Cryptography Reference
In-Depth Information
Algorithm Steps
1.
Padding
: The input message, denoted by
m
, is padded so that its bitlength
448(mod 512). If
is already 448 modulo 512,
before
padding, then
we still pad, in this case with 512 bits. The padded message is denoted
by
M
.
≡
2.
Appending
: A block of 64 bits is appended to
M
.
3.
Buffering
: A 160-bit buffer is employed to hold the intermediate and
final outputs of the algorithm. We represent the buffer having five 32-
bit registers, labelled
ABCDE
. (The buffer is initialized with specific
hexadecimal values that we will not cite here for the sake of simplicity.)
We will denote the five initialization values by
(
H
1
H
2
H
3
H
4
H
5
)
→
(
ABCDE
)
.
4.
Processing
: A module consisting of four rounds of 20 steps each employs
three different primitive logic functions. We will, for the sake of simplicity,
not describe their individual specific functions, rather we will call them
f
1
,
f
2
, and
f
3
. Each of these function inputs three 32-bit data strings or
words
and outputs 32-bit words. The notation is as follows.
We will assume that there is only one 512-bit block. The procedure can be
iterated to accommodate as many such blocks as necessary.
M
is divided
into sixteen 32-bit words, denoted by
m
j
for
j
=0
,
1
,...,
15. Then each
m
j
is put into temporary storage
m
j
→
X
j
. Then we expand the sixteen
32-bit words into eighty 32-bit words as follows.
First, we need some notation. Let
be addition modulo 2, and let
LS
k
be a circular shift left of
k
places, (for instance, see page 120, where we
used a slightly different notation for the
k
= 2 case in our description of
S-DES). For
j
=16
,
17
,...,
79, assign the following storage:
⊕
LS
1
(
X
j
−
16
⊕
X
j
−
14
⊕
X
j
−
8
⊕
X
j
−
3
)
→
X
j
.
5.
Rounds
: We need to employ four constants
c
i
for
i
=1
,
2
,
3
,
4. (These
have a certain hexadecimal representation that we need not cite here,
again forthe sake of simplicity.) Then each round operates on (the al-
ready initialized) buffer's so-called
chaining variables
ABCDE
, of 160
bits segmented into five 32-bit words, by updating the contents of the
buffer in each stepas follows, (where + denotes is addition modulo 2
32
):
Round 1
:For
j
=0
,
1
,...,
19, set,
(
LS
5
A
+
f
1
(
B,C,D
)+
E
+
X
j
+
c
1
,A,
LS
30
(
B
)
,C,D
)
→
(
A, B, C, D, E
)
.
Round 2
:For
j
=20
,
1
,...,
39, set,
(
LS
5
A
+
f
2
(
B,C,D
)+
E
+
X
j
+
c
2
,A,
LS
30
(
B
)
,C,D
)
→
(
A, B, C, D, E
)
.
Search WWH ::
Custom Search