Medern Symmetric -Key Ciphers (AES, DES)

Modern Symmetric-Key Ciphers(AES, DES)
B P Poddar Institute of Management & Technology
Presented
by
Guided by
Maharaj Dey (11500117099)
Sandeep Kumar(11500117059)
Pratap Chandra Mondal
Term paper presentation
Computer Science Engineering
April – 2022
Cryptography and Network Theory 1

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Modern Symmetric-Key Ciphers(AES, DES) April – 2022 2
Medern Symmetric-key ciphers

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Why Was the AES Encryption Algorithm necessary?

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What are the Features of AES?
 SP Network: It works on an SP network structure rather than a Feistel cipher structure
 Key Expansion: It takes a single key up during the first stage, which is later expanded to
multiple keys used in individual rounds.
 Byte Data: The AES encryption algorithm does operations on byte data instead of bit data.
 So it treats the 128-bit block size as 16 bytes during the encryption procedure.
 Key Length: The number of rounds to be carried out depends on the length of the key being
used to encrypt data.
 The 128-bit key size has ten rounds,
 the 192-bit key size has 12 rounds
 the 256-bit key size has 14 rounds.

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How Does AES Work?
 a single block is 16 bytes, a 4x4 matrix holds the data
in a single block, with each cell holding a single byte of
information.
 The matrix shown in the image above is known as a
state array. Similarly, the key being used initially is
expanded into (n+1) keys, with n being the number of
rounds to be followed in the encryption process. So for
a 128-bit key, the number of rounds is 16, with no. of
keys to be generated being 10+1, which is a total of 11
keys.

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The steps to be followed in AES

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Add Round Key
You pass the block data stored in the state array through an XOR
function with the first key generated (K0). It passes the resultant state
array on as input to the next step.

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Sub-Bytes
it converts each byte of the state array into hexadecimal, divided into two equal
parts. These parts are the rows and columns, mapped with a substitution box
(S-Box) to generate new values for the final state array.

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Shift Rows
It swaps the row elements among each other. It skips the first row. It shifts the
elements in the second row, one position to the left. It also shifts the elements
from the third row two consecutive positions to the left, and it shifts the last row
three positions to the left.

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Modern Symmetric-Key Ciphers(AES, DES) April – 2022
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Mix Columns
It multiplies a constant matrix with each column in the state array to get a
new column for the subsequent state array. Once all the columns are
multiplied with the same constant matrix, you get your state array for the next
step. This particular step is not to be done in the last round.

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Add Round Key
The respective key for the round is XOR’d with the state array is
obtained in the previous step. If this is the last round, the resultant state
array becomes the ciphertext for the specific block; else, it passes as
the new state array input for the next round.

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Example

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1
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Example

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Example
Add Round Key

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Example
•Sub-Bytes: It passes the elements through a 16x16 S-
Box to get a completely new state array.

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Example
•Shift Rows

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Example
Mix Columns

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Example
•Add Round Key
This state array is now
the final ciphertext for
this particular round.
This becomes the input
for the next round.
Depending on the key
length, you repeat the
above steps until you
complete round 10

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1
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Example

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What Are the Applications of AES?

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What are the Features of AES?
 The Data Encryption Standard (DES) is a symmetric-key block cipher published by the
National Institute of Standards and Technology (NIST).
 DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure.
 The block size is 64-bit.
 Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits
of the key are not used by the encryption algorithm (function as check bits only).

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General Structure of DES

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Content
Since DES is based on the Feistel Cipher, all that is required to
specify DES is −
Round function
Key schedule
Any additional processing − Initial and final permutation

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Initial and Final Permutation
The initial and final permutations are
straight Permutation boxes (P-boxes)
that are inverses of each other. They
have no cryptography significance in
DES

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Round Function
The heart of this cipher is the DES
function, f. The DES function applies a
48-bit key to the rightmost 32 bits to
produce a 32-bit output.

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Expansion Permutation Box
Since right input is 32-bit and round key is a 48-bit, we
first need to expand right input to 48 bits.
The graphically depicted
permutation logic is generally
described as table

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XOR (Whitener)
After the expansion permutation, DES does XOR
operation on the expanded right section and the round
key.

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Substitution Boxes
The S-boxes carry out the real mixing (confusion). DES
uses 8 S-boxes, each with a 6-bit input and a 4-bit output

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The S-box rule
•There are a total of eight S-box tables. The output of all
eight s-boxes is then combined in to 32 bit section.

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Straight Permutation
The 32 bit output of S-boxes is then subjected to the
straight permutation with rule shown in the following
illustration

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Key Generation

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DES Analysis
 The DES satisfies both the desired properties of block cipher. These two
properties make cipher very strong.
 Avalanche effect − A small change in plaintext results in the very great
change in the ciphertext.
 Completeness − Each bit of ciphertext depends on many bits of plaintext.
 During the last few years, cryptanalysis have found some weaknesses in
DES when key selected are weak keys. These keys shall be avoided.

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Differences Between AES & DES
DES Algorithm AES Algorithm
Key Length - 56 bits Key Length - 128, 192, 256 bits
Block Size - 64 bits Block size - 128 bits
Fixed no. of rounds
No. of rounds dependent on
key length
Slower and less secure Faster and more secure

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Conclusion
This tutorial explores the need & working of AES & DES Encryption, its origin and process
of encryption, all the way up to its applications, and a direct comparison in both the
algorithm.
Thank You

Medern Symmetric -Key Ciphers (AES, DES)

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