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PPT – SYMMETRIC KEY ALGORITHMS PowerPoint presentation | free to download - id: 7d6d84-ZGUwN

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SYMMETRIC KEY ALGORITHMS

Comparison of Symmetric and Asymmetric Encryption

BLOCK CIPHER DESIGN PRINCIPLES

- Two properties of operation of secure cipher
- Confusion make the relationship between the

statistics of the ciphertext and the value of the

encryption key as complex as possible. Confusion

is achieved through a complex substitution. - Diffusion dissipates the redundancies of the

plaintext by distributing over the ciphertext.

Diffusion is achieved through permutations. - Claude Shannons Papers of 1948/1949
- A Mathematical Theory of Communication
- Communication Theory of Secrecy Systems
- To thwart cryptanalysis based on statistical

analysis

SIMPLIFIED DES

- Developed 1996 as a teaching tool
- Santa Clara University
- Prof. Edward Schaefer
- Takes an 8-bit block plaintext, a 10 bit key and

produces an 8-bit block of ciphertext - Decryption takes the 8-bit block of ciphertext,

the same 10-bit key and produces the original

8-bit block of plaintext

Simplified DES scheme

- Five Functions to Encrypt
- IP an initial permutation
- fk - a complex, 2-input function
- SW a simple permutation that swaps the two

halves of data - fk - a complex, 2-input function again
- IP inverse permutation of the initial

permutation

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S-DES KEY GENERATION

S-DES KEY GENERATION

- 10-bit key be designated as
- (k1, k2,k3, k4, k5, k6, k7, k8, k9, k10)
- Then the permutation P10 is defined as
- P10(k1, k2, k3, k4, k5, k6, k7, k8, k9, k10)
- (k3, k5, k2, k7, k4, k10, k1,

k9, k8, k6)

S-DES KEY GENERATION

- Perform a circular shift (LS-1), or rotation,

separately on the 1st 5 bits and the 2nd 5 bits. - Next, we apply P8,permute 8 of the 10 bits as
- Result is subkey 1, K1 , of 8 bits

S-DES KEY GENERATION

- Go back to the pair of 5-bit strings produced by

the two LS-1 functions, - and perform a circular left shift of 2 bit

positions, LS-2 ,on each string - Finally, P8 is applied again to produce K2 ,the

subkey 2

S-DES ENCRYPTION

S-DES Encryption

- 8-bit block of plaintext (eg. 10111101)
- First permute using the IP(Initial Permutation)

function as - At the end, apply inverse permutation IP-1

Encryption Detail

S-DES Encryption

- The Function F and fk
- Divide the value after IP into two parts L,

R - fk(L,R) (L F(R,SK),R)
- where SK is a subkey and is the bit-by-bit

XOR operation

S-DES Encryption- F(R,SK)

- Rightmost 4 bits(n1,n2,n3,n4) as input to E/P
- Expand the 4-bit value and concatenate it twice

into an 8-bit value . Then permute it. - Create a matrix based on the result
- Row 1
- Row 2

S-DES Encryption- F(R,SK)

- 8-bit subkey K1 (k11, k12, k13, k14, k15, k16,

k17,k18) and perform an exclusive-OR function on

the matrix in prev. step

- Rename the resultant matrix as
- The first 4 bits (first row of the preceding

matrix) are fed into the S-box S0 to produce a

2-bit output, and the remaining 4 bits (second

row) are fed into S1 to produce another 2-bit

output

- The S-boxes are
- The first and fourth input bits are treated as a

2-bit number that specify a row of the S-box - and the second and third input bits specify a

column of the S-box - (P0,0 P0,3) (11) 3 (P0,1 P0,2)

(10) 2 - (P1,0 P1,3) (11) 3 (P1,1 P1,2)

(00) 0

- Concatenate S0 (e.g. 311) and S1 (e.g. 210)

into a 4-bit value (e.g. 1110) - Permute 4 bit value as P4
- Output of Function F
- fk(L,R) (L F(R,SK),R)
- 4 bits of L F(R,SK) and R are given to

SW - SW interchanges the L and R bits to next function

fk(L,R)

Encryption Detail

8 Bit Plaintext

I P

4

E/P

8

8

K 1

4

4

4

S1

S0

2

2

P4

4

SW

4

- 8 bits passes through next function fk
- Key used is K2
- Finally , apply inverse permutation IP-1

S-DES Decryption

- Reverse process of encryption
- Input is the ciphertext
- Key K2 is applied before K1
- Output is the plaintext.