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DATA^ ENCRYPTION
STANDARD
The^ Data^ Encryption
Standard^ (DES)
was^ designed
by^ IBM. DES^ ,^ adopted
in^1977 by^ National
Bureau^ of
Standards(NBS),
now^ National^ institute
of^ Standards
and^ Technology(NIST)
as^ Federal^ information
processing^ standard
It^ was^ unbroken
for^ more^ than^
10 years^ since^ its
publication^ and
some^ aspects^ of
its^ design^ were
kept^ secret^ by^ IBM
at^ the^ request^
of^ the^ US^ National
Security^ Agency
(NSA);^ some^ people
believed^ that
IBM^ and^ NSA^ had
hidden^ a^ trapdoor
in^ DES^ that^ only
they^ knew^ about
(that^ they^ could
use^ to^ crack^ DES)
DES^ ‐History
Under^ controversy,
how^ secure^ the^
DES^ is?
In^ late^ 1960,^ IBM
setup^ research^ in
computer^ crypto
led
by^ Horst^ Fiestel. Concluded^ in^ 1971,
with^ LUCIFER(
‐bit^ ,key^128 ‐bit),
use^ in^ cash‐dispensing
system.
Good^ results,^ decided
to^ develop^ a^ marketable
commercial^ encryption
product^ that^ could
implement^ on
single^ chip. Effort^ was^ headed
by^ Walter^ Tuchman
and^ Carl^ Meyer,
with^ advice^ from
NSA.
The^ outcome^ was
a^ refined^ version
of^ LUCIFER,^ more
resistant^ to^ cryptanalysis,
with^ reduced^ key
of^56 ‐bit,^ to
fit^ on^ single^ chip. In^ 1973,^ NBS,^ issue
proposal^ for^ national
chip^ standard^ ,
IBM^ submit^ project,
and^ was^ adopted
as^ DES^ in^ 1977.
DES‐wide^ use
DES^ flourish
in^ financial^ applications.
In^ 1994,^ NIST
reaffirm^ to^ use
it^ for^ more^5 years.
In^ 1999,^ NIST
issue^ a^ new^ version
of^ its^ standard
triple^ DES.
DES^ block^
cipherPlaintext (64) IP Subkey^1 (48)F Subkey^ 2...15^ (48)F Subkey^16 (48)F ‐ 1 IP Ciphertext (64)
DES^ Encryption(key) The right‐hand^ portion,^ shows
how^56 ‐bit^ key
is
used. Initially^ ,^ the^
key^ is^ passed^ through
a^ permutation
function. For^ each^ of^ the
16 rounds,^ a^ subkey (k
)^ isi
produced^ by^ the
combination^ of
the^ left^ circular
shift^ and^ the^ permutation.
the^ permutation
function^ is^ same
for^ each^ round,
but^ the^ different
subkey is^ produced
because^ of^ the
repeated^ shifts
of^ the^ key^ bits.
Initial^ Permutation:
IP(from^ last
tables)
-^ The^32 ‐bit^ input^ data
(message)^ block^ is^ first^
bitwise^ permutated^ (i.e.,
the^ bits within^ the^ block^ are^ rearranged) • This^ is^ done^ using^ the^ following
permutation^ table: Output Input 1 2 3 4 5 6 7 8
-^ Example:^ 35th^ bit^ of
output^ block^ is^ equal^ to
the^ 41st^ bit^ of^ the^ input
block. docsity.com
DES^ Cipher
FunctionInput^ (32)Subkey Output^ (32)
(48) F RoundFunction
(^32) E^48 4848 S^32 P^32
CipherFunction docsity.com
Detail^ of^ single
Round(continue..)
Expansion^ Permutation:
E(from^ last table)
-^ The^ expansion^ permutation
acts^ on^ the^32 ‐bit^ input^ to^ the
cipher^ function
-^ It^ expands^ the^32 ‐bit^ input
block^ to^ a^48 ‐bit^ output^ block
by^ duplicating^ some^ input^ bits
at^ specified positions • The^ permutation^ is^ given^ by^ the
following^ table: Output bit Input^ bit 1 2 3 4 5 6 32 1 2 3 4 5 7 8 9 10 11 12 4 5 6 7 8 9 13 14 15 16 17 18 8 9 10 11 12 1319 20 21 22 23 24 12 13 14 15 16 1725 26 27 28 29 30 16 17 18 19 20 2131 32 33 34 35 36 20 21 22 23 24 2537 38 39 40 41 42 24 25 26 27 28 2943 44 45 46 47 48 28 29 30 31 32 1
-^ Example:^ 46nd^ bit^ of^ output
block^ (counting^ from^ left,^ starting
from^ bit^ 1)^ is^ equal^ to^ the^ 31st
bit^ of
input^ block
Substitution
Boxes:^ S
•^ The^ substitution
boxes^ (S‐boxes)
map^ a^6 ‐bit
input^ block^ to
a^4 ‐bit^ output
block
•^ There^ are^8
S‐boxes,^ so^ the
48 ‐bit^ input^ block
is^ mapped^ to^ a
32 ‐bit^ output
block
Substitution
Boxes:^ S
S1^ S2^ S
S4^ S^
S6^ S7^ S
