12

5203AS–CRYPT–7/08

AT88SC0404CA

Figure 12-1. Password and Authentication Operations

Note:

Authentication and password verification may be attempted at any time and in any order. Exceeding corresponding authentica-

tion or password attempts trial limit renders subsequent authentication or password verification attempts futile.

12.5

Cryptographic Message Authentication Codes

AT88SC0404CA implements a data validity check function in the standard, authentication or

encryption modes of operation.

In the standard mode, data validity check is done through 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 authentication and encryption modes, the data validity check becomes more powerful since it

provides a bidirectional data integrity check and data origin authentication capability in the form

of a Message Authentication Codes (MAC). Only the host/device that carried out a valid authen-

tication is capable of computing a valid MAC. While operating in the authentication or encryption

modes, the use of MAC is required. For an ingoing command, if the device calculates a MAC dif-

ferent from the MAC transmitted by the host, not only is the command abandoned but the

security privilege is revoked. A new authentication and/or encryption activation will be required

to reactivate the MAC.

CS

VERIFY CS

Write DATA

VERIFY RPW

DATA

Checksum (CS)

VERIFY CS