Block ciphers encrypt data in blocks, typically 64 or 128 bits, using cryptographic keys and algorithms. The Data Encryption Standard (DES) is a symmetric-key algorithm that encrypts data in 64-bit blocks using a 56-bit key. DES encryption uses a Feistel cipher structure involving multiple rounds of substitutions, permutations, and XOR operations to encrypt plaintext into ciphertext. The security of the algorithm increases with a larger block and key size, more complex round functions, and a greater number of rounds.

1. 
BLOCK CIPHER
AND
DATA ENCRYPTION
STANDARD

2. 
BLOCK CIPHER
A block cipher is a method of encrypting text(to produce
cipher text) in which cryptographic keys and algorithms
are applied to a block of data.
At once as a group rather than to a one bit.
 Typically, a block size of 64 or 128 bits is used.

3. 
CIPHER TEXT
ENCRYPTION ALGORITHM
PLAIN TEXT
BLOCK CIPHER
b bits
b bits
Key
(k)

4. 
 A stream cipher is one that encrypts a digital data
stream one bit or one byte at a time.
 Since encryption if each digit is depend on current
state.
 Stream cipher is generally faster than block cipher.
STREAM CIPHER

5. 
Example of Stream
Cipher
Key
Stream
Plain text
Cipher text
Stream
Cipher text
Plain text
Key
(Example of stream encryption) (Example of stream decryption)

6. 
 Feistel cipher is proposed by the Claude Shannon and also
known as Feistel network.
 Feistel cipher is a symmetric structure used in the construction
of block cipher.
 Feistel cipher is iterative in nature and its internal functions
called round.
 Feistel networks are develop the concept of product cipher.
 The product cipher that combine the multiple rounds
of repeated operation, such as
Feistel Cipher

7. 
 Bit shifting(often called permutation box or P-box)
 Simple non-linear functions(often called substitution
boxes or S-box)
 Linear mixing(in the sense of modular algebra)
using Xor.
Feistel Cipher

8. 
 To produce function with large amount , Shannon
describe the Confusion and Diffusion.
 Confusion means that each character of the cipher
text should be depend on several part of the key.
 Confusion refers to making the relationship between
the cipher text and symmetric key.
Feistel Cipher

9. 
 Diffusion means if we change a character of the plain text,
the several character of the cipher text should be changed.
 Similarly ,if we change the character of the cipher text, the
several character of plain text should be changed.
 The simplest way to achieve the both diffusion and
confusion is to use a substitution-permutation network.
Feistel Cipher

10. 
 Substitution each plaintext element or group of
elements is uniquely replaced by a corresponding
cipher text element or group of elements.
 Permutation, a sequence of plaintext elements is
replaced by a permutation of that sequence.
 That is, no elements are added or deleted or replaced
in the sequence, rather the order in which the
elements appear in the sequence is changed.
Feistel Cipher

11. 
 In feistal cipher structure partitions input block into
two halves.
 The plaintext block is divided into two halves.
 The two halves of the data pass through rounds of
processing and then combine to produce the cipher
text block.
 Each round has as inputs and derived from the
previous round, as well as a sub key.
Feistel Cipher Structure

12. 
 All rounds have the same structure.
 A substitution is performed on the left half of the
data.
 This is done by applying a round function to the right
the data and the output of that function and the left
half of the data.
Cont..

13. 
Feistel Cipher Structure

14. 
 Block size: Larger block sizes mean greater security
(all other things being equal) but reduced
encryption/decryption speed for a given algorithm.
 Key size: Larger key size means greater security but
may decrease encryption/decryption speed.
 The greater security is achieved by greater resistance
to brute-force attacks and greater confusion.
Feistel Cipher Design
Principles

15. 
 Number of rounds: The essence of the Feistel cipher
is that a single round offers inadequate security but
that multiple rounds offer increasing security.
 A typical size is 16 rounds.
 Sub key generation algorithm: Greater complexity
in this algorithm should lead to greater difficulty of
cryptanalysis.
 Round function F: Again, greater complexity
generally means greater resistance to cryptanalysis.
Cont..

16. 
 The Data Encryption Standard (DES) has been the
most widely used data encryption algorithm.
 There are two main types of cryptography in use
symmetric or secret key cryptography and
asymmetric or public key cryptography.
 Secret key cryptography, it involves the use of only
one key or same key which is used for both
encryption and decryption.
DATA ENCRYPTION
STANDARD

17. 
DATA ENCRYPTION
STANDARD
E{P,K} D{C,K}
Plaintext(p)
Secret Key (K)
Cipher text (C)
Secret Key (K)
Plaintext (P)
Secrete key encryption

18. 
General Depiction of DES Encryption Algorithm
Cont..

19. 
 Block length is 64-bit.
 Key length is 56-bit.
 No of rounds is 16
 Each round is applied on one block cipher.
Cont..

20.  Substitution box(s-box) has extensively used in almost all
cryptographic algorithm, such as data encryption
standard(DES) and advance encryption standard(AES)
 S-box is basic component of symmetric key algorithm.
 The s-box provide the confusion.
 S-box is used to make the relation between the plain text
and the cipher text.
 the S-box takes m input and transform into n output bits.
 The s-box is an m*n substitution box that means we can
represent in matrix.
Substitution boxes

21. 
 PERMUTATION BOX(p-box) is a method of bit-
shuffling used to permute or transpose the bit across
s-box.
 A p-box is a permutation of all the bits.it take the
output of all the s-boxes of one round, permutes the
bits and feeds them into the s-box of the next round.
 At each round ,the round key is combined using
some group operations, typically xor.
PERMUTATION BOX

22. 
 DES (and most of the other major symmetric ciphers) is
based on a cipher known as the Feistel block cipher.
 It consists of a number of rounds where each round
contains bit-shuffling, non-linear substitutions (S-boxes)
and exclusive OR operations.
 As with most encryption schemes, DES expects two
inputs - the plaintext to be encrypted and the secret key.
Workings of DES

23.  The manner in which the plaintext is accepted, and the
key arrangement used for encryption and decryption,
both determine the type of cipher it is.
 DES is therefore a symmetric, 64 bit block cipher as it
uses the same key for both encryption and decryption and
only operates on 64 bit blocks of data at a time.
 The key size used is 56 bits, however a 64 bit (or eight-
byte)key is actually input.
 All blocks are numbered from left to right which makes
the eight bit
Cont..

24. 
 All blocks are numbered from left to right which
makes the eight bit
 Once a plain-text message is received to b encrypted,
it is arranged into 64 bit blocks required for input.
 If the number of bits in the message is not evenly
divisible by 64,then the last block will be padded.
Cont..

25. 