Abstract: Dependent on a variable start value (s), a random number according to the relationship v=f (K;s) is generated from a ciphering algorithm (f) implemented in the microprocessor of the chip card and from a stored secret cipher (K). This random number can be intermediately stored in a register and, when generating a new random number, can be logically operated with a variable input quantity (for example, with the variable start value (s), to form a modified variable start value (s'), for use in establishing protected communications between the chip card and a user terminal having a chip card reader.

Abstract: A secret cipher (Ki) that is the same for all user terminals is formed of two sub-components (KTi, KT'i) whereof the one sub-component (KTi) is deposited in an erasable, programmable read-only memory (EEPROM). For the second sub-component (KT'i), an encoded data block (E(KT'i)) is transmitted from the outside to a decoder means (DEC) provided in the security module, the decoded output signal thereof being deposited as second sub-component (KT'i) in a first sub-area of a write-read memory (RAM) present in the security module of the user terminal. An overall cipher (Ki) is calculated from the two sub-components (KTi, KT'i) and the result is deposited in a second sub-area of the write-read memory (RAM).

Abstract: A random number (v) generated in a customer chip card (KK) selects individual program parts (Pv) from the sensitive program data (P) for the data flow control, selecting these both in the customer card as well as in a customer terminal (KT). A respective authentication code (PACv) is formed from these program parts (Pv) with the assistance of an authentication algorithm (f) and of a secret cipher (KPC) and, finally, the authentication code (PACv) calculated in the customer chip card (KK) and the authentication code (PACv) calculated in the customer terminal (KT) are checked for identity in a comparison means (COMP) provided in the customer card (KK).