Conventional Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 51

K c

128 bits 2745 bits

⊕ Ciphertext frame BD ADDR E0 level 1 E0 level 2

CLK

Plaintext frame

Figure 2.31. The E0 keystream generator.

automaton for producing the keystream frame. Frames are finally used as a keystream
in the Vernam cipher.

E0 is very efficient in hardware. Its circuit is depicted in Fig. 2.32. The division
by two at the output from the second summator simply consists of dropping one bit.
Internal memory bit pairs for c t and c t − 1 are represented by the z − 1 symbol as a clock
time delay.

2.9 Brute Force Attacks

We now wonder what kind of security we can expect from symmetric encryption from
a generic point of view. Namely, we wonder about performances of generic attacks
using brute force which can apply to any encryption algorithm considered as a black
box. Security will be measured in terms of the encryption parameters such as the key
length and the message space size.

z t

⊕

c t + 1

c t

LFSR 1

z − 1

c t + 1

x t

c t

x t

×

LFSR 2

y t

+ +

y t

c t − 1

⊕

x t

LFSR 3

z − 1

c t − 1 ⊕

⊕

y t

s t + 1 s t + 1 LFSR 4

x t

Figure 2.32. One level of the E0 keystream generator.