Overview of Data Encryption Standard (DES)
Computer Security
IT423
Semester II
2017 - 2018
Dr. Saman Mirza Abdullah
saman.mirza@ishik.edu.iq
Data Encryption Standard
(DES)
2
3/4/2025
Computer Security - Ishik
Class Objective
•
The main objective of this class is to:
o
To review a short history of DES
o
To define the basic structure of DES
o
To describe the details of building elements of DES
o
To describe the round keys generation process
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Confusion and Diffusion
•
confusion
o
Make relation between statistics of ciphertext and the value of the
encryption key as complex as possible.
•
diffusion
o
Diffuse statistical property of plaintext digit across a range of ciphertext
digits
•
Each plaintext digits affects value of many ciphertext
digits, or has same relation with many ciphertext
digits.
4
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Feistel Cipher
•
It is a design model, not
specific method.
•
Uses same algorithm for
encryption and decryption.
•
Has multiple round of
processing plaintext.
•
Each round consists
S-box
and
P-box
step.
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5
DES
•
The
Data Encryption Standard
(DES) is
o
a symmetric-key block cipher published in 1973 by the
National Institute of
Standards and Technology
(NIST), as a proposal.
o
It has been accepted by
as a draft of the
Federal Information Processing
Standard
(FIPS), in March 1975
.
o
Although the draft was criticized, on 1977 the drafted has been published.
6
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Overview
•
DES takes:
o
A 64-bit plaintext and creates a 64-bit ciphertext; at the decryption site,
o
A 64-bit ciphertext and creates a 64-bit block of plaintext.
o
The same 56-bit cipher key is used for both encryption and decryption.
7
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3/4/2025
DES Structure
Encryption
•
Has two
P-box
es:
o
Initial and final permutation.
o
Each is inverse to the other.
•
Has 16 rounding or
(S-box).
•
Accepts 64 bit Plaintext.
•
Has 56 bit cipher key, 48
bit used.
o
Based on some predefined
algorithms.
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DES Structure
Encryption / (P-box )
•
Has two
P-box
es:
o
Initial and final permutation.
o
Each is inverse to the other.
•
It is applied directly on
64 bit of the plaintext.
•
They are keyless.
•
Increases the role of
hardware in
encryption process.
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DES Structure
Encryption / (P-box )
•
We have only the 15
th
bit and 64
th
bit are
one.
•
15
63
•
64
25
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•
Find the output of the
initial permutation box
when the input is
given in hexadecimal
0x0002 000 0000 0001
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DES Structure
Encryption / (P-box )
•
We have only the 25
th
bit and 63
rd
bit are
one.
•
63
15
•
25
64
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•
Find the output of the
Final permutation box
when the input is
given in hexadecimal
0x 0000 0080 0000 0002
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DES Structure
Encryption
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Done
Done
What are going
on inside the
rounds?
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DES Structure
Encryption / Rounds
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Computer Security - Ishik
•
This Called the DSE function.
•
It is the hart of the DES
structure.
•
It is apply a 48 bit key to the
32 bit input. Always the
rightmost bit.
•
It gives at output 32 bit.
•
It comes in different steps.
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•
These steps are occurred in
the DES Function
o
Expansion D-box.
o
Whitener XOR
o
S-Box
o
Straight D-box
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•
Why Expansion is needed?
•
Input to the round steps has 32 bits size, the Key has
48 bits. So, expansion needed.
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•
Input will be divided
into 8 four bit
sections.
•
Each 4 bit section
expends to 6 bits
section.
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•
S-boxes are doing real mixing. (Confusion).
•
DSE has 8 S-boxes, each.
o
Has 6 inputs and 4 outputs.
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How the process is going
on inside each
S-boxes
.
•
For each
S-box
, we
have a truth table.
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•
The truth table for
S-box 1 is differ from others
.
•
There is no similar truth table among the eight boxes.
•
Use the S-box below to get the output of the
110001
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•
The 1
st
bit is (1) and the 6
th
bit is (1), so the row is 11
which means (3)
•
The rest of the bits are (1000) which means (8).
•
The box with index (3,8) has a value of (05).
•
Converting (05) to binary, it will be (0101).
•
It is the final stage of DES function. Sometimes called as Final
Permutation.
•
It is the 32 bits input / 32 bites output permutation operation.
•
It follows a straight permutation truth table, as shown below:
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•
According to the above table, the 7
th
bit of the input
will be the 2
nd
bit of the output.
Key Generation
•
First step is Parity drop:
o
Removing the parity bit. (if Available).
o
Do permutation according to a give
table.
o
Output is 64 bits input becomes 56 bits.
•
Divide the 56 bits into two
parts, each 28 bits.
o
Left part and right part.
o
Do left bit shifting.
•
Given permutation /
compression table, do
permutation to reduce the
size to 48 bits.
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Key Generation
Key Generation
Example
Example
•
A 10 bits key is shared between sender and receiver. Use the
following information to generate K
1
and K
2
, which each has 8 bits
sizes.
o
K=(10100 00010) is the main key.
o
No parity bits are there.
o
P
10
permutation is (3 5 2 7 4 10 1 9 8 6)
o
P
8
permutation is (6 3 7 4 8 5 10 9).
