Cryptography Reference
In-Depth Information
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.
On pages 45 and 46, we learned that Qalqashandi was the first to introduce
the intermingled use of substitution and transposition in a single cipher. In a
more modern-day setting, we saw how such a combination of substitution and
transposition was used in the World War II ADFGVX field cipher, whose use
is illustrated on page 80. Even closer to the modern day is the DES cipher,
which employs some of the best combinations of transposition and substitution.
Although we mentioned DES on page 98, we gave no indication of how this
block cipher works. It is now time to learn about this cryptosystem in detail.
Although this cryptosystem is no longer used as the standard, the fact that this
block cipher ruled the roost for about a quarter century makes it deserving of
a closer look, if for no other reason than historical, in keeping with Chapters 1
and 2. Moreover, certain stronger ciphers derived from it are still valid and in
use.
DES is a symmetric-key block cipher, encrypting
octograms of bytes
3.2
with
a key based on a permutation, then sixteen substitutions followed by another
permutation. This is another way of saying that DES enciphers 64-bit blocks of
plaintext to produce 64-bit blocks of ciphertext, using the same key for encryp-
tion and decryption, a key that is based on a combination of substitution and
permutation techniques. However, in the interests of ease of presentation, we
are going to illustrate DES in a simplified form introduced by Ed Schaefer [232]
in 1996. This version uses only 8-bit plaintext and 10-bit keys to produce 8-bit
ciphertext, not secure, but pedagogically more satisfying for our purposes. (For
a complete description of the entire DES algorithm, see [169, pages 86-99].) We
look at each component of DES and build the edifice until the final construction
of the cryptosystem, dubbed
S-DES
.
3.2
A
byte
is an 8-bit binary integer, see Footnote 2.7 on page 84. Therefore, an octogram of
bytes is a collection of eight bytes, or a block consisting of a 64-bit integer. In what follows,
we will suppress base-integer subscripts. For example, (110)
2
will be written as (110) with
the context understood.
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