The Data Encryption Standard (DES) , a system developed for the U.S. government, was intended for use by the general public.

**The Data Encryption Standard**

The Data Encryption Standard
(DES) , a system developed for the
U.S. government, was intended for use by the general public. It has been
officially accepted as a cryptographic standard both in the United States and
abroad. Moreover, many hardware and software systems have been designed with
the DES. However, recently its adequacy has been questioned.

**Background and History**

In the early 1970s, the U.S.
National Bureau of Standards (NBS) recognized that the general public needed a
secure encryption technique for protecting sensitive information. Historically,
the U.S. Department of Defense and the Department of State had had continuing
interest in encryption systems; it was thought that these departments were home
to the greatest expertise in cryptology. However, precisely because of the
sensitive nature of the information they were encrypting, the departments could
not release any of their work. Thus, the responsibility for a more public
encryption technique was delegated to the NBS.

At the same time, several
private vendors had developed encryption devices, using either mechanical means
or programs that individuals or firms could buy to protect their sensitive
communications. The difficulty with this commercial proliferation of encryption
techniques was exchange: Two users with different devices could not exchange encrypted
information. Furthermore, there was no independent body capable of testing the
devices extensively to verify that they properly implemented their algorithms.

It soon became clear that
encryption was ripe for assessment and standardization, to promote the ability
of unrelated parties to exchange encrypted information and to provide a single
encryption system that could be rigorously tested and publicly certified. As a
result, in 1972 the NBS issued a call for proposals for producing a public encryption
algorithm. The call specified desirable criteria for such an algorithm:

·
able to provide a high level of security

·
specified and easy to understand

·
publishable so that security does not depend on the secrecy of the
algorithm

·
available to all users

·
adaptable for use in diverse applications

·
economical to implement in electronic devices

·
efficient to use

·
able to be validated

·
exportable

The NBS envisioned providing
the encryption as a separate hardware device. To allow the algorithm to be
public, NBS hoped to reveal the algorithm itself, basing the security of the
system on the keys (which would be under the control of the users).

Few organizations responded
to the call, so the NBS issued a second announcement in August 1974. The most
promising suggestion was the Lucifer algorithm on which IBM had been working
for several years. This idea had been published earlier, so the basic algorithm
was already public and had been open to scrutiny and validation. Although
lengthy, the algorithm was straightforward, a natural candidate for iterative
implementation in a computer program. Furthermore, unlike the MerkleHellman
(which we study in __Chapter 12__) and RSA
algorithms, which use arithmetic on 500- or 1,000- digit or longer binary
numbers (far larger than most machine instructions would handle as a single
quantity), Lucifer used only simple logical operations on relatively small
quantities. Thus, the algorithm could be implemented fairly efficiently in
either hardware or software on conventional computers.

The data encryption algorithm
developed by IBM for NBS was based on Lucifer, and it became known as the Data
Encryption Standard, although its proper name is DEA (Data Encryption
Algorithm) in the United States and DEA1 (**Data
Encryption Algorithm-1**) in other countries. Then, NBS called on the
Department of Defense through its National Security Agency (NSA) to analyze the
strength of the encryption algorithm. Finally, the NBS released the algorithm
for public scrutiny and discussion.

The DES was officially adopted
as a U.S. federal standard in November 1976, authorized by NBS for use on all
public and private sector unclassified communication. Eventually, DES was
accepted as an international standard by the International Standards
Organization.

**Overview of the DES Algorithm**

The DES algorithm is a
careful and complex combination of two fundamental building blocks of
encryption: substitution and transposition. The algorithm derives its strength
from repeated application of these two techniques, one on top of the other, for
a total of 16 cycles. The sheer complexity of tracing a single bit through 16
iterations of substitutions and transpositions has so far stopped researchers
in the public from identifying more than a handful of general properties of the
algorithm.

The algorithm begins by
encrypting the plaintext as blocks of 64 bits. The key is 64 bits long, but in
fact it can be any 56-bit number. (The extra 8 bits are often used as check
digits and do not affect encryption in normal implementations.) The user can
change the key at will any time there is uncertainty about the security of the
old key.

The algorithm, described in substantial detail
in __Chapter 12__, leverages the two
techniques Shannon identified to conceal information: confusion and diffusion.
That is, the algorithm accomplishes two things: ensuring that the output bits
have no obvious relationship to the input bits and spreading the effect of one
plaintext bit to other bits in the ciphertext. Substitution provides the
confusion, and transposition provides the diffusion. In general, plaintext is
affected by a series of cycles of a substitution then a permutation. The
iterative substitutions and permutations are performed as outlined in __Figure 2-8__.

DES uses only standard
arithmetic and logical operations on numbers up to 64 bits long, so it is
suitable for implementation in software on most current computers. Although
complex, the algorithm is repetitive, making it suitable for implementation on
a single-purpose chip. In fact, several such chips are available on the market
for use as basic components in devices that use DES encryption in an
application.

