Cryptography Reference
In-Depth Information
modes of operation such as Cipher Block Chaining (CBC) and Output Feedback
(OFB) mode (cf. Chap. 5).
Hardware
One design criterion for DES was its efficiency in hardware. Permutations such as
the
E
,
P
,
IP
and
IP
−
1
permutations are very easy to implement in hardware, as
they only require wiring but no logic. The small 6-by-4 S-boxes are also relatively
easily realizable in hardware. Typically, they are implemented with Boolean logic,
i.e., logic gates. On average, one S-box requires about 100 gates.
An area-efficient implementation of a single DES round can be done with less
than 3000 gates. If a high throughput is desired, DES can be implemented extremely
fast by fitting multiple rounds in one circuit, e.g., by using pipelining. On modern
ASICs and FPGAs throughput rates of several 100 Gbit/sec are possible. On the
other end of the performance spectrum, very small implementations with fewer than
3000 gates even fit onto lowcost radio frequency identification (RFID) chips.
3.7 DES Alternatives
There exist a wealth of other block ciphers. Even though there are many ciphers
which have security weaknesses or which are not well investigated, there are also
many block ciphers which appear very strong. In the following a brief list of ciphers
is given which can be of interest depending on the application needs.
3.7.1 The Advanced Encryption Standard (AES) and the AES
Finalist Ciphers
By now, the algorithm of choice for many, many applications has become the Ad-
vanced Encryption Standard (AES), which will be introduced in detail in the follow-
ing chapter. AES is with its three key lengths of 128, 192 and 256 bit secure against
brute-force attacks for several decades, and there are no analytical attacks with any
reasonable chance of success known.
AES was the result of an open competition, and in the last stage of the selection
process there were four other finalist algorithms. These are the block ciphers
Mars
,
RC6
,
Serpent
and
Twofish
. All of them are cryptographically strong and quite fast,
especially in software. Based on today's knowledge, they can all be recommended.
Mars, Serpent and Twofish can be used royalty-free.