==========================================
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o
No parity bits will be removed.
o
P
10
(K) = P10 (10100 00010) = (10000 01100).
o
Left part is (10000) and Right part is (01100).
o
Applying left shifting on both part L = 00001 and R = 11000.
o
Combining the L and R = 0000111000
o
P8( L and R) = P
8
(00001 11000) = (10100100)
o
Therefore, the sub-key k
1
is (10100100)
Key Generation
Key Generation
Example
Example
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25
o
No parity bits will be removed.
o
P
10
(K) = P10 (10100 00010) = (10000
01100).
o
Left part is (10000) and Right part is
(01100).
o
Applying left shifting on both part L =
00001 and R = 11000.
o
Combining the L and R = 0000111000
o
P
8
( L and R) = P
8
(00001 11000) =
(10100100)
o
Therefore, the sub-key k
1
is (10100100)
With which of the above information we
should start for computing the set values
of sub-key k
2
?
Key Generation
Key Generation
Example
Example
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26
o
No parity bits will be removed.
o
P
10
(K) = P10 (10100 00010) = (10000
01100).
o
Left part is (10000) and Right part is
(01100).
o
Applying left shifting on both part L =
00001 and R = 11000.
o
Do left shifting bit.
o
L=00010 and R= 10001
o
Combining the L and R =
00010 10001
o
P
8
permutation is (6 3 7 4 8 5 10 9).
o
P
8
( L and R) = P
8
(
00010 10001
) =
(
10010010
)
o
Therefore, the sub-key k
1
is (
10010010
)
Quiz ?
•
When need to find the sub-key K3,
o
Which information will be taken in consideration?
o
Dose the process of round 3 will go as round or not? If any change is there
show and highlight the change.
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Read More
•
The DES Algorithm Illustrated.pdf in the same folder.
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28
Class End
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Data Encryption Standard (DES) is a symmetric-key block cipher created in 1973 by NIST. It uses a 64-bit plaintext to generate a 64-bit ciphertext and vice versa. The same 56-bit cipher key is employed for encryption and decryption, utilizing rounds with S-box and P-box steps, ensuring confusion and diffusion. Learn about the history, structure, and objectives of DES in this comprehensive guide.
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Presentation Transcript
Computer Security IT423 Semester II 2017 - 2018 Dr. Saman Mirza Abdullah saman.mirza@ishik.edu.iq
Data Encryption Standard (DES) Computer Security - Ishik 3/4/2025 2
Class Objective The main objective of this class is to: o To review a short history of DES o To define the basic structure of DES o To describe the details of building elements of DES o To describe the round keys generation process Computer Security - Ishik 3/4/2025 3
Confusion and Diffusion confusion o Make relation between statistics of ciphertext and the value of the encryption key as complex as possible. diffusion o Diffuse statistical property of plaintext digit across a range of ciphertext digits Each plaintext digits affects value of many ciphertext digits, or has same relation with many ciphertext digits. Computer Security - Ishik 3/4/2025 4
Feistel Cipher It is a design model, not specific method. Uses same algorithm for encryption and decryption. Has multiple round of processing plaintext. Each round consists S-box and P-box step. Computer Security - Ishik 3/4/2025 5
DES The Data Encryption Standard (DES) is o a symmetric-key block cipher published in 1973 by the National Institute of Standards and Technology (NIST), as a proposal. o It has been accepted by as a draft of the Federal Information Processing Standard (FIPS), in March 1975 . o Although the draft was criticized, on 1977 the drafted has been published. Computer Security - Ishik 3/4/2025 6
Overview DES takes: o A 64-bit plaintext and creates a 64-bit ciphertext; at the decryption site, o A 64-bit ciphertext and creates a 64-bit block of plaintext. o The same 56-bit cipher key is used for both encryption and decryption. Computer Security - Ishik 3/4/2025 7
DES Structure Encryption Has two P-boxes: o Initial and final permutation. o Each is inverse to the other. Has 16 rounding or (S-box). Accepts 64 bit Plaintext. Has 56 bit cipher key, 48 bit used. o Based on some predefined algorithms. Computer Security - Ishik 3/4/2025 8
DES Structure Encryption / (P-box ) Has two P-boxes: o Initial and final permutation. o Each is inverse to the other. It is applied directly on 64 bit of the plaintext. They are keyless. Increases the role of hardware in encryption process. Computer Security - Ishik 3/4/2025 9
DES Structure Encryption / (P-box ) Find the output of the initial permutation box when the input is given in hexadecimal 0x0002 000 0000 0001 We have only the 15th bit and 64th bit are one. 15 63 64 25 Computer Security - Ishik 3/4/2025 10
