1.2 Vertraulichkeit - Stromchiffre mit wechselnden Sitzungsschlüsseln

Enciphering

The purpose of this security service is to ensure the privacy of the user information carried in both traffic and signalling channels and of user-related signalling elements on the radio path. The activation of this service is controlled by the network. It is started by the base station by sending a start cipher command to the MS.

A standard cipher algorithm called A5 is contained as dedicated hardware in mobile equipment and base stations. A5 is a stream cipher14, and because of its high encryption rate is suitable for real time applications as telephony. The plain text is organised into blocks of 114 bits as this is the amount of data which is transmitted during a time slot. The key stream, which is Key generation and enciphering a sequence of bits to be XORed (modulo 2 addition) with the data block, is produced by the algorithm A5 as an output block of 114 bits. The generation of the key stream from the A5 algorithm is controlled by the key Kc (input parameter). This key is derived in the SIM as part of the authentication process using the network operator specific key generation algorithm A8 and the same RAND and Ki as in the authentication algorithm A3. The process of cipher key generation and enciphering is shown in the animation.

Ki: geheimer Schlüssel des Mobilfunkteilnehmers (128 Bit)

RAND: Zufallsbitfolge (128 Bit)

Kc: Sitzungsschlüssel (64 Bit)

Kc = A8(Ki, RAND)

TDMA: Rahmennummern der zu verschlüsselnden Daten

: Modulo-2-Addition des Ausgabebitstroms A5(TDMA, Kc) und des Klartextbitstroms

Der Algorithmus A8 ist netzbetreiberabhängig und nicht europaweit standardisiert.

Der Algorithmus A5 ist ein Pseudozufallszahlengenarator, der europaweit standardisiert und nur den Herstellern zugänglich ist.