time and in any order. If the trials limit has been reached (after four consecutive incorrect attempts), the password verification or authentication process will not be taken into account.
Checksum
The AT88SC0204C 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 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 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.
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.
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 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 AT88SC0204C device, the secure code is $E5 47 47. 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.
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