Data Security - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 219

Key K

Transmission data Received data

K K ′

94 0E 5F 85 FF 32 94 0E 5F 85 FF 32

Cipher data

38 F0 17 33 05 62

???

Attacker

Figure 8.4 By encrypting the data to be transmitted, this data can be effectively protected
from eavesdropping or modification

ciphers are generally very calculation intensive, they play a less important role in RFID
systems. Therefore the emphasis is placed on sequential ciphers in what follows.
A fundamental problem of all cryptological procedures is the secure distribution of
the secret key K , which must be known by the authorised communication participants
prior to the start of the data transfer procedure.

8.3.1 Stream cipher

Sequential ciphers or stream ciphers are encryption algorithms in which the sequence
of plain text characters is encrypted sequentially using a different function for every
step (Fumy, 1994). The ideal realisation of a stream cipher is the so-called one-time
pad , also known as the Vernam cipher after its discoverer (Longo, 1993).
In this procedure a random key K is generated, for example using dice, prior to
the transmission of encrypted data, and this key is made available to both parties
(Figure 8.5). The key sequence is linked with the plain text sequence by the addition
of characters or using XOR gating. The random sequence used as a key must be at
least as long as the message to be encrypted, because periodic repetitions of a typically
short key in relation to the plain text would permit cryptoanalysis and thus an attack
on the transmission. Furthermore, the key may only be used once, which means that an
extremely high level of security is required for the secure distribution of keys. Stream
ciphering in this form is completely impractical for RFID systems.
To overcome the problem of key generation and distribution, systems have been
created based upon the principle of the one-time pad stream cipher, that use a so-
called pseudorandom sequence instead of an actual random sequence. Pseudorandom
sequences are generated using so-called pseudorandom generators.
Figure 8.6 shows the fundamental principle of a sequential cipher using a pseudo-
random generator: because the encryption function of a sequential cipher can change
(at random) with every character, the function must be dependent not only upon the