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530 Chapter 9 • Basis of Cryptography

be increased. Both of these solutions tend to increase the processing
power required to encrypt and decrypt data and slow down the encryp-
tion/decryption speed, because of the increased number of mathemat-
ical operations required. Examples of modified DES include Triple Data
Encryption Standard (3DES) and DESX.

Each round consists of a substitution phase, wherein the data is substituted with
pieces of the key, and a permutation phase, wherein the substituted data is scrambled
(re-ordered). Substitution operations, sometimes referred to as confusion operations,
occur within S-boxes. Similarly, permutation operations, sometimes called diffusion oper-
ations, are said to occur in P-boxes. Both of these operations occur in the “F
Module” of the diagram.The security of DES lies in the fact that since the substi-
tution operations are non-linear, the resulting ciphertext does not resemble the
original message.The permutation operations add another layer of security by
scrambling the already partially encrypted message.

TEST DAY TIP

Keep in mind that all of the operations mentioned above (substitution,
permutation, and so forth) are being performed on the data in binary
form. Instead of trading out letters and numbers, 1’s and 0’s are being
manipulated because these manipulations can be performed at high
speeds.

Triple DES (3DES) and DESX are methods that attempt to use the DES cipher
in a way that increases its security.Triple DES uses three separate 56-bit DES keys
as a single 168-bit key, though sometimes keys 1 and 3 are identical, yielding 112-
bit security. DESX adds an additional 64 bits of key data. Both 3DES and DESX
are intended to strengthen DES against brute-force attacks. It would take many
years to decrypt 3DES encrypted date (depending on available computing power).
However, 3DES is inefficient because it requires two to three times the processing
overhead as a single DES.

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