Cryptography Reference
In-Depth Information
Fig. 3.1
Round
transformation
in the Feistel scheme
3.2 The Data Encryption Standard
The Data Encryption Standard (DES) is a block cipher that was designed during
the 1970s. In May 1973, the US National Bureau of Standards called for proposals
for an encryption algorithm that could be used by companies to secure their com-
munications. At that time, no candidate was retained as an acceptable encryption
standard. Following a second call in August 1974, IBM submitted a cipher called
Lucifer designed by Feistel and his colleagues. After a few modifications of the
design by the US National Security Agency, the cipher was selected as an official
standard in 1976. The use of DES quickly became internationally widespread even
though it was soon identified to be vulnerable to exhaustive key search due to its short
key length. To tackle this issue, Triple-DES was proposed as a replacement for DES,
which consists of three successive applications of DES with different keys. Although
a new Advanced Encryption Standard (AES) was adopted in 2002, Triple-DES is
still widely used, for instance, in the smart card protocol EMV.
1
DES uses a 56-bit key (usually represented on 64 bits including 8 parity check
bits) and it operates on 64-bit message blocks. It has an iterative structure apply-
ing the same round transformation
F
16 times, which is preceded by an initial
bit-permutation
IP
and followed by a final bit-wise permutation
FP
. Every round
transformation is parameterized by a 48-bit round key
K
r
that is derived from the
secret key
K
through a key schedule process. To summarize, a ciphertext
C
is com-
puted from a plaintext
P
as
1
F
K
r
16
r
C
=
FP
◦
◦
IP
(
P
).
=
The round transformation follows a Feistel scheme: the block is split into two 32-bit
parts,
L
(the left half) and
R
(the right half), and
F
is defined as
)
=
R
)
,
F
K
r
(
L
,
R
,
L
⊕
f
K
r
(
R
where
f
is a function mapping 32 bits to 32 bits and is parameterized with a 48-bit
round key. This structure is illustrated in Fig.
3.1
.
1
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