**Double and Triple DES**

As you know, computing power
has increased rapidly over the last few decades, and it promises to continue to
do so. For this reason, the DES 56-bit key length is not long enough for some
people to feel comfortable. Since the 1970s, researchers and practitioners have
been interested in a longer-key version of DES. But we have a problem: The DES
algorithm is fixed for a 56-bit key.

**Double DES**

To address the discomfort, some researchers
suggest using a double encryption for greater secrecy. The double encryption
works in the following way. Take two keys, k_{1} and k_{2}, and
perform two encryptions, one on top of the other: E(k_{2}, E(k_{1},m)).
In theory, this approach should multiply the difficulty of breaking the encryption, just as
two locks are harder to pick than one.

Unfortunately, that assumption is false. Merkle
and Hellman showed that two encryptions
are no better than one. The basis of their argument is that the cryptanalyst
works plaintext and ciphertext toward each other. The analyst needs two pairs
of plaintext (call them P_{1} and P_{2}) and corresponding
ciphertext, C_{1} and C_{2}, but not the keys used to encrypt
them. The analyst computes and saves P_{1} encrypted

under each possible key. The analyst then tries
decrypting C_{1} with a single key and looking for a match in the saved
Ps. A match is a possible pair of double keys, so the analyst checks the match
with P_{2} and C_{2}. Computing all the Ps takes 2^{56}
steps, but working backward from C_{1} takes only the same amount of
time, for a total of 2 * 2^{56} or 2^{57}, equivalent to a
57-bit key. Thus, the double encryption only doubles the work for the attacker.
As we soon see, some 56 -bit DES keys have been derived in just days; two times
days is still days, when the hope was to get months if not years for the effort
of the second encryption.

**Triple DES**

However, a simple trick does
indeed enhance the security of DES. Using three keys adds significant strength.

The so-called **triple DES** procedure is C = E(k_{3}, E(k_{2}, E(k_{1},m))). That is, you encrypt
with one key, decrypt with the second, and encrypt

with a third. This process gives a strength
equivalent to a 112-bit key (because the double DES attack defeats the strength
of one of the three keys).

A minor variation of triple
DES, which some people also confusingly call triple DES, is C = E(k_{1}, D(k_{2}, E(k_{1},m))). That is, you encrypt

with one key, decrypt with
the second, and encrypt with the first again. This version requires only two
keys. (The second decrypt step also makes this process work for single
encryptions with one key: The decryption cancels the first encryption, so the
net result is one encryption.) This approach is subject to another tricky
attack, so its strength is rated at only about 80 bits.

In summary, ordinary DES has
a key space of 56 bits, double DES is scarcely better, but two -key triple DES
gives an effective length of 80 bits, and three-key triple DES gives a strength
of 112 bits. Now, over three decades after the development of DES, a 56-bit key
is inadequate for any serious confidentiality, but 80- and 112-bit effective
key sizes provide reasonable security.

**Security of the DES**

Since its was first
announced, DES has been controversial. Many researchers have questioned the
security it provides. Much of this controversy has appeared in the open
literature, but certain DES features have neither been revealed by the
designers nor inferred by outside analysts.

In 1990, Biham and Shamir invented a technique, **differential cryptanalysis**, that
investigates the change in algorithmic strength when an encryption algorithm is
changed in some way. In 1991 they applied their technique to DES, showing that
almost any change to the algorithm weakens it. Their changes included cutting
the number of iterations from 16 to 15, changing the expansion or substitution
rule, or altering the order of an iteration. In each case, when they weakened
the algorithm, Biham and Shamir could break the modified version.

Thus, it seems as if the
design of DES is optimal.

However, Diffie and Hellman argued in 1977 that a 56-bit key is too
short. In 1977, it was prohibitive to test all 2^{56 }(approximately 10^{15})
keys on then current computers. But they argued that over time, computers would
become more powerful and the DES algorithm would remain unchanged; eventually,
the speed of computers would exceed the strength of DES. Exactly that has
happened. In 1997 researchers using over 3,500 machines in parallel were able
to infer a DES key in four months' work. And in 1998 for approximately
$100,000, researchers built a special "DES cracker" machine that
could find a DES key in approximately four days.

Does this mean that the DES is insecure? No,
not yet. No one has yet shown serious flaws in the DES. The 1997 attack
required a great deal of cooperation, and the 1998 machine is rather expensive.
Triple DES is still well beyond the power of these attacks. Nevertheless, to anticipate
the increasing power of computers, it was clear a new, stronger algorithm was
needed. In 1995, the U.S. National Institute of Standards and Technology (NIST,
the renamed NBS) began the search for a new, strong encryption algorithm. The
response to that search has become the **Advanced
Encryption Standard, or AES**.

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Security in Computing : Elementary Cryptography : The Data Encryption Standard(DES) |

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