Cryptography Reference
In-Depth Information
in the 4G mobile standard called LTE (Long Term Evolution). ZUC uses a
16-word LFSR where each word is of 31 bits. The key size is 128 bits and
the keystream is generated in 32-bit words. A finite state machine works as a
non-linear core for this cipher.
A few non-LFSR-based hardware stream ciphers have also been designed.
For examples, the work [128] presents a family of stream ciphers called Vest,
based on non-linear parallel feedback shift registers (NLFSR) and non-linear
Residue Number System (RNS) based counters.
2.4 Software Stream Ciphers
A software stream cipher typically consists of two modules. The first
module takes as input the secret key (and optionally an IV) and expands it
into a secret internal state. This is called key scheduling. The second module
generates the keystream from the internal state. At every state transition,
a pseudo-random keystream bit or byte or word is output depending on the
design of the cipher.
Here we discuss RC4, the most widely used software stream cipher. Vari-
ants of RC4 are discussed in Chapter 9. The eSTREAM finalist HC-128 can
be considered as the next generation of software stream cipher evolution after
RC4 and RC4-like ciphers. It is not easy to maintain an array of 2
32
loca-
tions to implement a 32-bit instance of RC4. More security margin for 32-bit
cipher obviously requires more time/memory. HC-128 is a 32-bit cipher that
satisfies these requirements. Use of arrays and modular addition in HC-128 is
in similar line as RC4, but many more additional operations are incorporated.
We devote Chapter 10 to the discussion of HC-128 in detail.
2.4.1 RC4 Stream Cipher
The RC4 stream cipher was designed by Ron Rivest for RSA Data Security
in 1987. It is believed to be a propriety algorithm. In 1994, it was allegedly
revealed on the internet [6]. Currently, RC4 stands out to be one of the most
popular among state-of-the-art software stream ciphers with varied industrial
applications. It is used for encrypting the internet tra
c in network protocols
such as Secure Sockets Layer (SSL), Transport Layer Security (TLS), Wired
Equivalent Privacy (WEP), Wi-Fi Protected Access (WPA) etc. The cipher
is also used in Microsoft Windows, Lotus Notes, Apple Open Collaboration
Environment (AOCE), and Oracle Secure SQL.
RC4 data structure consists of an S-Box
S = (S[0],...,S[N −1])
of length N = 2
n
, where each entry is an n-bit integer. Typically, n = 8 and