DES Structure Encryption / (P-box ) Find the output of the Final permutation box when the input is given in hexadecimal 0x 0000 0080 0000 0002 We have only the 25th bit and 63rd bit are one. 63 15 25 64 Computer Security - Ishik 3/4/2025 11
DES Structure Encryption Done What are going on inside the rounds? Done Computer Security - Ishik 3/4/2025 12
DES Structure Encryption / Rounds The input to the rounds will be divided into two parts (Left and Right). The round has Swapperand Mixer processes. Swapper inverted the right half of the input with left. Mixer uses a Function to mix the first half with a key. ?(?? 1,??) Computer Security - Ishik 3/4/2025 13
DES Structure / Rounds/?(?? 1,??) This Called the DSE function. It is the hart of the DES structure. It is apply a 48 bit key to the 32 bit input. Always the rightmost bit. It gives at output 32 bit. It comes in different steps. Computer Security - Ishik 3/4/2025 14
DES Structure / /?(?? 1,??)/????? These steps are occurred in the DES Function o Expansion D-box. o Whitener XOR o S-Box o Straight D-box Computer Security - Ishik 3/4/2025 15
?(??1,??)/????????? Why Expansion is needed? Input to the round steps has 32 bits size, the Key has 48 bits. So, expansion needed. Computer Security - Ishik 3/4/2025 16
?(??1,??)/????????? Input will be divided into 8 sections. Each 4 bit section expends to 6 bits section. four bit Computer Security - Ishik 3/4/2025 17
?(??1,??)/Whitener XOR Computer Security - Ishik 3/4/2025 18
? ??1,?? /? ????? S-boxes are doing real mixing. (Confusion). DSE has 8 S-boxes, each. o Has 6 inputs and 4 outputs. How the process is going on inside each S-boxes. Computer Security - Ishik 3/4/2025 19
? ??1,?? /? ????? For each S-box, we have a truth table. The truth table for S-box 1 is differ from others. There is no similar truth table among the eight boxes. Computer Security - Ishik 3/4/2025 20
? ??1,?? /? ????? Use the S-box below to get the output of the 110001 The 1st bit is (1) and the 6th bit is (1), so the row is 11 which means (3) The rest of the bits are (1000) which means (8). The box with index (3,8) has a value of (05). Converting (05) to binary, it will be (0101). Computer Security - Ishik 3/4/2025 21
? ??1,?? ?????? ? ??????????? It is the final stage of DES function. Sometimes called as Final Permutation. It is the 32 bits input / 32 bites output permutation operation. It follows a straight permutation truth table, as shown below: According to the above table, the 7th bit of the input will be the 2nd bit of the output. Computer Security - Ishik 3/4/2025 22
Key Generation First step is Parity drop: o Removing the parity bit. (if Available). o Do permutation according to a give table. o Output is 64 bits input becomes 56 bits. Divide the 56 bits into two parts, each 28 bits. o Left part and right part. o Do left bit shifting. Given permutation / compression table, do permutation to reduce the size to 48 bits. Computer Security - Ishik 3/4/2025 23
Key Generation Example A 10 bits key is shared between sender and receiver. Use the following information to generate K1 and K2, which each has 8 bits sizes. K=(10100 00010) is the main key. No parity bits are there. P10 permutation is (3 5 2 7 4 10 1 9 8 6) P8 permutation is (6 3 7 4 8 5 10 9). o o o o ========================================== o No parity bits will be removed. o P10 (K) = P10 (10100 00010) = (10000 01100). o Left part is (10000) and Right part is (01100). o Applying left shifting on both part L = 00001 and R = 11000. o Combining the L and R = 0000111000 o P8( L and R) = P8 (00001 11000) = (10100100) o Therefore, the sub-key k1 is (10100100) Computer Security - Ishik 3/4/2025 24
Key Generation Example o No parity bits will be removed. o P10 (K) = P10 (10100 00010) = (10000 01100). o Left part is (10000) and Right part is (01100). o Applying left shifting on both part L = 00001 and R = 11000. o Combining the L and R = 0000111000 o P8( L and R) = P8 (00001 11000) = (10100100) o Therefore, the sub-key k1 is (10100100) With which of the above information we should start for computing the set values of sub-key k2? Computer Security - Ishik 3/4/2025 25
Key Generation Example o No parity bits will be removed. o P10 (K) = P10 (10100 00010) = (10000 01100). o Left part is (10000) and Right part is (01100). o Applying left shifting on both part L = 00001 and R = 11000. o Do left shifting bit. o L=00010 and R= 10001 o Combining the L and R = 00010 10001 o P8 permutation is (6 3 7 4 8 5 10 9). o P8( L and R) = P8 (00010 10001) = (10010010) o Therefore, the sub-key k1 is (10010010) Computer Security - Ishik 3/4/2025 26
Quiz ? When need to find the sub-key K3, o Which information will be taken in consideration? o Dose the process of round 3 will go as round or not? If any change is there show and highlight the change. Computer Security - Ishik 3/4/2025 27
Read More The DES Algorithm Illustrated.pdf in the same folder. Computer Security - Ishik 3/4/2025 28
Class End Computer Security - Ishik 3/4/2025 29
